frust_core/

app.rs

//! The render root: the object each platform shell drives each frame.
//!
//! It owns the widget [`WidgetTree`] and the previous [`View`], and exposes the
//! three framework passes in Masonry order (the subset relevant to v0):
//!
//! * [`RenderRoot::rebuild`] — run the root build closure, diff against the previous view,
//!   producing/mutating the retained widget.
//! * [`RenderRoot::layout`] — hand the root widget window-sized constraints and
//!   record the size it returns.
//! * [`RenderRoot::paint`] — emit the root widget's draw commands into a scene.
//!
//! v0 is single-root: the root component's build closure returns one `impl View<State>` whose concrete
//! type is fixed, so the root's previous view and element are stored typed.
//! ViewSequence / multiple children are explicitly out of scope for now.

use std::any::Any;
use std::cell::Cell;
use std::num::NonZeroU64;
use std::sync::atomic::{AtomicU64, Ordering};

use kurbo::{Point, Rect, Size};

use crate::anim::FrameTime;
use crate::event::{
    ContactFrame, ContactPass, CursorIcon, EventCtx, EventOutcome, EventResult, FileDropEvent,
    FileDropPhase, ImeState, InputEvent, OverlayEvent, OverlayEventKind, PointerButton,
    PointerEvent, PointerId, PointerPhase, RequestPass,
};
use crate::insets::WindowInsets;
use crate::layout::BoxConstraints;
use crate::overlay::{
    OutsideTap, OverlayEntry, OverlayHit, OverlayInput, OverlayKey, OverlayPaintPass,
    sort_into_paint_order,
};
use crate::selection_toolbar::{
    SelectionToolbarActions, SelectionToolbarPass, SelectionToolbarRequest,
};
use crate::semantics::{ROOT_NODE_ID, SemanticsCtx, SemanticsUpdate};
use crate::tree::{InspectNode, WidgetPod, WidgetTree};
use crate::view::{BuildCtx, ChangeFlags, View, WidgetId};
use crate::widget::{LayoutCtx, PaintCtx, PaintOutcome, PaintScene, PlatformViewFrame};

/// The window's shape and platform-occlusion state, delivered to app code as a
/// plain [`provide_context`](reactive_graph::owner::provide_context)-carried
/// value — logical size, device-pixel scale, a
/// [derived](Orientation::from_size) orientation, and the current
/// [`WindowInsets`].
///
/// # Plain value, not a signal
///
/// `WindowMetrics` is delivered exactly like `Theme` and [`WindowInsets`]
/// already are: a shell calls `provide_context` with a freshly-built value on
/// change, and app code recovers it with `use_context::<WindowMetrics>()`
/// inside `Component::build`. It is **not** an `RwSignal` — only `deep_link`
/// and `back` are true signals in `frust-reactive`; every other host-signal
/// carrier (theme, insets, and now this) is a re-provided plain value.
///
/// # Alongside `WindowInsets`, not superseding it
///
/// `WindowInsets` already reaches `Component::build` on Android and iOS today
/// (each shell's `push_insets` calls `provide_context(insets)` independently
/// of anything here — desktop has no such arm for either value yet).
/// `WindowMetrics` is additive: a shell that starts providing it keeps
/// providing the standalone `WindowInsets` context too, so an existing
/// `use_context::<WindowInsets>()` call site never breaks. `insets` on this
/// type is a **copy** of that same value for convenience (a widget laying
/// itself out around window shape wants size/scale/orientation/insets
/// together), not a replacement for the independent context.
///
/// # Orientation is derived, not platform-sourced
///
/// No platform callback in either mobile shell carries an orientation enum —
/// Android's `nativeOnSurfaceChanged` and iOS's `frust_resize` each hand the
/// shell only a `(width, height, scale)` triple. [`Orientation`] is therefore
/// always computed from `size` via [`Orientation::from_size`]
/// (portrait when `height >= width`, so an exact square reads as portrait);
/// it never tracks a device orientation-lock setting or a platform rotation
/// event directly.
///
/// # Context is not reactive
///
/// `provide_context` is a plain insert into the owner's context map — it
/// notifies nothing — and `use_context` inside `Component::build` (or the
/// root build closure) creates no subscription, so re-providing a changed
/// `WindowMetrics` does not itself mark anything dirty or wake a frame. A new
/// value becomes visible only on the next rebuild, which the resize or inset
/// change that produced it already drives; do not write a shell that assumes
/// a `provide_context` write triggers one. A shell wiring this up (see
/// `docs/SHELLS_ARCHITECTURE.md`) must still re-provide `WindowMetrics` only
/// on an actual change (mirroring `RenderRoot::set_insets`'s
/// `PartialEq`-guarded no-op) — the reason is cost at the FFI boundary (a
/// lock write plus an allocation every frame), not a rebuild storm.
/// Separately, there is no per-component rebuild skipping in this framework
/// (a component always re-runs `build` on any rebuild it does take part in),
/// which is affordable only because builds are cheap by construction.
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct WindowMetrics {
    /// The window's logical (density-independent) size.
    pub size: Size,
    /// The device-pixel scale factor (logical → physical px multiplier).
    pub scale: f64,
    /// The orientation derived from `size` — see the type's doc for why this
    /// is computed, never platform-sourced.
    pub orientation: Orientation,
    /// A copy of the window's current insets — see the type's doc for why
    /// this does not replace the standalone `WindowInsets` context.
    pub insets: WindowInsets,
}

impl WindowMetrics {
    /// Construct a [`WindowMetrics`] from its transported fields, deriving
    /// [`orientation`](Self::orientation) from `size` rather than accepting it
    /// as an input — see the type's doc for why orientation is never
    /// platform-sourced.
    pub fn new(size: Size, scale: f64, insets: WindowInsets) -> Self {
        Self {
            size,
            scale,
            orientation: Orientation::from_size(size),
            insets,
        }
    }
}

/// A window's derived portrait/landscape orientation.
///
/// Always computed from a [`WindowMetrics::size`] via [`Orientation::from_size`]
/// — see [`WindowMetrics`]'s doc for why no platform callback carries this as
/// an enum directly.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Orientation {
    /// `size.height >= size.width`, including the exact-square case.
    Portrait,
    /// `size.height < size.width`.
    Landscape,
}

impl Orientation {
    /// Derives orientation from a logical window size: portrait when
    /// `height >= width` (an exact square reads as portrait), landscape
    /// otherwise.
    pub fn from_size(size: Size) -> Self {
        if size.height >= size.width {
            Orientation::Portrait
        } else {
            Orientation::Landscape
        }
    }
}

/// How many [`InputEvent::Housekeeping`] flush passes one
/// [`RenderRoot::rebuild`] will run before deferring the rest to the next frame.
///
/// A flushed pop-result callback may itself push or pop, queueing another
/// callback — so the flush/re-diff cycle has to be allowed to iterate, but it
/// must never be allowed to spin: a pair of callbacks that push each other would
/// otherwise hang the frame. Three passes covers every shape observed in
/// practice (a result that navigates once, and that page's own result), while
/// keeping the worst case at four build-closure runs per frame — the closure is
/// cheap by construction (see [`RenderRoot::rebuild`]).
///
/// Past the cap the mark stays raised and one more frame is requested, so the
/// remaining work lands next frame instead of being lost.
const MAX_PENDING_RESULT_FLUSH_PASSES: usize = 3;

/// Change-guarded write of the shell-facing IME surface: replaces `slot` and
/// bumps `generation` **only** when the value actually moves ([`ImeState`] is
/// `PartialEq`).
///
/// A free function over the two fields rather than a `&mut self` method, so the
/// change guard has exactly one implementation whatever borrows its caller
/// happens to hold; [`RenderRoot::store_ime_state`] is the `&mut self` form, and
/// is what every current caller goes through.
///
/// The change guard is load-bearing, not an optimisation: the paint pass
/// re-publishes the focused widget's IME surface every frame, so an
/// unconditional bump would make the shell's `focus_or_ime_changed` edge fire on
/// every vsync for the whole life of a focus session — the level-input behavior
/// the generation exists to replace.
fn store_ime_state_in(slot: &mut Option<ImeState>, generation: &mut u64, next: Option<ImeState>) {
    if *slot != next {
        *slot = next;
        *generation = generation.wrapping_add(1);
    }
}

/// Release the whole focus/IME session: drop `focus_active` **and** the
/// shell-facing surface together, bumping `generation` **exactly once** if
/// either actually moved.
///
/// The paired form of [`RenderRoot::set_focus_active`]`(false)` +
/// [`RenderRoot::store_ime_state`]`(None)`, and the single primitive every
/// release site goes through — the blur-on-outside-tap `Down`, an explicit
/// [`EventCtx::release_focus`](crate::event::EventCtx::release_focus), a widget
/// publishing an *inactive* surface (see [`RenderRoot::paint`]), and the
/// generic-unmount orphan drain in [`RenderRoot::rebuild`]. Keeping them on one
/// primitive is what makes "a session ends" mean the same thing everywhere,
/// rather than four hand-assembled pairs that can drift apart.
///
/// **One release is one edge.** The two field writers bump on each field's own
/// change, so calling them in sequence would move
/// [`focus_ime_generation`](RenderRoot::focus_ime_generation) *twice* for the
/// ordinary release (focus `true`→`false` and surface `Some`→`None`). A shell
/// only ever compares the value, so two bumps and one bump raise the same single
/// `focus_or_ime_changed` edge — but a counter that moves once per observable
/// transition is the contract the field doc states, and is what the release
/// tests pin. The change guard itself is unchanged: an already-released root
/// writes the same values back and moves nothing.
///
/// **A release ends the session's identity too.** On an actual move it advances
/// `focus_epoch` and republishes it, which strands every recorded focus link at
/// once — the chain the session ran through, and a floated surface's link that
/// no container's blur sweep can reach. That is why the release has to own the
/// epoch rather than leave it to the caller: a session cleared without moving
/// its identity leaves links behind that still name it.
///
/// A free function over the fields (not a `&mut self` method) for the same
/// reason [`store_ime_state_in`] is — one implementation of the contract,
/// whatever borrows the caller holds. [`RenderRoot::release_focus_session`] is
/// the method form, and is what every current caller goes through.
fn release_focus_session_in(
    focus_active: &mut bool,
    ime_slot: &mut Option<ImeState>,
    generation: &mut u64,
    focus_epoch: &mut u64,
    root_identity: u64,
) {
    let moved = *focus_active || ime_slot.is_some();
    *focus_active = false;
    *ime_slot = None;
    if moved {
        *generation = generation.wrapping_add(1);
        // A session that ends strands every link recorded against it, wherever
        // in (or off) the tree it sits — the one clearing sweep no container can
        // be asked to run. Only on an actual release: a `Down` on already-blurred
        // chrome is the commonest event there is and must move nothing.
        *focus_epoch = advance_focus_epoch(*focus_epoch);
        crate::widget::set_live_focus_session(root_identity, *focus_epoch, *focus_epoch);
    }
}

/// The allocator behind [`RenderRoot::root_identity`], handing every root a
/// value no other root shares.
///
/// Starts at `1` so `0` stays available as "no root" (a pod that has never held a
/// claim, and the at-rest published hover link — see
/// `crate::event::set_live_hover_link`).
///
/// A module-level static rather than an associated const/`static` inside the
/// generic `impl`: the latter is monomorphized per `<State, V>` pair, which would
/// hand two roots of different concrete types the same identity — exactly the
/// collision this counter exists to remove. `Relaxed` is enough because the value
/// is only ever compared for equality, never used to order anything.
static NEXT_ROOT_IDENTITY: AtomicU64 = AtomicU64::new(1);

/// The next focus epoch after `epoch`, skipping `0`.
///
/// `0` is reserved twice over — it is a never-claimed
/// [`ChildPod`](crate::widget::ChildPod)'s stamp, and it is the epoch half of
/// the `(0, 0)` pair a dispatch driven with no root at all compares against — so
/// a root that wrapped onto it would hand every unclaimed pod in the tree a live
/// link at once.
fn advance_focus_epoch(epoch: u64) -> u64 {
    match epoch.wrapping_add(1) {
        0 => 1,
        next => next,
    }
}

/// What [`RenderRoot`]'s overlay pre-pass decided about one incoming event.
///
/// The pre-pass runs before anything else [`RenderRoot::event`] does, and has
/// exactly two answers: the event belonged to a floated surface (or was swallowed
/// by a modal light-dismiss) and the main tree must not see it, or it did not and
/// today's dispatch continues. Both arms carry an [`EventOutcome`], because even
/// the "continue" answer may already have produced one — an
/// [`OutsideTap::Notify`]`{ consume: false }` surface is told about the press
/// *and* lets it through, and the redraw that notification asked for must not be
/// dropped on the floor when the main dispatch's own outcome replaces it.
enum OverlayRoute {
    /// The overlay layer consumed the event; return this outcome unchanged.
    Consumed(EventOutcome),
    /// The event continues into today's dispatch; merge this outcome into
    /// whatever that produces.
    Continue(EventOutcome),
}

/// Owns the retained tree and drives the rebuild/layout/paint passes for a
/// single-root application.
///
/// Generic over the application `State` and the concrete root view type `V`
/// returned by the build closure.
pub struct RenderRoot<State: 'static, V: View<State>> {
    tree: WidgetTree,
    root_id: Option<WidgetId>,
    /// The previous view, retained to diff against on the next rebuild.
    prev_view: Option<V>,
    /// Monotonic widget-id counter, borrowed by each `BuildCtx`.
    next_id: u64,
    window_size: Size,
    /// The **claimant** of the pointer capture in flight, if any: the contact
    /// whose `Down` requested capture. Set on that `Down`, cleared only by the
    /// claimant's own `Up`/`Cancel` — another contact's release never touches
    /// it (rule (c) of [`InputEvent::PointerContact`]'s multi-contact contract).
    /// Root-level mirror of the per-container `active` path bookkeeping, which
    /// is keyed on the same claimant (see [`crate::widget::ChildPod::set_active`]).
    capture_claimant: Option<PointerId>,
    /// Whether the live capture's captor opted into the gesture's other
    /// contacts ([`EventCtx::capture_contacts`]) on the `Down` it captured
    /// with. Meaningless — and kept `false` — while nothing is captured.
    /// Cleared early when a container takes the gesture over from that captor
    /// ([`EventCtx::release_captured_child`]).
    capture_contacts: bool,
    /// Whether the live opt-in was made by the root widget itself rather than
    /// by a pod below it — in which case a non-claimant contact is handed to the
    /// root widget directly instead of walking the active path (see
    /// [`crate::widget::ChildPod::event_child`]). Same lifetime as
    /// `capture_contacts`.
    contacts_captor_is_root: bool,
    /// Whether some widget in the tree currently holds focus. Root-level mirror of
    /// the per-container `focused` path bookkeeping (the focus analog of
    /// `capture_claimant`): set when a dispatch requested focus, cleared by a
    /// session release — a blur-on-outside-tap `Down`, an explicit focus release,
    /// a widget publishing an inactive IME surface, or the generic-unmount orphan
    /// drain in [`RenderRoot::rebuild`] (see [`release_focus_session_in`]).
    focus_active: bool,
    /// Which branch the live focus/IME session belongs to: `None` for the main
    /// tree, `Some(key)` for the floated surface whose pod holds the recorded
    /// focus path. The identity `focus_active` deliberately does not carry — one
    /// bool cannot say *whose* session it is, and a surface's chain and the main
    /// tree's are not siblings any container's blur sweep can reach across.
    ///
    /// **Resolved from links that can be shown to be live.** The only
    /// authoritative view of a pod's recorded link the root ever gets is the pod
    /// itself, which it holds for exactly the length of
    /// [`RenderRoot::paint_overlays`] — so that pass writes this, from
    /// [`crate::widget::ChildPod::holds_live_focus`], and the event pass only ever
    /// *clears* it (a hit-tested claim is the main tree's by construction, and a
    /// release ends the session outright). An overlay-pass focus request cannot
    /// be attributed at the root: the bubble is a bare flag, and a field in the
    /// main tree re-claiming its own session through a floated toolbar raises
    /// exactly the same one as an editable inside the surface claiming it for the
    /// first time.
    ///
    /// The liveness half is what makes the record worth keeping. A pod's raw
    /// `focused` flag survives the session moving away from it — nothing visits
    /// an abandoned branch to clear one — so a record resolved from the flag
    /// alone latched on the first surface that ever took focus and never let go.
    /// Resolved from the stamp instead, it answers a question the code can
    /// falsify, and it names nobody the moment the session leaves every surface.
    ///
    /// **It no longer gates the tree's paint seed**, which is the other half of
    /// the same correction: seeding is per-link, against `focus_epoch`, so a
    /// branch proves its own claim rather than the root vouching for it from one
    /// frame behind. What is left here is *provenance* — whether an overlay
    /// dispatch's IME publish is the session owner's — plus the cross-surface
    /// retirement in [`RenderRoot::event`], both of which genuinely need a name
    /// rather than a per-link answer.
    focus_surface: Option<OverlayKey>,
    /// The identity of the live focus session — what a
    /// [`ChildPod`](crate::widget::ChildPod) stamps beside its recorded focus
    /// link, and the only thing that tells a link on the session the root has
    /// now from one a moved session left behind.
    ///
    /// The focus analog of `hover_epoch`, with one difference that follows from
    /// focus having no per-pass rhythm: this advances **around a dispatch**
    /// rather than at the end of one. [`RenderRoot::event`] moves it forward
    /// before it dispatches and puts it back afterwards unless the dispatch
    /// actually recorded a claim — so a claim is stamped with a value nothing
    /// older carries, and a pass that moved no focus leaves every standing link
    /// exactly as it found it. Published to the pods through
    /// [`crate::widget::set_live_focus_session`], which is where a container
    /// deciding routing, and a pod's own destructor, read it.
    ///
    /// Starts at `1`, not `0`: a freshly built pod's stamp is `0`, and `(0, 0)`
    /// is what a dispatch driven with no root at all sees, so a real root must
    /// never publish that pair.
    ///
    /// Wrapping is deliberate and harmless — the value is only ever compared for
    /// equality, never ordered — but it skips `0` on the way round (see
    /// [`advance_focus_epoch`]).
    focus_epoch: u64,
    /// Whether the last completed hover pass left some widget in the tree holding
    /// the hover link. Root-level mirror of the per-pod hover stamp (the hover
    /// analog of `focus_active`), seeded into every event/paint pass so nothing
    /// below can read as hovered while the root says nothing is.
    hover_active: bool,
    /// The live hover epoch: the identity of the most recent completed hover pass.
    ///
    /// Advanced by exactly one per hover pass — an **uncaptured**
    /// [`PointerPhase::Move`] (which may record a claim), or the `Down`/`Up`/
    /// `Cancel` that ends a hover outright (which may not) — and by nothing else,
    /// so a scroll, key, IME, or housekeeping pass leaves a live hover standing.
    /// A [`crate::widget::ChildPod`]'s recorded stamp counts as hovered only while
    /// it equals this, which is what strands the previous claimant's path with no
    /// container having to clear it (see the [`crate::event`] module docs).
    ///
    /// Starts at `1`, not `0`: a freshly built pod's stamp is `0`, and starting the
    /// epoch past it means a never-claimed pod cannot match the live epoch by
    /// accident before the first hover pass ever runs.
    hover_epoch: u64,
    /// This root's process-unique identity, assigned once at construction from
    /// [`NEXT_ROOT_IDENTITY`] and never reused.
    ///
    /// It exists for exactly one comparison: the hover-orphan channel
    /// (`crate::event`'s `mark_hover_orphaned`/`take_hover_orphaned`) is a
    /// thread-local a *destructor* writes, so a second root driving passes on the
    /// same thread can otherwise see a mark that is none of its business.
    /// `hover_epoch` cannot tell them apart — every root's counter starts at `1`
    /// and advances per hover pass, so two roots hold colliding integers as a rule
    /// rather than as a fluke. Publishing and draining `(identity, epoch)` is what
    /// keeps one root's unmounting claimant from ending another's live hover.
    root_identity: u64,
    /// The cursor the last cursor pass resolved — hover's sibling channel, and
    /// the value a desktop shell reads through [`RenderRoot::cursor`].
    ///
    /// Deliberately **not** derived from `hover_active`: that mirror is
    /// identity-free (it knows *that* something is hovered, not which widget or
    /// what shape it wants), so a request travels its own pass-scoped slot
    /// ([`EventCtx::set_cursor`]) and is resolved here.
    ///
    /// Re-resolved on every pointer [`PointerPhase::Move`], captured or not:
    /// whatever the pass requested, or [`CursorIcon::Default`] when it requested
    /// nothing. Every other pass leaves it standing — see [`RenderRoot::event`]
    /// for why a `Down`/`Up` must not reset it. There is no generation counter
    /// beside it: the shell compares the value it last applied (see
    /// [`RenderRoot::cursor`]).
    cursor: CursorIcon,
    /// The text the tree last asked the shell to put on the host clipboard, or
    /// `None` once drained — the cursor's write-only sibling, resolved from the
    /// same kind of per-pass slot ([`EventCtx::write_clipboard`]) by the same
    /// [`RequestPass`] bracket.
    ///
    /// **One-shot, unlike [`cursor`](RenderRoot::cursor).** A cursor is a *level*
    /// (a standing shape a shell re-applies when it differs); a clipboard write
    /// is an *edge* (a thing to do once), so the accessor
    /// [`RenderRoot::take_clipboard_write`] drains it and a shell that forgets to
    /// call it merely delays the write rather than repeating it.
    ///
    /// A pass that writes replaces whatever stood here undrained — the newest
    /// copy is the one the user meant, and the shell is expected to drain after
    /// every dispatch — while a pass that writes nothing leaves it alone rather
    /// than silently discarding a write nobody has taken yet.
    pending_clipboard_write: Option<String>,
    /// Whether the tree has asked the shell to read the host clipboard back to it
    /// ([`EventCtx::request_paste`]), until drained by
    /// [`RenderRoot::take_paste_request`].
    ///
    /// The data-free twin of [`pending_clipboard_write`](RenderRoot::pending_clipboard_write),
    /// and one-shot for the same reason. Raised by any pass in which a widget
    /// asked and lowered only by the drain, so a shell that skips a drain answers
    /// late rather than losing the paste.
    pending_paste_request: bool,
    /// The IME surface the focused widget last published (via
    /// [`EventCtx::publish_ime_state`]), surfaced to the shell by
    /// [`RenderRoot::ime_state`]. Persists across rebuilds/events until refreshed
    /// by a new publish or dropped by a release (a blur, a focus release, an
    /// inactive publish, or a generic-unmount orphan drain — see
    /// [`release_focus_session_in`]).
    ///
    /// Only ever `None` or an **active** surface: an inactive publish is a
    /// release, never a stored value (see [`RenderRoot::ime_state`]).
    ime_state: Option<ImeState>,
    /// A monotonically-increasing generation bumped on every **actual** change
    /// of `focus_active` or `ime_state` — the focus/IME session's edge signal,
    /// read by a shell through [`RenderRoot::focus_ime_generation`].
    ///
    /// The mobile frame gate turns this into an *edge* input
    /// (`FrameInputs::focus_or_ime_changed`): a shell caches the last value it
    /// saw and runs a frame when it moves. A *level* input ("something holds
    /// focus") forced a frame every vsync for as long as a field stayed focused,
    /// which made caret pacing unreachable — measured at 62–120 fps on a static
    /// screen whose only live input was focus (Xiaomi 12).
    ///
    /// Same-value writes deliberately do **not** bump it (see
    /// [`RenderRoot::set_focus_active`]/[`RenderRoot::store_ime_state`]): the
    /// paint pass republishes the focused widget's IME surface on *every* frame,
    /// so bumping on write rather than on change would re-create exactly the
    /// per-vsync forcing this edge exists to remove.
    focus_ime_gen: u64,
    /// The [`PlatformViewFrame`]s the tree published during the most recent
    /// [`RenderRoot::paint`], surfaced to the shell via
    /// [`RenderRoot::platform_view_frames`]. Unlike `ime_state` above, this is
    /// REPLACED wholesale every pass (never merged with the previous one), so
    /// a pass that publishes none yields an empty `Vec` — a culled/removed
    /// slot from the prior frame does not linger as a stale frame. Core stays
    /// dumb here: the shell's differ owns absent-means-hide/dispose semantics.
    platform_view_frames: Vec<PlatformViewFrame>,
    /// The z-shield rects the tree reported during the most recent
    /// [`RenderRoot::paint`] (via [`crate::widget::PaintCtx::report_input_shield`]),
    /// surfaced to the shell via [`RenderRoot::input_shields`].
    ///
    /// Exactly the `platform_view_frames` discipline above — REPLACED wholesale
    /// every pass, so a pass whose shields stopped painting reports none. Core
    /// stays dumb: it never associates a shield with a slot, that is the
    /// shell-side differ's job.
    input_shields: Vec<Rect>,
    /// Dirtiness accumulated since the last [`RenderRoot::take_change_flags`] —
    /// merged from each rebuild so a shell can decide, in one place, whether a
    /// frame needs layout/paint at all.
    pending: ChangeFlags,
    /// The app's active theme, stored type-erased so `frust-core` needs no
    /// `frust-theme` dependency (the concrete `Theme` is boxed by the shell —
    /// see [`RenderRoot::set_theme`]). Lent as `Option<&dyn Any>` into each
    /// [`LayoutCtx`]/[`PaintCtx`]; `None` until a shell sets one (a supported
    /// state — bare-core tests and pre-theme apps run without a theme).
    theme: Option<Box<dyn Any>>,
    /// The window's insets ([`WindowInsets`]), delivered by the shell via
    /// [`RenderRoot::set_insets`] and threaded into every subsequent
    /// layout/paint pass (recovered by widgets through
    /// [`crate::widget::LayoutCtx::window_insets`]/
    /// [`crate::widget::PaintCtx::window_insets`]). Unlike the theme this is a
    /// concrete core-owned type (only `f64` scalars), stored by value — no
    /// `Box<dyn Any>` erasure needed. Defaults to the zero inset until a shell
    /// pushes one (a supported state — bare-core tests and pre-insets apps).
    insets: WindowInsets,
    /// The shell's running count of frames the render thread has actually
    /// presented, threaded into every subsequent paint pass and recovered by
    /// widgets through [`crate::widget::PaintCtx::presented_frames`]. A plain
    /// `u64` core stores by value (like the insets). `None` until a shell pushes
    /// one via [`RenderRoot::set_presented_frames`] — a supported state
    /// (bare-core tests and pre-wiring shells run without it), so widgets can
    /// fall back to a paint-cadence measure. Unlike the theme/insets this is a
    /// pure observation: [`RenderRoot::set_presented_frames`] deliberately marks
    /// NO [`ChangeFlags`] and bumps NO semantics generation (see its doc), so a
    /// ticking presented count never forces a relayout or feeds the mobile frame
    /// gate.
    presented_frames: Option<u64>,
    /// Whether the shell created a translucent (alpha-channel, "Mode B") GPU
    /// surface, threaded into every subsequent paint pass and recovered by
    /// widgets through [`crate::widget::PaintCtx::is_translucent`]. A plain
    /// `bool` core stores by value (like the insets); `false` (opaque, "Mode A")
    /// until a shell pushes one via [`RenderRoot::set_surface_translucent`] — the
    /// supported default for every desktop app and bare-core test. The
    /// platform-view hole-punch is the sole reader: a slot clears its rect only
    /// on a translucent surface (see `frust-widgets`' `PlatformViewWidget`).
    surface_translucent: bool,
    /// The persistent, never-reused per-pod semantics base-id allocator's next
    /// value. Seeded at `2` (ids `0`/`1` reserved: `0` keeps
    /// `NonZeroU64` valid, `1` is the [`ROOT_NODE_ID`] window node), advanced as
    /// [`ChildPod`](crate::widget::ChildPod)s are assigned bases on their first
    /// semantics visit, and carried across passes so a pod that first appears on a
    /// later frame never collides with an already-assigned one. A `Cell` because
    /// [`RenderRoot::semantics`] runs behind `&self`.
    semantics_alloc: Cell<u64>,
    /// The root widget's stable semantics base id (the root pod is arena-backed,
    /// not a [`ChildPod`](crate::widget::ChildPod), so it caches its base here
    /// rather than in a pod). Lazily assigned on the first semantics pass.
    root_semantics_id: Cell<Option<NonZeroU64>>,
    /// A monotonically-increasing generation bumped whenever a rebuild or theme
    /// swap could have changed the semantics tree, so a shell can cheaply skip
    /// re-pulling + re-pushing an unchanged accessibility tree (the semantics
    /// dirty gate — see [`RenderRoot::semantics_if_changed`]). v1 recompute is
    /// acceptable; this is the seam a shell gates on.
    semantics_gen: u64,
    /// Set by [`RenderRoot::rebuild`] when the deferred-callback flush owes the
    /// shell a frame, and folded into the next [`RenderRoot::paint`]'s
    /// [`PaintOutcome::needs_frame`] (then cleared). Two raisers, both in the
    /// flush loop: hitting [`MAX_PENDING_RESULT_FLUSH_PASSES`] with work still
    /// owed, and a dispatched [`InputEvent::Housekeeping`] whose
    /// [`EventOutcome::needs_redraw`] came back set.
    ///
    /// The frame-request half of the deferral: `pending |= PAINT` already tells
    /// the mobile frame gate to run its next tick, but the desktop loop is
    /// dirty-driven (`ControlFlow::Wait`) and schedules off `needs_frame`, so the
    /// deferral has to surface there too — otherwise the remaining flush would
    /// wait for whatever input happens to arrive next, which is the exact
    /// failure this whole mechanism exists to remove.
    deferred_frame: bool,
    /// The routing half of the overlay entries the **last** [`RenderRoot::paint`]
    /// registered, in paint order (`Floating` band then `Tooltip`, registration
    /// order within each) — what [`RenderRoot::event`]'s overlay pre-pass
    /// hit-tests before the main tree ever sees a pointer.
    ///
    /// Replaced wholesale every paint, exactly like `platform_view_frames`: an
    /// owner keeps a surface routable by registering it again each frame, so a
    /// surface whose owner stopped registering (or was unmounted) stops taking
    /// input after the next paint with nothing to unregister.
    ///
    /// Carries **no pod handle** by construction (see
    /// [`OverlayHit`](crate::overlay::OverlayHit)): the owner owns the pod, and a
    /// root holding a clone of it between passes would both outlive the owner and
    /// invite a borrow held across a pass boundary.
    ///
    /// One frame of lag is inherent and intended: input is routed against where
    /// the surfaces were painted, which is the only place the user could have
    /// seen them.
    overlay_hits: Vec<OverlayHit>,
    /// The selection-toolbar request the focused field published during the most
    /// recent [`RenderRoot::paint`], surfaced to the shell through
    /// [`RenderRoot::selection_toolbar`] for the platform edit-menu route.
    ///
    /// Resolved per paint pass: a pass in which nothing published clears it, which
    /// is what puts the menu away when a selection collapses. A session release
    /// clears it too (see [`RenderRoot::release_focus_session`]), so the menu can
    /// never outlive the focus the selection belonged to — the event pass's blur
    /// lands a whole frame before the paint that would otherwise notice.
    selection_toolbar: Option<SelectionToolbarRequest>,
    /// A monotonically-increasing generation bumped on every **actual** change of
    /// `selection_toolbar` — the same change-guarded edge signal
    /// `focus_ime_gen` is, and for the same reason: the publishing field
    /// re-publishes an unchanged request every single frame its selection stands,
    /// so bumping on write rather than on change would ask the shell to re-present
    /// the platform menu on every vsync.
    ///
    /// Kept beside the value rather than inside it so the counter survives a
    /// clear: a shell diffs the generation to notice the menu went *away* just as
    /// much as to notice it appeared.
    selection_toolbar_gen: u64,
    _state: core::marker::PhantomData<fn(&mut State)>,
}

impl<State: 'static, V: View<State>> RenderRoot<State, V> {
    /// Create an empty render root with no widget yet built.
    pub fn new() -> Self {
        Self {
            tree: WidgetTree::new(),
            root_id: None,
            prev_view: None,
            next_id: 0,
            window_size: Size::ZERO,
            capture_claimant: None,
            capture_contacts: false,
            contacts_captor_is_root: false,
            focus_active: false,
            focus_surface: None,
            // Past a fresh pod's `0` stamp, and past the `(0, 0)` a rootless
            // dispatch sees — see the field doc.
            focus_epoch: 1,
            hover_active: false,
            // Past a fresh pod's `0` stamp — see the field doc.
            hover_epoch: 1,
            root_identity: NEXT_ROOT_IDENTITY.fetch_add(1, Ordering::Relaxed),
            cursor: CursorIcon::Default,
            pending_clipboard_write: None,
            pending_paste_request: false,
            ime_state: None,
            focus_ime_gen: 0,
            platform_view_frames: Vec::new(),
            input_shields: Vec::new(),
            pending: ChangeFlags::NONE,
            theme: None,
            insets: WindowInsets::default(),
            presented_frames: None,
            surface_translucent: false,
            // Ids 0 and 1 are reserved (see the field doc); pods start at 2.
            semantics_alloc: Cell::new(2),
            root_semantics_id: Cell::new(None),
            semantics_gen: 0,
            deferred_frame: false,
            overlay_hits: Vec::new(),
            selection_toolbar: None,
            selection_toolbar_gen: 0,
            _state: core::marker::PhantomData,
        }
    }

    /// Store the app's active theme, threaded into every subsequent
    /// layout/paint pass as `Option<&dyn Any>` and recovered by widgets via
    /// [`crate::widget::PaintCtx::theme_as`]/[`crate::widget::LayoutCtx::theme_as`].
    ///
    /// The theme is boxed **type-erased** (`Box<dyn Any>`) so this crate stays
    /// independent of `frust-theme`; the shell boxes the concrete `Theme`
    /// (and re-boxes it on a live appearance change, e.g. dark-mode toggle).
    /// Calling again replaces the stored theme.
    ///
    /// Marks `LAYOUT | PAINT` pending (drained by
    /// [`RenderRoot::take_change_flags`]): a theme swap can change baked-in
    /// paint state a widget resolves at layout time (e.g. `Text`'s themed
    /// glyph color, cached into its `TextLayout` — see
    /// `frust-widgets::text`), so a shell that later gates layout/paint on
    /// this seam must still see a bare `set_theme` as dirty even though no
    /// view changed.
    pub fn set_theme(&mut self, theme: Box<dyn Any>) {
        self.theme = Some(theme);
        self.pending |= ChangeFlags::LAYOUT | ChangeFlags::PAINT;
        // A theme swap can change semantics-visible state (e.g. a relabelled or
        // re-bounded node once layout re-runs); treat it as semantics-dirty too.
        self.semantics_gen = self.semantics_gen.wrapping_add(1);
    }

    /// Store the window's insets ([`WindowInsets`]), threaded into every
    /// subsequent layout/paint pass and recovered by widgets via
    /// [`crate::widget::LayoutCtx::window_insets`]/
    /// [`crate::widget::PaintCtx::window_insets`].
    ///
    /// Mirrors [`RenderRoot::set_theme`]'s dirty-tracking contract: a change
    /// marks `LAYOUT | PAINT` pending (drained by
    /// [`RenderRoot::take_change_flags`]) so a shell gating layout/paint on that
    /// seam still relayouts when the insets move — a `SafeArea` widget resolves
    /// its inset at layout time, so the mobile layout-skip gate must see a bare
    /// `set_insets` as dirty even though no view changed (the same reasoning as
    /// the theme swap — see `docs/ARCHITECTURE.md`'s Theme delivery and Frame
    /// gate). A change also bumps the semantics generation, since a moved inset
    /// shifts laid-out node bounds.
    ///
    /// No-op guarded by [`WindowInsets`]'s `PartialEq`: pushing the current
    /// value marks nothing dirty, so a shell that polls the platform insets
    /// every frame and forwards unconditionally never forces a needless
    /// relayout. (A shell may also skip the call itself by comparing first —
    /// this is the same guard, held on the core side.)
    pub fn set_insets(&mut self, insets: WindowInsets) {
        if self.insets == insets {
            return;
        }
        self.insets = insets;
        self.pending |= ChangeFlags::LAYOUT | ChangeFlags::PAINT;
        // A moved inset shifts laid-out node bounds once layout re-runs; treat
        // it as semantics-dirty too (mirrors `set_theme`).
        self.semantics_gen = self.semantics_gen.wrapping_add(1);
    }

    /// The window's insets currently threaded into the layout/paint passes.
    pub fn insets(&self) -> WindowInsets {
        self.insets
    }

    /// Store the shell's running count of frames the render thread has actually
    /// presented, threaded into every subsequent paint pass and recovered by
    /// widgets via [`crate::widget::PaintCtx::presented_frames`]. A shell loads
    /// the atomic its render side increments (once per presented frame) and
    /// pushes it here once per UI frame, before `paint`.
    ///
    /// **Deliberately dirties nothing.** Unlike [`RenderRoot::set_theme`] and
    /// [`RenderRoot::set_insets`] — which mark `LAYOUT | PAINT` pending because a
    /// widget bakes their value in at layout time — this setter marks NO
    /// [`ChangeFlags`] and bumps NO semantics generation. The presented count is
    /// a paint-only *observation* a widget reads live every paint (never baked at
    /// layout), so treating it as dirty would be wrong twice over: it would force
    /// a needless relayout, and — critically — on the mobile shells a
    /// monotonically ticking counter would keep the frame gate's pending-flags
    /// input perpetually true, so the menu would never idle (the 32s-idle
    /// behavior must survive). Keeping this setter dirt-free
    /// is exactly what keeps the frame gate unaware of it (see
    /// `docs/ARCHITECTURE.md`'s Frame gate).
    pub fn set_presented_frames(&mut self, presented: u64) {
        self.presented_frames = Some(presented);
    }

    /// The presented-frame count currently threaded into the paint pass, or
    /// `None` if no shell has pushed one.
    pub fn presented_frames(&self) -> Option<u64> {
        self.presented_frames
    }

    /// Store whether the shell's GPU surface is translucent (alpha-channel,
    /// "Mode B"), threaded into every subsequent paint pass and recovered by
    /// widgets through [`crate::widget::PaintCtx::is_translucent`]. A shell
    /// pushes the surface's **resolved** translucency here — what the GPU
    /// backend reports after the surface is installed, not what the app
    /// requested via `frust-shell-common::surface_mode`'s latch: a translucency
    /// request the platform refuses must degrade to the opaque contract, or
    /// every `platform_view` slot punches a hole in an opaque swapchain
    /// (black rectangles). Every desktop app leaves the default `false`
    /// (opaque, "Mode A").
    ///
    /// Marks `PAINT` pending on an actual change (`PartialEq`-guarded, mirroring
    /// [`RenderRoot::set_insets`]'s no-op guard): translucency is read purely at
    /// paint time (the hole-punch runs in `paint`, never baked at layout), so a
    /// flip must repaint but need not relayout. A flip is rare but **real**: a
    /// surface (re)install can resolve differently from the previous one, and
    /// both mobile shells re-push this every frame (the no-op-if-unchanged
    /// guard is what makes that free).
    pub fn set_surface_translucent(&mut self, translucent: bool) {
        if self.surface_translucent == translucent {
            return;
        }
        self.surface_translucent = translucent;
        self.pending |= ChangeFlags::PAINT;
    }

    /// Whether the shell's GPU surface is currently marked translucent.
    pub fn is_surface_translucent(&self) -> bool {
        self.surface_translucent
    }

    /// Whether a captured pointer gesture is currently in flight.
    pub fn is_pointer_captured(&self) -> bool {
        self.capture_claimant.is_some()
    }

    /// The contact that claimed the pointer capture in flight — the only one
    /// whose `Up`/`Cancel` can end it — or `None` while nothing is captured.
    pub fn pointer_capture_claimant(&self) -> Option<PointerId> {
        self.capture_claimant
    }

    /// Whether the capture in flight routes the gesture's **other** contacts to
    /// its captor — the captor opted in with [`EventCtx::capture_contacts`] on
    /// the `Down` it captured with, and no container has since taken the
    /// gesture over from it ([`EventCtx::release_captured_child`]). `false`
    /// while nothing is captured.
    pub fn pointer_capture_contacts(&self) -> bool {
        self.capture_contacts
    }

    /// Whether some widget in the tree currently holds keyboard/IME focus.
    pub fn is_focus_active(&self) -> bool {
        self.focus_active
    }

    /// Whether some widget in the tree currently holds the hover link — i.e.
    /// whether the last hover pass (an uncaptured [`PointerPhase::Move`]) left the
    /// pointer over a widget that claimed it.
    ///
    /// The hover analog of [`RenderRoot::is_focus_active`], and a level accessor
    /// like it: hover is not a session (nothing has to be released), so there is no
    /// generation counterpart. `false` for any app whose widgets never call
    /// [`EventCtx::claim_hover`](crate::event::EventCtx::claim_hover). A touch app
    /// can still see it go `true` transiently: nothing distinguishes a touch
    /// contact from a mouse here, so an uncaptured touch drag over a
    /// non-capturing claimant is an ordinary hover pass — ended by the `Up` at
    /// lift (see `docs/LIMITATIONS.md`'s `hover-window-leave-standing`).
    ///
    /// A [`RenderRoot::rebuild`] that removes the claimant ends the link too, so
    /// this never reports a hover held by a widget that no longer exists — the
    /// hover counterpart of the unmount focus release (see that method).
    pub fn is_hover_active(&self) -> bool {
        self.hover_active
    }

    /// The cursor the tree last asked the host to show — what a desktop shell
    /// pushes to its window (`frust-shell-desktop` maps it onto winit's own
    /// cursor icons).
    ///
    /// A **level** accessor like [`RenderRoot::is_hover_active`], not an edge one:
    /// the value re-resolves on every pointer [`PointerPhase::Move`] and stands
    /// unchanged through every other pass, so a shell caches what it last applied
    /// and calls the platform only when this differs. There is deliberately no
    /// generation counter — a cursor is a *value*, not a session, and an unmoved
    /// cursor is indistinguishable from one re-resolved to the same shape.
    ///
    /// [`CursorIcon::Default`] before the first `Move`, and after any `Move` in
    /// which no widget called
    /// [`EventCtx::set_cursor`](crate::event::EventCtx::set_cursor) — so any app
    /// whose widgets never request a cursor reads `Default` forever, and the mobile
    /// shells never read this at all regardless of what resolves here.
    ///
    /// **Residual:** a widget that is torn down (or moves out from under a
    /// stationary pointer) while its request stands leaves the last shape in
    /// place until the next `Move` re-resolves it — the same self-correction
    /// window hover has, and for the same reason: nothing re-resolves without
    /// pointer motion.
    pub fn cursor(&self) -> CursorIcon {
        self.cursor
    }

    /// Take (and clear) the text the tree asked the shell to put on the host
    /// clipboard — the drain a shell performs immediately after every
    /// [`RenderRoot::event`], beside [`cursor()`](RenderRoot::cursor) and
    /// [`ime_state()`](RenderRoot::ime_state).
    ///
    /// `Some` exactly when some widget called
    /// [`EventCtx::write_clipboard`](crate::event::EventCtx::write_clipboard)
    /// during a pass since the last drain (answering a
    /// [`EditCommand::Copy`](crate::event::EditCommand::Copy)/[`Cut`](crate::event::EditCommand::Cut),
    /// or a chord the widget decoded itself). The shell hands the text to its host
    /// clipboard — winit's `arboard` on desktop, `ClipboardManager` on Android,
    /// `UIPasteboard` on iOS — and does nothing at all on `None`.
    ///
    /// **Destructive**, unlike [`cursor()`](RenderRoot::cursor): a clipboard write
    /// is an edge, not a standing level, so a caller that drains and drops the
    /// result loses that write. Draining twice after one pass yields `None` the
    /// second time.
    ///
    /// A widget that never copies leaves this `None` forever, so a shell with no
    /// clipboard (the mobile shells before their own clipboard work lands) may
    /// call it and discard the result, or not call it at all.
    pub fn take_clipboard_write(&mut self) -> Option<String> {
        self.pending_clipboard_write.take()
    }

    /// Take (and clear) whether the tree asked the shell to read the host
    /// clipboard back to it — drained beside
    /// [`take_clipboard_write`](RenderRoot::take_clipboard_write) after every
    /// [`RenderRoot::event`].
    ///
    /// `true` exactly when some widget called
    /// [`EventCtx::request_paste`](crate::event::EventCtx::request_paste) during a
    /// pass since the last drain. The shell answers by reading its host clipboard
    /// and dispatching
    /// [`InputEvent::EditCommand`]`(`[`EditCommand::Paste`](crate::event::EditCommand::Paste)`(text))`
    /// — a *new* dispatch, because the read may be asynchronous and the pass that
    /// asked is over. That answer carries no identity of its own and is
    /// focus-routed to whoever holds focus when it lands: a release in between
    /// drops it harmlessly, but a focus *move* in between lands it in the new
    /// field rather than the one that asked. A synchronous read has no such
    /// window; an asynchronous one snapshots
    /// [`focus_epoch`](RenderRoot::focus_epoch) at this drain and discards an
    /// answer whose epoch no longer matches — not
    /// [`focus_ime_generation`](RenderRoot::focus_ime_generation), which also
    /// moves within a single session.
    ///
    /// **Destructive**, for [`take_clipboard_write`](RenderRoot::take_clipboard_write)'s
    /// reason. A pass may both write and request (a cut that immediately re-reads,
    /// or a widget answering two chords) — the two drains are independent.
    pub fn take_paste_request(&mut self) -> bool {
        std::mem::take(&mut self.pending_paste_request)
    }

    /// The IME surface the focused widget published, for the shell to drive the
    /// platform input method (winit `set_ime_cursor_area`, Android
    /// `updateSelection`, iOS `inputDelegate`). `None` when nothing is focused or
    /// the focused widget publishes no IME surface.
    ///
    /// Written by the focused widget through [`EventCtx::publish_ime_state`] during
    /// the event pass and refreshed on every event; it survives a rebuild (so the
    /// shell can query it between frames) and is cleared when focus is lost.
    ///
    /// # `None` is the only "no session" form — an inactive surface is never stored
    ///
    /// A widget publishing `ImeState { active: false, .. }` is ending the session,
    /// not describing it, so both publish paths turn that into a full release
    /// (see [`RenderRoot::paint`]) and this returns `None` rather than
    /// `Some(inactive)`. A shell therefore never has to distinguish the two, and
    /// `is_some()` means "a live IME session" with no second check.
    ///
    /// **The platform still sees the keyboard-hide.** All three shells already
    /// map `None` onto the inactive form on the way out, so the observable wire
    /// behavior is unchanged: `frust-shell-android`'s `ime_state_to_json` returns
    /// `ImeJsonState::default()` (`active:false`, empty text, `-1` indices, null
    /// caret, `"normal"`) and `frust-shell-ios`' returns the byte-identical
    /// `ime_state_json(false, "", -1, -1, -1, -1, None, "normal")` — exactly what
    /// the navigator's own cleared surface serialised to before. Kotlin's
    /// `pollImeAfterDispatch` and Swift's `syncImeFocus` both branch on `active`
    /// alone (an inactive surface's text/caret/content-type are ignored), and the
    /// desktop shell's `sync_ime` reads `is_some_and(|s| s.active)`. Dropping the
    /// inactive surface's payload also stops a disabled *secret* field's text
    /// riding to the platform after its session ended.
    pub fn ime_state(&self) -> Option<ImeState> {
        self.ime_state.clone()
    }

    /// The focus/IME session generation — bumped on every **actual** change of
    /// [`is_focus_active`](RenderRoot::is_focus_active) or
    /// [`ime_state`](RenderRoot::ime_state), and on nothing else.
    ///
    /// The *edge* counterpart of those two level accessors, for a shell that
    /// needs "did the focus/IME session move since I last looked?" rather than
    /// "is something focused?". A shell caches the value it last saw and
    /// compares (mirroring [`semantics_generation`](RenderRoot::semantics_generation)'s
    /// cheap dirty gate) — that comparison is the mobile frame gate's
    /// `FrameInputs::focus_or_ime_changed` input.
    ///
    /// A same-value write never moves it: re-publishing an identical IME
    /// surface (which the paint pass does on every frame a field stays focused)
    /// or re-blurring an already-blurred root is not an edge. Wrapping is
    /// deliberate and harmless — a comparison, never an ordering.
    ///
    /// # Not the session's identity
    ///
    /// This counts *changes to the published surface*, not *sessions*, and the
    /// two come apart in both directions — see
    /// [`focus_epoch`](RenderRoot::focus_epoch), which is what to reach for when
    /// the question is "is this still the same focus session?". Answering that
    /// one from this counter is wrong whenever focus moves between two fields
    /// without the published value changing.
    pub fn focus_ime_generation(&self) -> u64 {
        self.focus_ime_gen
    }

    /// The live focus session's **identity** — advanced once per honoured focus
    /// claim and once per session release, and by nothing else.
    ///
    /// The neighbour of [`focus_ime_generation`](RenderRoot::focus_ime_generation)
    /// and easy to mistake for it, so: that one counts *changes to the published
    /// surface* (the focus flag, or the [`ImeState`] value), this one counts
    /// *sessions*. They come apart in both directions, which is why both exist:
    ///
    /// * Focus moving from one field to another moves this one and can leave
    ///   that one completely still. Claiming focus while some field already
    ///   holds it writes `true` over `true`, and the surface the new field
    ///   publishes may compare equal to the old field's ([`ImeState`] is
    ///   `{active, editing, caret, content_type}` and names no widget) — or may
    ///   not be published at all, since a widget is free to take focus and
    ///   publish nothing, which leaves the previous field's surface standing.
    /// * An edit landing, a caret moving, or the field being repositioned under
    ///   the user moves that one and leaves this one still: the session is the
    ///   same session throughout.
    ///
    /// So a caller binding an asynchronous answer to the session that asked for
    /// it wants this one; a caller asking "must I run a frame, or re-sync the
    /// platform IME?" wants that one.
    ///
    /// **Never `0`.** The counter is built at `1` and steps *past* `0` on wrap,
    /// because `0` is a never-claimed [`ChildPod`](crate::widget::ChildPod)'s
    /// stamp and a root publishing it would hand every unclaimed pod in the tree
    /// a live link. A caller is therefore free to use `0` as its own "no root /
    /// no answer" sentinel with no risk of colliding with a live value. Wrapping
    /// is otherwise deliberate and harmless: the value is compared for equality,
    /// never ordered.
    pub fn focus_epoch(&self) -> u64 {
        self.focus_epoch
    }

    /// Set the root's focus flag, bumping [`RenderRoot::focus_ime_generation`]
    /// only when the value actually moves.
    ///
    /// One of the two writers of `focus_active` outside construction (the other
    /// is [`release_focus_session_in`], which clears it together with the IME
    /// surface as one edge): every focus/blur arm of [`RenderRoot::event`] goes
    /// through one of them, so the edge generation cannot drift from the state it
    /// describes. In practice this one only ever *sets* focus — a clear is always
    /// a session release.
    fn set_focus_active(&mut self, active: bool) {
        if self.focus_active != active {
            self.focus_active = active;
            self.focus_ime_gen = self.focus_ime_gen.wrapping_add(1);
        }
    }

    /// Store (or clear) the shell-facing IME surface, bumping
    /// [`RenderRoot::focus_ime_generation`] only when the stored value actually
    /// moves — the `&mut self` form of [`store_ime_state_in`], for the event
    /// pass (the paint pass holds disjoint field borrows and calls that
    /// function directly).
    fn store_ime_state(&mut self, ime: Option<ImeState>) {
        store_ime_state_in(&mut self.ime_state, &mut self.focus_ime_gen, ime);
    }

    /// End the focus/IME session: clear `focus_active` and drop the shell-facing
    /// surface together, moving [`RenderRoot::focus_ime_generation`] exactly once
    /// if either was set. The `&mut self` form of [`release_focus_session_in`]
    /// (whose doc carries the full contract), for the event and rebuild passes;
    /// the paint pass holds disjoint field borrows and calls that function
    /// directly.
    ///
    /// Idempotent: releasing an already-released root writes the same values
    /// back and fires no edge.
    fn release_focus_session(&mut self) {
        release_focus_session_in(
            &mut self.focus_active,
            &mut self.ime_state,
            &mut self.focus_ime_gen,
            &mut self.focus_epoch,
            self.root_identity,
        );
        // No session, no owner. A paint pass that releases re-resolves the owner
        // from the pods before it ends anyway (see
        // `RenderRoot::resolve_session_surface`), so this write is the event and
        // rebuild passes' own.
        self.focus_surface = None;
        // A selection toolbar describes the *focused* field's selection, so the
        // session ending is the toolbar ending — and it must end on the event
        // pass that blurred, not a frame later when the next paint happens to
        // publish nothing. Change-guarded like every other edge here: releasing
        // an already-toolbarless root moves no generation.
        if self.selection_toolbar.take().is_some() {
            self.selection_toolbar_gen = self.selection_toolbar_gen.wrapping_add(1);
        }
    }

    /// Publish this root's live focus session so the pods can compare their own
    /// stamps against it — the focus counterpart of the `(root, epoch)` pair
    /// `crate::event::set_live_hover_link` publishes for hover.
    ///
    /// Called at the head of every pass, not only when the session moves: the
    /// channel mirrors one root, so a second root driving passes on the same
    /// thread would otherwise leave this one's pods comparing against a session
    /// that is none of their business. Re-publishing an unchanged triple costs a
    /// `Cell` write and notifies nothing.
    ///
    /// Publishes the session "at rest" — the live epoch and the epoch a claim
    /// would take are the same value. [`RenderRoot::event`] publishes the two
    /// apart for the length of its dispatch; see that method.
    fn publish_focus_session(&self) {
        crate::widget::set_live_focus_session(
            self.root_identity,
            self.focus_epoch,
            self.focus_epoch,
        );
    }

    /// End the standing hover link outright, outside any hover pass: advance the
    /// epoch (which strands every stamp in the tree at once, so no container has
    /// to be told) and clear the mirror.
    ///
    /// Hover's analog of [`RenderRoot::release_focus_session`], and idempotent in
    /// the same way — ending a hover nothing holds writes the same mirror back and
    /// costs one epoch. There is no generation counter to move: hover is not a
    /// session a shell mirrors (see [`RenderRoot::is_hover_active`]).
    ///
    /// The one caller is [`RenderRoot::rebuild`]'s severed-claimant drain; a hover
    /// pass ends its own link inline, where it also decides the *new* one.
    fn end_hover_link(&mut self) {
        self.hover_epoch = self.hover_epoch.wrapping_add(1);
        self.hover_active = false;
        crate::event::set_live_hover_link(self.root_identity, 0);
    }

    /// The [`PlatformViewFrame`]s published during the most recent
    /// [`RenderRoot::paint`], in paint order.
    ///
    /// Replaced wholesale every pass (see the `platform_view_frames` field
    /// doc), so a pass with no publishers yields an empty slice — a shell
    /// never sees a stale frame for a slot that stopped painting.
    pub fn platform_view_frames(&self) -> &[PlatformViewFrame] {
        &self.platform_view_frames
    }

    /// The z-shield rects reported during the most recent [`RenderRoot::paint`]
    /// (see [`crate::widget::PaintCtx::report_input_shield`]), in paint order.
    ///
    /// Replaced wholesale every pass, exactly like
    /// [`RenderRoot::platform_view_frames`] — a shell feeds both into the same
    /// differ ingest call, and the differ intersects these against each
    /// interactive slot's rect.
    pub fn input_shields(&self) -> &[Rect] {
        &self.input_shields
    }

    /// Drain the slot ids whose `platform_view` widgets were torn down since the
    /// last call (`View::teardown` ran on them — see
    /// [`crate::widget::report_retired_slot`]).
    ///
    /// The prompt-teardown channel: a shell calls this once per frame, right
    /// after its rebuild, and retires each id in its platform-view differ
    /// (`PlatformViewState::retire`) so a disposed slot's native view goes away
    /// immediately instead of waiting out the differ's missing-streak
    /// heuristic. Draining is destructive, mirroring
    /// [`RenderRoot::take_change_flags`]: an id is reported exactly once, so a
    /// shell that drains and drops the result loses the prompt path (the
    /// missing-streak backstop still covers it).
    ///
    /// A merely *culled* slot (scrolled offscreen, a parent skipping paint)
    /// never appears here — culling doesn't run `teardown` — which is what
    /// keeps the camera keep-alive contract intact.
    pub fn take_retired_platform_views(&mut self) -> Vec<u64> {
        crate::widget::take_retired_slots()
    }

    /// Take (and clear) the dirtiness accumulated since the last call.
    ///
    /// A shell can consult this to skip the layout/paint passes when nothing has
    /// changed and no redraw was requested (a desktop optimisation; the mobile
    /// continuous-loop shells may ignore it and repaint every tick). Each
    /// [`RenderRoot::rebuild`] merges its result here; this drains it.
    pub fn take_change_flags(&mut self) -> ChangeFlags {
        let flags = self.pending;
        self.pending = ChangeFlags::NONE;
        flags
    }

    /// Non-draining peek at the dirtiness accumulated since the last
    /// [`RenderRoot::take_change_flags`] — `true` when any `LAYOUT`/`PAINT`
    /// bit is pending, without clearing it.
    ///
    /// Complements [`take_change_flags`](RenderRoot::take_change_flags) for a
    /// shell frame gate: the gate reads this as one of its
    /// "should this frame run" inputs *before* deciding, so a frame it chooses
    /// to skip leaves `pending` intact for the next non-skipped frame to drain
    /// and act on. Draining stays the job of `take_change_flags`, called only
    /// on a frame that actually runs its layout/paint passes. No behavioral
    /// change to rebuild/layout/paint.
    pub fn has_pending_change_flags(&self) -> bool {
        !self.pending.is_empty()
    }

    /// The root widget id, once built.
    pub fn root_id(&self) -> Option<WidgetId> {
        self.root_id
    }

    /// Shared access to the retained tree (for the shell / tests).
    pub fn tree(&self) -> &WidgetTree {
        &self.tree
    }

    /// A read-only, pre-order snapshot of the retained tree for tooling: per
    /// node an id, its parent and children, the concrete widget's type name, an
    /// optional debug label, and its absolute border box in logical px.
    ///
    /// Computed on demand in O(nodes) and takes `&self` — no per-frame
    /// bookkeeping, no mutation, and nothing here participates in
    /// build/layout/paint. Bounds reflect the **last layout pass**, so call it
    /// after one (before the first, every rect is zero-sized).
    ///
    /// Scope: the walk covers the [`WidgetTree`] arena *and* the
    /// [`ChildPod`](crate::widget::ChildPod)s containers own, reached through
    /// [`Widget::visit_children`](crate::widget::Widget::visit_children) — so it
    /// is the real retained hierarchy, not just the arena (which holds little
    /// more than the root pod). A container that leaves that seam defaulted
    /// reads as a leaf.
    pub fn inspect(&self) -> Vec<InspectNode> {
        self.tree.inspect()
    }

    /// Run the build closure, then build (first call) or rebuild (subsequent calls)
    /// the root widget, returning what changed.
    ///
    /// the build closure is expected to be cheap and re-entrant: it is
    /// re-run in full every rebuild.
    ///
    /// # Deferred-callback flush
    ///
    /// The view diff itself is state-free (`rebuild_view` below takes no
    /// `State`), so a widget applying a structural op there — the navigator
    /// draining its queued `push`/`pop` is the shipped case — cannot run an app
    /// callback that needs `&mut State`. It instead queues the callback and calls
    /// [`mark_pending_result_flush`](crate::event::mark_pending_result_flush);
    /// this method drains that flag and dispatches an
    /// [`InputEvent::Housekeeping`] broadcast through the ordinary
    /// [`event`](RenderRoot::event) plumbing, where `state` *is* in scope. This
    /// is the only unconditional per-frame pass that holds `&mut State`, which is
    /// why the dispatch lives here and not in a shell (flushing on the next
    /// real input meant waiting seconds for a touch, or forever when the next
    /// touch went to chrome outside the navigator).
    ///
    /// A flushed callback mutates `State`, so the view built before it ran is
    /// stale — the build closure + `rebuild_view` cycle therefore re-runs after
    /// each flush, and the same frame shows the result. Results can queue further
    /// nav ops, so the loop is **bounded**; past the cap the flag is left standing
    /// and one more frame is requested rather than spinning (see
    /// `MAX_PENDING_RESULT_FLUSH_PASSES`, this module's private cap constant).
    ///
    /// The broadcast's [`EventOutcome`] is propagated, not discarded: a
    /// `needs_redraw` coming back from the dispatch folds into this rebuild's
    /// [`ChangeFlags::PAINT`] and the deferred frame request, so a callback
    /// whose only effect is [`EventCtx::request_redraw`]
    /// — invisible to the re-diff, since no view-visible state changed — still
    /// wakes both the mobile frame gate and the desktop `Wait` loop.
    pub fn rebuild(
        &mut self,
        build: &mut impl FnMut(&mut State) -> V,
        state: &mut State,
    ) -> ChangeFlags {
        // Republish this root's focus session before the diff runs: a pod
        // severed by it compares its own stamp against the channel from its
        // destructor, and the channel mirrors one root at a time.
        self.publish_focus_session();
        let view = build(state);
        let mut flags = self.rebuild_view(view);

        // Deferred-callback convergence loop (see the method doc). Each pass:
        // drain the flag, run the queued callbacks against real state, then
        // re-diff so this frame reflects them.
        let mut passes = 0usize;
        while crate::event::take_pending_result_flush() {
            if passes >= MAX_PENDING_RESULT_FLUSH_PASSES {
                // Cap reached. Put the flag back — the work is still owed — and
                // ask for one more frame instead of spinning inside this one.
                // `pending |= PAINT` is what the mobile frame gate reads
                // (`has_pending_change_flags`); `deferred_frame` is what surfaces
                // on the next `paint` as `needs_frame`, which is how the desktop
                // `ControlFlow::Wait` loop learns to wake.
                crate::event::mark_pending_result_flush();
                flags |= ChangeFlags::PAINT;
                self.deferred_frame = true;
                break;
            }
            // The dispatch's own outcome is load-bearing, not noise: a flushed
            // callback whose *only* effect is `EventCtx::request_redraw` (no
            // signal write, no state the next build-closure run reads) leaves the
            // re-diff below reporting `ChangeFlags::NONE`, so nothing else in
            // this method would ever mark the frame dirty and the requested
            // redraw would be dropped on the floor. Fold it into exactly the
            // wake the cap branch above raises: `PAINT` reaches `self.pending`,
            // which is what the mobile frame gate reads
            // (`has_pending_change_flags`), and `deferred_frame` surfaces on the
            // next `paint` as `needs_frame`, which is how the desktop
            // `ControlFlow::Wait` loop learns to schedule a frame. Both
            // Housekeeping producers need it (a navigator pop-result callback
            // and `frust-widgets`' gesture long-press latch), and without it a
            // redraw-only effect waits for whatever input happens to arrive
            // next — exactly the failure this mechanism exists to remove.
            //
            // Non-empty flags also bump the semantics generation below, which
            // is correct: the callback just mutated real `State` through a live
            // `EventCtx`, so the accessibility tree may genuinely have changed,
            // and every other paint-class path here bumps it the same way (a
            // spurious bump costs one recompute of an unchanged tree, a missed
            // one strands a stale tree).
            let outcome = self.event(state, &InputEvent::Housekeeping);
            if outcome.needs_redraw {
                flags |= ChangeFlags::PAINT;
                self.deferred_frame = true;
            }
            let view = build(state);
            flags |= self.rebuild_view(view);
            passes += 1;
        }

        // Generic-unmount focus release. A reconciler that tears down (or
        // type-swaps, or clears the `focused` flag of) a child pod holding the
        // recorded focus path *on the live focus chain* has severed that path,
        // but runs over a `BuildCtx` with no `RenderRoot` in scope — so it raises
        // `mark_focus_orphaned` and this drain performs the release the
        // reconciler could not. "On the live chain" is what `rebuild_view`'s seed
        // buys: a mark means a live session lost its owner, never that some stale
        // flag deep in an already-blurred branch went away (see
        // `mark_focus_orphaned`). Without it the root's mirror stays standing over
        // a widget that no longer exists: `is_focus_active()` keeps reporting
        // true and `ime_state()` keeps handing the shell a surface for a dead
        // field, self-correcting only on the next event pass — which never
        // arrives on a screen the user has stopped touching (the pop-into-idle
        // case this whole seam exists for).
        //
        // Drained *after* the flush loop so one release covers every pass: a
        // flushed callback that navigates re-diffs, and either diff may orphan
        // the focus. `Housekeeping` claims no focus of its own (its root arm is
        // inert), so nothing the loop dispatched can be undone here.
        //
        // The release marks no `ChangeFlags` of its own: the structural change
        // that severed the path already flagged `LAYOUT | PAINT`, and the
        // generation bump is what wakes the mobile frame gate's
        // `focus_or_ime_changed` edge for the one repaint the release needs.
        if crate::event::take_focus_orphaned() {
            self.release_focus_session();
        }

        // Generic-unmount hover release, the same shape one channel over: a diff
        // that dropped the `ChildPod` holding the live hover link has severed a
        // path the epoch mechanism cannot strand, because stranding needs a hover
        // pass and the dead claimant will never see another one. Without this the
        // mirror stands over a widget that no longer exists — `is_hover_active()`
        // reporting a link nothing holds — and every surviving ancestor of the
        // claimant keeps painting hover chrome off its own still-matching stamp
        // until some later `Move` re-derives, which never comes on a pointer the
        // user has stopped moving.
        //
        // The mark is raised by the pod's destructor rather than by the
        // reconcilers (the stamp has no setter for a container to cooperate
        // through — see `mark_hover_orphaned`), which is what makes this cover
        // every removal route, including hand-rolled containers outside this
        // workspace. That reach is also why the mark is qualified by
        // `root_identity`: a destructor fires whenever a pod happens to die, so
        // an unqualified mark could be a second root's on this thread. Drained
        // after the flush loop for the focus release's reason: any pass of the
        // loop may re-diff, and one end covers them all.
        //
        // Unlike that release this one flags `PAINT` of its own. The reconciler
        // that dropped the claimant usually reported `LAYOUT | PAINT` already,
        // but "usually" is not a contract this drain can rest on: the destructor
        // route deliberately covers containers outside this workspace (that is
        // its whole reason for existing), and one of those can drop a pod while
        // reporting whatever flags it likes. Ending a hover always changes what
        // paints, so the correction states its own need for the frame it rides
        // on — idempotent where the reconciler already said so.
        if crate::event::take_hover_orphaned(self.root_identity) && self.hover_active {
            self.end_hover_link();
            flags |= ChangeFlags::PAINT;
        }

        self.pending |= flags;
        // A rebuild that changed layout/paint could have changed the semantics
        // tree (added/removed/relabelled nodes); bump the dirty gate a shell polls
        // via `semantics_if_changed`.
        if !flags.is_empty() {
            self.semantics_gen = self.semantics_gen.wrapping_add(1);
        }
        flags
    }

    /// The rebuild body, split out so [`RenderRoot::rebuild`] can accumulate the
    /// result into [`RenderRoot::pending`] in one place.
    ///
    /// # Seeding the diff's focus chain
    ///
    /// The root is where the effective focus chain ([`BuildCtx::has_focus`])
    /// starts: the root widget sits in no `ChildPod`, so its "link above" is the
    /// root's own session mirror. A reconciler deep in the diff ANDs its pod's
    /// `focused` flag onto this seed and marks an orphan only if the whole chain
    /// holds — which is why the seed is "is there a session to lose" rather than
    /// `focus_active` alone: an active surface parked without the flag is still a
    /// live session `release_focus_session` would move. With neither set there is
    /// nothing to release, so the seed is `false` and the diff marks nothing.
    fn rebuild_view(&mut self, view: V) -> ChangeFlags {
        // Read before the `&mut self.next_id` borrow below (disjoint fields, but
        // spelled out for the reader).
        let session_live = self.focus_active || self.ime_state.is_some();
        match (self.root_id, self.prev_view.take()) {
            // Reconcile against the previous view of the same type.
            (Some(root_id), Some(prev)) => {
                let mut ctx = BuildCtx::new(&mut self.next_id);
                ctx.set_has_focus(session_live);
                let flags = {
                    let pod = self
                        .tree
                        .pod_mut(root_id)
                        .expect("root pod present when root_id is set");
                    let element = pod
                        .widget_mut()
                        .downcast_mut::<V::Element>()
                        .expect("root widget type matches its originating view");
                    view.rebuild(&prev, element, &mut ctx)
                };
                if let Some(pod) = self.tree.pod_mut(root_id) {
                    pod.merge_flags(flags);
                }
                self.prev_view = Some(view);
                flags
            }
            // First build: materialise the widget and insert it as the root.
            _ => {
                let mut ctx = BuildCtx::new(&mut self.next_id);
                // A first build tears nothing down, so the seed is moot — set it
                // anyway so the rule is "the root always seeds the chain", with no
                // arm exempt.
                ctx.set_has_focus(session_live);
                let id = ctx.alloc_id();
                let element = view.build(&mut ctx);
                // `new_typed` boxes the element exactly like `new` would, and
                // additionally records `V::Element`'s type name for
                // introspection — the concrete type is only nameable here.
                let pod = WidgetPod::new_typed(id, element);
                let root_id = self.tree.insert_root(pod);
                self.root_id = Some(root_id);
                self.prev_view = Some(view);
                ChangeFlags::LAYOUT | ChangeFlags::PAINT
            }
        }
    }

    /// Lay out the root widget against `window_size` and record its geometry.
    ///
    /// The root receives loose constraints (zero up to the window size) and is
    /// placed at the origin. Returns the size the root chose. No text context is
    /// threaded in (use [`RenderRoot::layout_with_text`] when the tree contains
    /// text widgets); the stored theme, if any, is still threaded down.
    pub fn layout(&mut self, window_size: Size) -> Size {
        self.layout_inner(window_size, None)
    }

    /// Lay out the root widget, threading a shared text-shaping context down to
    /// text widgets.
    ///
    /// `text_ctx` is the shell-owned `frust_text::TextContext`, passed
    /// type-erased so this crate needs no `frust-text` dependency. Text
    /// widgets recover it via [`crate::widget::LayoutCtx::text_context`]. The
    /// stored theme, if any, is threaded down alongside it.
    pub fn layout_with_text(&mut self, window_size: Size, text_ctx: &mut dyn Any) -> Size {
        self.layout_inner(window_size, Some(text_ctx))
    }

    /// Shared layout body: hands the root loose window constraints, lends the
    /// optional text context and the stored theme into a [`LayoutCtx`], and
    /// records the size the root returns.
    fn layout_inner(&mut self, window_size: Size, text_ctx: Option<&mut dyn Any>) -> Size {
        self.window_size = window_size;
        let Some(root_id) = self.root_id else {
            return Size::ZERO;
        };
        let bc = BoxConstraints::loose(window_size);
        // Disjoint field borrows: the theme (immut) and the tree (mut) are
        // different fields of `self`, so both borrows coexist through the layout.
        let theme = self.theme.as_deref();
        // Copied out before the `&mut self.tree` borrow below (a disjoint,
        // `Copy` field read).
        let insets = self.insets;
        let Some(pod) = self.tree.pod_mut(root_id) else {
            return Size::ZERO;
        };
        let mut ctx = LayoutCtx::with_resources(text_ctx, theme);
        // Thread the window insets down; one layout context reaches the whole
        // tree, so the global insets are set once here (see `crate::insets`).
        ctx.set_window_insets(insets);
        // Thread the window's own size down the same way — global and
        // origin-independent like the insets. A widget floating an overlay pod
        // lays it out against this rather than against its own constraints (see
        // `LayoutCtx::window_size`).
        ctx.set_window_size(window_size);
        let size = pod.widget_mut().layout(&mut ctx, &bc);
        pod.set_layout(Point::ZERO, size);
        size
    }

    /// Paint the root widget into `scene`, returning whether the tree wants
    /// another frame to continue an animation.
    ///
    /// A widget whose paint advances animation state (e.g. a scroll fling) signals
    /// [`PaintCtx::request_frame`]; that flag bubbles up through the container
    /// [`ChildPod`](crate::widget::ChildPod)s and out here as
    /// [`PaintOutcome::needs_frame`], which the shell honors by scheduling the next
    /// frame (desktop `window.request_redraw()`; the mobile continuous loops
    /// already do so). Mirrors how [`RenderRoot::event`] surfaces `needs_redraw`.
    ///
    /// A widget whose animation changes its *layout* signals
    /// [`PaintCtx::request_layout`] instead (or as well); that bubbles up the same
    /// way and is folded here into the render root's pending [`ChangeFlags`]
    /// (`LAYOUT`), so the *next* frame's
    /// [`take_change_flags`](RenderRoot::take_change_flags)`().needs_layout()`
    /// reports it and the mobile intra-frame layout skip relayouts while the
    /// animation is in flight. It is also surfaced on the returned
    /// [`PaintOutcome::needs_layout`].
    ///
    /// `frame_time` is the shell's shared monotonic clock for this frame
    /// (time enters `frust-core` from the shell, never `Instant::now()` here). It
    /// is seeded onto the root [`PaintCtx`] and threaded unchanged to every child
    /// ([`crate::widget::ChildPod::paint_child`]), so an animating widget advances
    /// against one consistent timestamp — see [`PaintCtx::frame_time`].
    ///
    /// # The overlay post-pass
    ///
    /// Painting the main tree is only the first half. Widgets registering a
    /// floated surface during that walk ([`PaintCtx::register_overlay`]) are
    /// drained here and painted **after** it, in band order — which is the only
    /// way a popover, menu or tooltip escapes its owner's paint order and every
    /// ancestor's clip. Their routing rects are retained (see
    /// `RenderRoot::overlay_hits`) for the next event pass to hit-test first, and
    /// their paint outcomes merge into this pass's own, so an animating overlay
    /// keeps the frames coming exactly like an animating widget in the tree.
    pub fn paint(&mut self, scene: &mut dyn PaintScene, frame_time: FrameTime) -> PaintOutcome {
        // Open the two paint-pass channels for the whole pass. Entering CLEARS
        // each slot, which is what makes "the registry is empty at the start of
        // every paint" true by construction rather than by everyone remembering
        // to unregister; `Drop` hands an enclosing pass its own back.
        let overlay_pass = OverlayPaintPass::enter();
        let toolbar_pass = SelectionToolbarPass::enter();
        // Republish this root's focus session before anything reads it: the
        // channel mirrors one root, and the pods about to be visited must
        // compare their stamps against *this* root's session.
        self.publish_focus_session();

        let (mut outcome, main_tree_ime) = self.paint_main_tree(scene, frame_time);

        // Drain what the main tree registered and paint it above everything.
        // Sorting is stable, so the band decides and registration order breaks
        // ties within a band (see `crate::overlay::sort_into_paint_order`).
        let mut entries = overlay_pass.take();
        sort_into_paint_order(&mut entries);
        // Retain the routing half — never the pods — for the next event pass.
        self.overlay_hits = entries.iter().map(OverlayHit::of).collect();
        let (holder, holder_ime) = self.paint_overlays(&entries, scene, frame_time, &mut outcome);
        // Release the owners' pod clones before the pass ends: the root holds no
        // overlay pod at rest, so a pod's lifetime stays exactly its owner's.
        drop(entries);

        // Both halves have now spoken, so the one question neither of them can
        // answer alone — whose surface the shell is configured with — is settled
        // in one place, with both answers in hand.
        self.resolve_session_surface(main_tree_ime, holder, holder_ime);

        // Resolve the selection-toolbar publish last, so a field that published
        // while painting *inside* a floated pod (a text input hosted in a
        // popover) is resolved by the same rule as one in the main tree.
        self.resolve_selection_toolbar(toolbar_pass.take());

        outcome
    }

    /// Settle which branch's IME surface the shell is configured with, from the
    /// two halves of the paint pass: what the main tree published, and what the
    /// floated surface holding the live focus link published (`holder`/
    /// `holder_ime`, both resolved from the pods' own stamps — see
    /// [`RenderRoot::paint_overlays`]).
    ///
    /// # Why it is decided here and not where the publish happens
    ///
    /// A publish is a claim about *the* session, and the session has exactly one
    /// owner. The main tree paints first, so at the moment it publishes, nothing
    /// yet knows whether a floated surface is about to prove that the session is
    /// no longer the tree's. Storing it there and correcting later is what
    /// produced a frame of lag with a secure field's surface standing in it —
    /// and the record the correction had to be driven from was one nothing could
    /// falsify. Deferring the decision by the width of one pass removes both.
    ///
    /// # The rules
    ///
    /// * the owner's own publish wins, whether that owner is a surface or the
    ///   tree; a publish from anywhere else was already dropped by the half that
    ///   collected it;
    /// * an **inactive** publish from the owner is the session ending, not a
    ///   value — the same reading every other release site gives it;
    /// * an owner that published **nothing** leaves a standing surface standing,
    ///   *unless* ownership moved this pass: then what stands belongs to the
    ///   branch that just lost the session, and goes with it rather than
    ///   remaining as the shell's idea of a live one. This is the rule that stops
    ///   a popover taking focus from leaving the platform keyboard configured for
    ///   the secure field underneath it — in both directions, since the session
    ///   coming back to the tree moves ownership just as much as it leaving.
    fn resolve_session_surface(
        &mut self,
        main_tree_ime: Option<ImeState>,
        holder: Option<OverlayKey>,
        holder_ime: Option<ImeState>,
    ) {
        let published = if holder.is_some() {
            holder_ime
        } else {
            main_tree_ime
        };
        let ownership_moved = holder != self.focus_surface;
        match published {
            Some(ime) if ime.active => self.store_ime_state(Some(ime)),
            Some(_) => self.release_focus_session(),
            None if ownership_moved => self.store_ime_state(None),
            None => {}
        }
        // Resolved from the links themselves, never from the focus request that
        // opened the session — see the field's doc.
        self.focus_surface = if self.focus_active { holder } else { None };
    }

    /// Paint the main widget tree — everything [`RenderRoot::paint`] does before
    /// the floated overlay pods get their turn.
    ///
    /// Split out from [`RenderRoot::paint`] purely for borrow scoping: this body
    /// holds `&mut self.tree` (beside disjoint borrows of the theme) for its whole
    /// length, while painting an overlay pod needs the root's fields again to
    /// merge outcomes and extend the per-pass channels. Nothing about the pass
    /// itself changed when it moved here.
    ///
    /// Reports, beside the outcome, the IME surface the tree published this pass
    /// (if any) — handed back rather than stored, because whether the tree still
    /// owns the session is not knowable until the floated pods have been visited
    /// (see [`RenderRoot::resolve_session_surface`]).
    fn paint_main_tree(
        &mut self,
        scene: &mut dyn PaintScene,
        frame_time: FrameTime,
    ) -> (PaintOutcome, Option<ImeState>) {
        let Some(root_id) = self.root_id else {
            return (PaintOutcome::default(), None);
        };
        // Disjoint field borrows: the theme (immut) vs the tree (mut).
        let theme = self.theme.as_deref();
        // Copied out before the `&mut self.tree` borrow (a disjoint `Copy` read).
        let insets = self.insets;
        // Same disjoint `Copy` read: the presented-frame count threaded to widgets.
        let presented_frames = self.presented_frames;
        // Same disjoint `Copy` read: the surface-translucency flag the
        // platform-view hole-punch reads (see `PaintCtx::is_translucent`).
        let surface_translucent = self.surface_translucent;
        if let Some(pod) = self.tree.pod_mut(root_id) {
            let mut ctx = PaintCtx::new(pod.origin(), pod.size());
            // Seed the shared shell clock so the whole paint pass sees one time.
            ctx.set_frame_time(frame_time);
            // Lend the stored theme (type-erased) into the paint pass; widgets
            // recover it via `PaintCtx::theme_as`.
            ctx.set_theme(theme);
            // Thread the window insets down (global — see `crate::insets`).
            ctx.set_window_insets(insets);
            // Thread the shell's presented-frame count down (global; a widget
            // measuring FPS differences it — see `PaintCtx::presented_frames`).
            ctx.set_presented_frames(presented_frames);
            // Thread the surface-translucency flag down (global; the
            // platform-view hole-punch gates its rect-clear on it — see
            // `PaintCtx::is_translucent`).
            ctx.set_translucent(surface_translucent);
            // Seed the root widget's paint-time focus from the session mirror so
            // a leaf-root editable observes its own focus, and thread the live
            // session's identity down beside it: deeper focus is resolved
            // per-pod by `ChildPod::paint_child`, which counts a recorded link
            // only while its stamp names this session.
            //
            // The mirror alone, deliberately. Narrowing the seed by *which
            // branch* the root believes owns the session was the previous shape,
            // and it de-seeded the whole tree off a record no pass could
            // falsify: a link a floated pod recorded is reached by no container's
            // blur sweep, so once one existed the tree never got seeded again.
            // The epoch answers the same question where it can actually be
            // answered — at each link, against the session that link was
            // recorded for.
            //
            // The arena root has no recorded link of its own (it is a
            // `WidgetPod`, not a `ChildPod`, and carries no stamp), so a
            // *leaf-root* editable is still seeded from the bare mirror. Every
            // real tree puts a container there, and the first `ChildPod` below it
            // composes the stamp back in.
            ctx.set_has_focus(self.focus_active);
            ctx.set_focus_epoch(self.focus_epoch);
            // Thread the hover mirror + live epoch the same way: the root widget's
            // own hover comes from the mirror (a leaf root can claim hover itself),
            // and deeper links are resolved per-pod by `ChildPod::paint_child`
            // against this epoch.
            ctx.set_hovered(self.hover_active);
            ctx.set_hover_epoch(self.hover_epoch);
            pod.widget_mut().paint(&mut ctx, scene);
            pod.clear_flags();
            // A focused editable republishes its IME surface during paint (which
            // runs after every rebuild), so a controlled change applied by the
            // rebuild — e.g. a submit clearing the field — refreshes the
            // shell-facing `ime_state` that the event pass alone would leave
            // stale.
            //
            // Collected, not stored: what the tree published is only the
            // session's if the session is still the tree's, and the pods that
            // could say otherwise have not been visited yet.
            // `resolve_session_surface` decides once both halves have spoken,
            // and routes the store through the change guard so an *unchanged*
            // republish — the overwhelmingly common case, a focused field
            // re-publishing the same surface frame after frame — moves no
            // generation and therefore fires no `focus_or_ime_changed` edge at
            // the shell.
            //
            // An **inactive** publish is not a surface refresh at all: it is the
            // publishing widget saying "this session is over" — the navigator's
            // post-pop `cleared_ime_state`, `PatternSwitcher`'s equivalent, and
            // a `TextInput` turned disabled/read-only under a live focus are the
            // three shipped producers, and every one of them is an unmount or a
            // de-focus. Storing it as `Some(inactive)` and leaving `focus_active`
            // standing is what leaked the session after a pop: the shells' gate
            // saw `ime_state().is_some()`, `is_focus_active()` kept lying, and the
            // next real focus interaction started from a corrupt baseline. So the
            // resolver takes the *full* release instead — the same one a
            // blur-on-outside-tap `Down` performs, firing exactly one edge.
            //
            // The `self.focus_active` guard stays and is applied here, at the
            // point of collection: a publish arriving when no session is live
            // describes nothing, so a widget whose pod focus was just cleared by
            // a container-routed blur (but whose internal flag lags one frame)
            // can never resurrect the `ime_state` that blur dropped — even
            // before it observes the blur via `PaintCtx::has_focus`.
            //
            // Provenance below that guard is the chain's own job now: a link
            // reads focused only while its stamp names the live session, so a
            // branch the session has left publishes nothing to collect.
            let main_tree_ime = if self.focus_active {
                ctx.take_ime_state()
            } else {
                None
            };
            // Replace (never merge) the whole platform-view collection with
            // whatever this pass published — unlike `ime_state` above there is
            // no single "the" published instance to guard behind a focus
            // check, and a pass that publishes none must clear out every
            // stale frame from the previous one (see the field's doc comment).
            self.platform_view_frames = ctx.take_platform_views();
            // Same replace-per-pass discipline for the z-shield channel: a pass
            // whose shields stopped painting reports none, so a stale shield can
            // never keep stealing input from an interactive slot (see
            // `PaintCtx::report_input_shield`).
            self.input_shields = ctx.take_input_shields();
            // Fold a bubbled `request_layout` into `pending` so the *next* frame
            // relayouts. `pending` survives to the next frame and feeds both the
            // frame gate (`has_pending_change_flags`) and the Android layout-skip
            // (`take_change_flags().needs_layout()`), so no shell change is needed
            // on any platform. Deliberately opt-in: `request_frame` alone never
            // sets LAYOUT, keeping paint-only animations layout-free.
            let needs_layout = ctx.needs_layout();
            if needs_layout {
                self.pending |= ChangeFlags::LAYOUT;
            }
            // A rebuild that ran out of flush passes owes one more frame; surface
            // it here (and clear it) so a dirty-driven shell schedules the frame
            // that finishes the flush — see the `deferred_frame` field doc.
            let deferred_frame = std::mem::take(&mut self.deferred_frame);
            let outcome = PaintOutcome {
                needs_frame: ctx.needs_frame() || deferred_frame,
                needs_layout,
                // Aggregate tick class: paced-only iff a frame was requested and
                // every request was CosmeticLoop-class. The mobile frame gate
                // may throttle such a frame; any Transition request
                // (including the LAYOUT-implying `request_layout` above) leaves
                // this false so the frame runs every vsync.
                needs_frame_paced_only: ctx.needs_frame_paced_only(),
                // ...and, when it IS paceable, how fast it asked to be re-run:
                // the MIN over every paced request this pass (`Duration::ZERO`
                // / `None` meaning the theme's own cosmetic rate). The gate
                // resolves it against the live theme's cap — see
                // `PaintCtx::request_frame_paced_at`.
                paced_interval: ctx.paced_interval(),
            };
            (outcome, main_tree_ime)
        } else {
            (PaintOutcome::default(), None)
        }
    }

    /// Paint the pods registered during this pass, above the main tree, and fold
    /// each one's paint outcome back into `outcome`.
    ///
    /// `entries` arrives in paint order (`Floating` band first, then `Tooltip`,
    /// registration order within each). Each pod is painted through a
    /// [`PaintCtx`] whose absolute origin is its own registered
    /// [`window_rect`](crate::overlay::OverlayEntry::window_rect) — not its
    /// owner's origin, which is the whole point of floating — carrying the same
    /// clock, theme, insets, presented count, translucency and focus/hover
    /// seeding the root pod's context carries, so a widget inside a pod cannot
    /// tell it is not in the tree.
    ///
    /// The pod borrow is taken per entry and released before the next: the root
    /// must never hold one across a pass boundary, nor across another entry's
    /// paint (two entries may belong to the same owner).
    ///
    /// It is also the one pass that can act on a floated pod's focus link at
    /// all, because it is the one pass holding the pods. Two things follow, and
    /// both happen here:
    ///
    /// * it **retires** a link the live session has already stranded
    ///   ([`ChildPod::retire_stale_focus_link`](crate::widget::ChildPod::retire_stale_focus_link)),
    ///   so the raw flag consumers that cannot consult an epoch — an owner's own
    ///   `is_focused()` read, a hand-written container's routing — stop seeing a
    ///   record of a session that has moved on;
    /// * it **resolves** which surface, if any, holds a link on the live session,
    ///   and collects that one surface's IME publish. Every other pod's publish
    ///   is dropped where it is taken: paint descends into every pod
    ///   unconditionally and the bubble up `paint_child` carries a published
    ///   surface whatever the publisher's link says, so without this the last pod
    ///   painted would decide what the shell is configured with — a surface
    ///   belonging to a field it has nothing to do with, secure-text
    ///   configuration and all.
    ///
    /// Returns `(holder, holder's publish)` for
    /// [`RenderRoot::resolve_session_surface`] to settle against the main tree's.
    /// **Two holders resolve to none:** a second live link is a contradiction the
    /// mechanism is supposed to make impossible (one claim, one chain, one
    /// stamp), and answering it by picking the topmost would let a surface speak
    /// for a session on evidence that has already failed. Reporting no holder
    /// instead means neither surface's publish is carried out of this pass —
    /// the resolver then reads the pass exactly as it reads one where nothing is
    /// floated at all.
    fn paint_overlays(
        &mut self,
        entries: &[OverlayEntry],
        scene: &mut dyn PaintScene,
        frame_time: FrameTime,
        outcome: &mut PaintOutcome,
    ) -> (Option<OverlayKey>, Option<ImeState>) {
        if entries.is_empty() {
            // Nothing is floated, so nothing floated owns the session.
            return (None, None);
        }
        // The same disjoint field borrows the main pass takes, for the same
        // reason: the theme is lent immutably into each context while other
        // fields of `self` are written.
        let theme = self.theme.as_deref();
        let presented_frames = self.presented_frames;
        let surface_translucent = self.surface_translucent;
        let hover_active = self.hover_active;
        let hover_epoch = self.hover_epoch;
        let focus_active = self.focus_active;
        let focus_epoch = self.focus_epoch;

        // The surface whose pod holds a link on the LIVE session, resolved as the
        // pods go past, together with the surface that pod published — one pair,
        // so the publish can never be attributed to an entry that did not make
        // it. A second live holder sets `contested` and the pair is discarded.
        let mut holder: Option<(OverlayKey, Option<ImeState>)> = None;
        let mut contested = false;

        for entry in entries {
            let mut ctx = PaintCtx::new(entry.window_rect.origin(), entry.window_rect.size());
            ctx.set_frame_time(frame_time);
            ctx.set_theme(theme);
            ctx.set_window_insets(entry.insets);
            ctx.set_presented_frames(presented_frames);
            ctx.set_translucent(surface_translucent);
            // Seeded from the root's own mirrors exactly as the root pod's
            // context is, so a focused editable inside a floated pod observes its
            // focus (and a hovered one its hover) through the ordinary
            // `ChildPod::paint_child` composition.
            //
            // The mirror alone, deliberately: the pod's own link is ANDed onto it
            // inside `paint_child`, which is the same composition that decides
            // the main tree's, so a pod that holds no link reads unfocused
            // whatever the mirror says.
            ctx.set_has_focus(focus_active);
            ctx.set_focus_epoch(focus_epoch);
            ctx.set_hovered(hover_active);
            ctx.set_hover_epoch(hover_epoch);
            // Retired and read before the paint, in the same borrow: nothing in a
            // paint pass moves the focus path, and the answer is what decides
            // whether this pod may describe the session below.
            let pod_holds_focus = {
                let mut pod = entry.pod.borrow_mut();
                pod.retire_stale_focus_link();
                let holds = pod.holds_live_focus();
                pod.paint_child(&mut ctx, scene);
                holds
            };

            // Fold this pod's continuation-frame request into the frame's outcome
            // on the two lattices the tree's own aggregation uses: `needs_frame`
            // ORs, the class is a max-lattice (any unpaced request makes the whole
            // frame unpaced), and the paced interval is a MIN-lattice. The
            // standing aggregate's class is recovered from the outcome itself —
            // `needs_frame && !needs_frame_paced_only` is precisely "something
            // unpaced asked" — which also preserves the deferred-flush frame the
            // main pass may have folded in.
            let stood_unpaced = outcome.needs_frame && !outcome.needs_frame_paced_only;
            let entry_unpaced = ctx.needs_frame() && !ctx.needs_frame_paced_only();
            outcome.needs_frame |= ctx.needs_frame();
            outcome.needs_frame_paced_only =
                outcome.needs_frame && !(stood_unpaced || entry_unpaced);
            outcome.paced_interval = match (outcome.paced_interval, ctx.paced_interval()) {
                (Some(standing), Some(asked)) => Some(standing.min(asked)),
                (standing, asked) => standing.or(asked),
            };
            // A layout-animating widget inside a pod relayouts the next frame the
            // same way one in the tree does.
            if ctx.needs_layout() {
                outcome.needs_layout = true;
                self.pending |= ChangeFlags::LAYOUT;
            }
            // A focused editable inside a pod republishes its IME surface on every
            // paint, exactly like one in the tree, so the same rules apply
            // verbatim: collect a publish only while a session is actually active
            // (never resurrect a surface a blur cleared), and only from the pod
            // that holds a link on THAT session — the provenance a child of the
            // tree gets for free from its chain. A publish taken from any other
            // pod is dropped here, which is why the take is unconditional: the
            // per-entry context is about to be discarded either way, and leaving
            // a surface in it would only invite a later reader to trust it.
            //
            // An *inactive* publish is refused on exactly the same terms rather
            // than treated as a release: ending a session is a claim about it
            // too, and a pod that does not hold it makes neither.
            let published = ctx.take_ime_state();
            if focus_active && pod_holds_focus {
                if holder.is_some() {
                    // Two pods claiming one session. See the method doc: the
                    // contradiction is answered by attributing the session to
                    // nobody, not by ranking the claimants.
                    contested = true;
                } else {
                    holder = Some((entry.key, published));
                }
            }
            // EXTEND the two replace-per-pass channels rather than replacing them:
            // `paint_main_tree` already put this pass's tree-published frames and
            // shields there, and a platform-view slot or z-shield that happens to
            // paint inside a floated pod must survive beside them (see
            // `PaintCtx::publish_platform_view`).
            self.platform_view_frames.extend(ctx.take_platform_views());
            self.input_shields.extend(ctx.take_input_shields());
        }

        match holder {
            Some((key, published)) if !contested => (Some(key), published),
            _ => (None, None),
        }
    }

    /// Resolve this paint pass's selection-toolbar publish into the shell-facing
    /// slot, moving [`RenderRoot::selection_toolbar_generation`] only on a
    /// **menu edge**.
    ///
    /// The request carries two shapes of fact, and they are resolved differently
    /// (see [`SelectionToolbarRequest`]):
    ///
    /// * The whole request is stored as a **level**, newest wins. A shell reads
    ///   [`SelectionToolbarRequest::anchor`] on every tick it has a menu on
    ///   screen, so the stored anchor has to be the current one, not the one the
    ///   generation last moved for.
    /// * The generation moves on the **menu-significant** part alone:
    ///   [`SelectionToolbarRequest::present_menu`] and
    ///   [`SelectionToolbarRequest::actions`], with an absent request reading as
    ///   "no menu, no verbs" so appearing and disappearing are edges on the same
    ///   comparison. An anchor that merely moved is deliberately **not** an edge:
    ///   a focused field recomputes its anchor every painted frame, and a
    ///   selection dragged wider moves it on every touch sample — bumping there
    ///   would ask the platform to re-present its menu per sample.
    ///
    /// The publish is refused outright while no focus session is active, mirroring
    /// `paint`'s refusal to let a paint-time publish resurrect a cleared IME
    /// surface: the request describes the focused field, so one arriving after the
    /// blur describes a field that no longer holds anything.
    fn resolve_selection_toolbar(&mut self, published: Option<SelectionToolbarRequest>) {
        let next = if self.focus_active { published } else { None };
        // "Nothing published" is the same statement as "no menu wanted, no verbs
        // enabled" — which is what lets one comparison cover a change between two
        // requests, a first appearance, and a clearing alike.
        let menu_edge = |request: &Option<SelectionToolbarRequest>| {
            request.map_or((false, SelectionToolbarActions::default()), |request| {
                (request.present_menu, request.actions)
            })
        };
        if menu_edge(&self.selection_toolbar) != menu_edge(&next) {
            self.selection_toolbar_gen = self.selection_toolbar_gen.wrapping_add(1);
        }
        self.selection_toolbar = next;
    }

    /// The selection-toolbar request the focused field published during the most
    /// recent [`RenderRoot::paint`], or `None` when no field is focused at all.
    ///
    /// The shell half of the platform edit-menu route
    /// ([`SelectionToolbarPolicy::Native`](crate::selection_toolbar::SelectionToolbarPolicy::Native)):
    /// a shell reads it beside [`RenderRoot::ime_state`], answers "may I offer
    /// this verb?" from [`SelectionToolbarRequest::actions`] whenever the platform
    /// asks, and presents the host's own menu at
    /// [`SelectionToolbarRequest::anchor`] when
    /// [`SelectionToolbarRequest::present_menu`] says so. A **level**, not an edge
    /// — re-read it as often as you like; pair it with
    /// [`RenderRoot::selection_toolbar_generation`] to notice the changes worth
    /// presenting or dismissing for.
    ///
    /// Present for a focused field with no selection at all, which is not a
    /// wasted answer: paste applies to a bare caret, and a platform asking
    /// whether it may offer one needs a reply before any bar exists.
    ///
    /// A field under the framework policy publishes this too (it floats its own
    /// toolbar through [`crate::overlay`] as well), so a shell that drives the
    /// platform menu must decide on the policy, not on the presence of a request.
    pub fn selection_toolbar(&self) -> Option<SelectionToolbarRequest> {
        self.selection_toolbar
    }

    /// A monotonically-increasing generation bumped on every change to the
    /// **menu-significant** part of [`RenderRoot::selection_toolbar`] — its
    /// [`present_menu`](SelectionToolbarRequest::present_menu) flag and its
    /// [`actions`](SelectionToolbarRequest::actions) — including the clearing that
    /// a blur produces, so a shell sees the menu going away as an edge too.
    ///
    /// A moved [`anchor`](SelectionToolbarRequest::anchor) is **not** an edge: it
    /// is republished (and recomputed) every painted frame, so a shell re-reads it
    /// from [`RenderRoot::selection_toolbar`] rather than waiting for this to move
    /// — bumping on it would re-present a menu on every touch sample of a drag
    /// that widens a selection.
    ///
    /// The `focus_ime_generation` contract one channel over: a shell caches the
    /// last value it acted on and acts only when it moves, which is what keeps a
    /// standing selection — republished every single frame — from asking the
    /// platform to re-present its menu on every vsync.
    pub fn selection_toolbar_generation(&self) -> u64 {
        self.selection_toolbar_gen
    }

    /// Collect the accessibility tree for the current frame,
    /// returning a [`SemanticsUpdate`] a platform adapter (`accesskit_*`)
    /// can consume.
    ///
    /// Pull-based and stateless: the shell calls this when a platform a11y client
    /// asks for the tree (or after a change), *never* per frame — this crate owns
    /// no scheduling. Must run **after** [`RenderRoot::layout`], since node bounds
    /// come from the pods' post-layout geometry.
    ///
    /// The result is always rooted at a synthetic [`accesskit::Role::Window`]
    /// node covering the window, whose children are whatever the root widget
    /// contributed. An unbuilt tree yields a bare window node with no children.
    pub fn semantics(&self) -> SemanticsUpdate {
        let mut ctx = SemanticsCtx::new(self.window_size, self.semantics_alloc.get());
        let window = self.window_size;
        let root_pod = self.root_id.and_then(|id| self.tree.pod(id));
        // The root pod is arena-backed (not a `ChildPod`), so it caches its stable
        // base id in `root_semantics_id` rather than in a pod — assigned on first
        // pass and reused thereafter, exactly like `ChildPod::semantics_base`.
        let root_widget_base = match self.root_semantics_id.get() {
            Some(id) => id,
            None => {
                let id = ctx.alloc_base();
                self.root_semantics_id.set(Some(id));
                id
            }
        };
        let root_node = ctx.push_container_with_id(
            ROOT_NODE_ID,
            accesskit::Role::Window,
            |node| {
                node.set_bounds(accesskit::Rect {
                    x0: 0.0,
                    y0: 0.0,
                    x1: window.width,
                    y1: window.height,
                });
            },
            |ctx| {
                if let Some(pod) = root_pod {
                    // The root pod sits at its recorded origin (ZERO today) with
                    // its laid-out size; descend into that geometry and its stable
                    // id scope, mirroring `ChildPod::semantics_child`.
                    ctx.descend_into_pod(
                        root_widget_base,
                        pod.origin().to_vec2(),
                        pod.size(),
                        |ctx| {
                            pod.widget().semantics(ctx);
                        },
                    );
                }
            },
        );
        // Persist the allocator's high-water mark so the next pass keeps handing
        // out fresh, never-reused bases to pods that first appear later.
        self.semantics_alloc.set(ctx.next_base());
        ctx.finish(root_node)
    }

    /// The current semantics generation — bumped by every rebuild/theme swap that
    /// could have changed the accessibility tree (the semantics dirty gate).
    ///
    /// A shell records the value it last pushed and compares; see
    /// [`RenderRoot::semantics_if_changed`].
    pub fn semantics_generation(&self) -> u64 {
        self.semantics_gen
    }

    /// Pull a fresh [`SemanticsUpdate`] **only if** the semantics tree may have
    /// changed since generation `last_seen`.
    ///
    /// Returns `None` when nothing relevant changed, letting a shell skip both the
    /// tree walk and the platform `accesskit_*` push. Call it post-layout (bounds
    /// must be valid). A shell threads its stored generation in and, on `Some`,
    /// updates it from [`RenderRoot::semantics_generation`]. v1 pushes the whole
    /// tree when it does recompute (stable ids make that valid); finer-grained
    /// diffing is a later optimization.
    pub fn semantics_if_changed(&self, last_seen: u64) -> Option<SemanticsUpdate> {
        (self.semantics_gen != last_seen).then(|| self.semantics())
    }

    /// Deliver an input event to the widget tree, returning what happened.
    ///
    /// Builds a root [`EventCtx`] over the (type-erased) `state`, dispatches to
    /// the root widget — which routes the event down through its container
    /// children — and folds the result into an [`EventOutcome`]. The outcome's
    /// `needs_redraw` is set whenever a widget consumed the event or explicitly
    /// requested a redraw; the shell turns that into a `window.request_redraw()`.
    ///
    /// Root capture bookkeeping mirrors the per-container `active`-child model: a
    /// `Down` whose dispatch requested capture marks a gesture in flight and
    /// latches the contact that sent it as the gesture's claimant; the
    /// claimant's `Up` and `Cancel` release it (never a window-leave, and never
    /// another contact's release).
    ///
    /// Pointer contacts are gated **first**, before anything else runs: an
    /// [`InputEvent::PointerContact`] is unwrapped into the plain
    /// [`InputEvent::Pointer`] every widget matches on, with
    /// [`EventCtx::pointer_id`](crate::event::EventCtx::pointer_id) reporting its
    /// id, and a contact the multi-contact contract does not route — an
    /// additional contact with nothing captured, or one the captor did not opt
    /// into — is dropped here with an empty outcome (see that variant's
    /// *Multi-contact contract*). A bare `InputEvent::Pointer` is the mouse.
    ///
    /// Root hover bookkeeping is the third recorded path, and the one this pass
    /// *derives* rather than merely mirrors: an **uncaptured** `Move` opens a hover
    /// pass (widgets on the hit-tested path may claim it — see
    /// [`EventCtx::claim_hover`](crate::event::EventCtx::claim_hover)), a
    /// `Down`/`Up`/`Cancel` ends whatever hover stood, and every other event leaves
    /// it alone. There is nothing to release and no generation to bump: the epoch
    /// advance strands the previous claimant's path by itself, and the outcome's
    /// `needs_redraw` carries the one repaint **no widget can ask for** — a hover
    /// that ended with nothing taking it. A hover that *begins* or *moves from one
    /// claimant to another* is repainted by the new claimant's own change-gated
    /// `request_redraw`, which is why keeping an internal hover flag is part of the
    /// consumer contract rather than an optimization (see `claim_hover`).
    /// **No shell change is required for hover** — the desktop shell already
    /// dispatches a `Move` on every cursor move.
    ///
    /// The cursor is hover's sibling channel and the fourth thing this pass
    /// resolves: any pointer `Move` (captured included) re-resolves
    /// [`RenderRoot::cursor`] from the pass's last
    /// [`EventCtx::set_cursor`](crate::event::EventCtx::set_cursor), defaulting to
    /// [`CursorIcon::Default`] when nothing asked. It deliberately does **not**
    /// fold into the outcome's `needs_redraw`: applying a cursor is a platform
    /// call a desktop shell makes straight after this pass returns, with no frame
    /// involved, and folding it in would repaint the tree on every hover move.
    ///
    /// The **clipboard channel** rides the same bracket and is the fifth thing
    /// this pass resolves: whatever the dispatch asked for through
    /// [`EventCtx::write_clipboard`](crate::event::EventCtx::write_clipboard) and
    /// [`EventCtx::request_paste`](crate::event::EventCtx::request_paste) lands in
    /// [`RenderRoot::take_clipboard_write`] / [`RenderRoot::take_paste_request`],
    /// which a shell drains immediately after this returns, beside
    /// [`RenderRoot::cursor`] and [`RenderRoot::ime_state`]. Unlike the cursor,
    /// both commit on **every** pass rather than on a pointer `Move` alone — a
    /// copy can be answered from a key chord, a context-menu tap, or an
    /// [`InputEvent::EditCommand`] — and both are one-shot drains rather than
    /// standing levels. Like the cursor, neither folds into `needs_redraw`:
    /// talking to the host clipboard paints nothing (a `Cut` that mutates the
    /// document asks for its own redraw, for the mutation).
    ///
    /// # Reentrancy
    ///
    /// This pass **never rebuilds or repaints**. Event handlers mutate `state`
    /// synchronously through the context; the shell is expected to run a single
    /// [`RenderRoot::rebuild`] (then layout/paint) *after* the event pass returns,
    /// driven by the outcome. Rebuilding re-entrantly here would invalidate the
    /// widget references the dispatch still holds and turn the event→state→view
    /// feedback into recursion.
    ///
    /// [`RenderRoot::rebuild`] calls this itself with
    /// [`InputEvent::Housekeeping`] to flush deferred state-bearing callbacks.
    /// That is *sequential*, not re-entrant — the dispatch fully returns before
    /// the next diff starts — so the rule above is intact.
    ///
    /// # File-drag follow-up
    ///
    /// Every [`InputEvent::FileDrop`] whose phase is [`FileDropPhase::Drop`]
    /// or [`FileDropPhase::Cancel`] is followed, within this same call and
    /// whether or not its hit-tested dispatch was handled, by a
    /// [`FileDropPhase::Ended`] broadcast to the whole tree; the returned
    /// outcome folds both. A `Hover` and an `Ended` dispatched directly get
    /// no follow-up. See [`FileDropEvent`]'s *Broadcast follow-up* for why.
    pub fn event(&mut self, state: &mut State, event: &InputEvent) -> EventOutcome {
        let Some(root_id) = self.root_id else {
            return EventOutcome::default();
        };

        // The multi-contact gate (`InputEvent::PointerContact`'s contract). A
        // contact is unwrapped into the plain `Pointer` every widget matches on,
        // so everything below sees one pointer shape; a bare `Pointer` is the
        // mouse. Any other event dispatches under the contact the enclosing pass
        // carries (the overlay pre-pass re-enters this method with a broadcast
        // that must keep its gesture's id), or the mouse at the top level.
        let unwrapped;
        let (event, pointer_id) = match event {
            InputEvent::PointerContact {
                pointer_id,
                event: pointer,
            } => {
                unwrapped = InputEvent::Pointer(*pointer);
                (&unwrapped, *pointer_id)
            }
            InputEvent::Pointer(_) => (event, PointerId::MOUSE),
            _ => (event, crate::event::current_pointer_id()),
        };
        let secondary = if matches!(event, InputEvent::Pointer(_)) {
            match self.contact_route(pointer_id) {
                Some(secondary) => secondary,
                // Rule (b), or rule (c) for a captor that did not opt in: the
                // contact reaches nothing and moves no root state.
                None => return EventOutcome::default(),
            }
        } else {
            false
        };
        // Published for the whole dispatch: `EventCtx::new` seeds
        // `pointer_id()` from it (so the id survives a component boundary),
        // `capture_contacts` records its opt-in into it, and `ChildPod::set_active`
        // reads its `secondary` mark to keep every container's active link on
        // the claimant. Restored on drop, so a nested pass scopes its own.
        let contact_pass = ContactPass::enter(pointer_id, secondary);

        // The overlay pre-pass runs before every other thing this method does —
        // before the hover derivation, before the cursor bracket, before the
        // dispatch — because a pointer over a floated surface must reach none of
        // them: not the main tree's hit test, not its hover pass, and above all
        // not its blur rule. See `route_overlay`.
        let carried = match self.route_overlay(state, event) {
            OverlayRoute::Consumed(outcome) => return outcome,
            OverlayRoute::Continue(outcome) => outcome,
        };

        // Open a candidate focus session for this dispatch, BEFORE anything is
        // routed. A claim recorded below is stamped with this new epoch, which
        // nothing older carries — so the branch the session is leaving is
        // stranded by arithmetic rather than by a clearing sweep that would have
        // to visit it, and a floated pod no container owns is retired on exactly
        // the same terms as a field in the tree.
        //
        // Candidate, not committed: `self.focus_epoch` is left alone and the
        // pass settles it below, because whether this dispatch recorded anything
        // is only known once it has run. A `Move` over a focused field, a
        // housekeeping broadcast, a press inside a surface that claims nothing —
        // none of those may disturb a standing link.
        //
        // Both epochs are published because a claim recorded on the way back up
        // has to be observable to the container still unwinding around it (a
        // blur sweep asking which child kept focus, a portal noticing its surface
        // took the session) while a link recorded *before* this dispatch still
        // has to read live. See `crate::widget::set_live_focus_session`.
        //
        // After `route_overlay`, deliberately: an event that belongs to a floated
        // surface is re-dispatched through the front door and opens its own
        // candidate session there, and this call returns that outcome untouched.
        let live_focus_epoch = self.focus_epoch;
        let candidate_focus_epoch = advance_focus_epoch(live_focus_epoch);
        crate::widget::set_live_focus_session(
            self.root_identity,
            live_focus_epoch,
            candidate_focus_epoch,
        );
        let focus_active_before = self.focus_active;

        let Some(pod) = self.tree.pod_mut(root_id) else {
            // Nothing to dispatch into, but an outside-tap notification may
            // already have produced an outcome; returning it rather than the
            // default keeps that redraw.
            self.publish_focus_session();
            return carried;
        };

        // Hover is derived per **uncaptured** pointer `Move`: that pass, and only
        // that pass, may record a claim, so a captured drag can never paint hover
        // under the pointer. Every other pointer phase — `Down`, `Up`, `Cancel` —
        // is an epoch-advancing pass that *ends* whatever hover stood without
        // opening a new one: a press is not a hover, a touch `Down` must not
        // inherit one, and a lift is the only signal a touch contact leaving the
        // screen ever produces (no further `Move` follows it, so a tint claimed
        // during an uncaptured touch drag would otherwise stand indefinitely).
        // Ending on `Up` costs a mouse the hover tint between a click's release
        // and its next motion — the same standing-until-next-move class as the
        // press-then-hold-still gap, and traded deliberately for a touch link that
        // cannot outlive the finger (`docs/LIMITATIONS.md`'s
        // `hover-window-leave-standing`). A scroll, key, IME, or the housekeeping
        // broadcast leaves a live hover exactly as it was.
        //
        // A non-claimant contact delivered down a live capture (`secondary`) is
        // neither: it is not a gesture of its own, so it leaves hover exactly as
        // the claimant's gesture had it.
        let hover_pass = matches!(event, InputEvent::Pointer(p) if p.phase == PointerPhase::Move)
            && self.capture_claimant.is_none();
        let hover_ends = !secondary
            && matches!(
                event,
                InputEvent::Pointer(p)
                    if matches!(
                        p.phase,
                        PointerPhase::Down | PointerPhase::Up | PointerPhase::Cancel
                    )
            );
        let hover_epoch = self.hover_epoch;
        let hover_was_active = self.hover_active;
        let root_identity = self.root_identity;

        // The cursor pass is hover's pass widened by one case: **any** pointer
        // `Move`, captured included, because a captured `Move` routes only to the
        // capturing widget and that is exactly how a drag keeps its own cursor
        // while the pointer is outside its bounds. Every other pass leaves the
        // resolved cursor standing — notably `Down`/`Up`, whose handlers have no
        // reason to restate a cursor and whose reset would blink the shape back to
        // `Default` for the length of a click.
        //
        // The slot is cleared here rather than trusted to be empty: a `set_cursor`
        // from a dispatch no root drove (a reconciler's synthesized `Cancel`)
        // must not leak into this pass's resolution. The clear and the drain below
        // are one bracket (`CursorPass`) rather than two bare calls, so a dispatch
        // that re-entered this method could not silently eat the enclosing pass's
        // request — see that guard.
        //
        // The claimant's moves only: a second finger moving under a captured
        // drag says nothing about the drag's own cursor.
        let cursor_pass =
            !secondary && matches!(event, InputEvent::Pointer(p) if p.phase == PointerPhase::Move);
        // The same bracket carries the clipboard channel (a write and a paste
        // request), which differs only in when it commits: every pass, not the
        // pointer-move subset, since a copy can be answered from a key chord or an
        // `EditCommand` that never moved a pointer. See `RequestPass`.
        let request_slot = RequestPass::enter();

        // Whether the root widget itself holds the live opt-in (rather than a
        // pod below it), and whether it opted in during this dispatch.
        let contacts_captor_is_root = self.capture_claimant.is_some()
            && self.capture_contacts
            && self.contacts_captor_is_root;
        let root_opted_in;

        let (handled, needs_redraw, captured, hover_claimed, focus_req, focus_rel, ime) = {
            let state_any: &mut dyn Any = state;
            let mut ctx = EventCtx::new(state_any, pod.origin(), pod.size());
            // Seed the root widget's focus flag so a leaf-root editable that holds
            // focus can observe `has_focus()`; deeper focus is threaded per-pod.
            ctx.set_has_focus(self.focus_active);
            // Same for the hover link, plus the live epoch every pod compares its
            // stamp against and the eligibility gate that decides whether a claim
            // is recordable at all this pass.
            ctx.set_hovered(hover_was_active);
            ctx.set_hover_epoch(hover_epoch);
            ctx.set_hover_eligible(hover_pass);
            // Stamped onto whichever pod records a claim, so that pod's
            // destructor can tell this root's link from another root's
            // identically-numbered epoch (see `root_identity`).
            ctx.set_hover_root(root_identity);
            // The root widget's own contact frame: attributes a
            // `capture_contacts` made by the root widget itself (not by a pod
            // below it) to the root, and marks the dispatch as running under the
            // live opt-in's holder when the root widget is that holder.
            let frame = ContactFrame::enter(contacts_captor_is_root);
            let result = if secondary && !contacts_captor_is_root {
                // A non-claimant contact walks the recorded active path
                // forward-only (see `ChildPod::event_child`): the root widget is
                // handed the inert carrier and the real event rides beside it,
                // so only the captor that opted in runs its pointer handling. A
                // walk that never reached a pod on the path (the root widget
                // does not forward broadcasts to its captured child) falls back
                // to the ordinary delivery.
                let widget = pod.widget_mut();
                let carrier = crate::event::secondary_walk_carrier();
                let (_, delivered) = crate::event::run_secondary_walk(event.clone(), || {
                    widget.event(&mut ctx, &carrier)
                });
                match delivered {
                    Some(result) => result,
                    None => crate::event::without_secondary_walk(|| widget.event(&mut ctx, event)),
                }
            } else {
                pod.widget_mut().event(&mut ctx, event)
            };
            root_opted_in = frame.close().0;
            let handled = matches!(result, EventResult::Handled);
            (
                handled,
                ctx.needs_redraw() || handled,
                ctx.is_pointer_captured(),
                ctx.is_hover_claimed(),
                ctx.is_focus_requested(),
                ctx.is_focus_released(),
                ctx.take_ime_state(),
            )
        };

        // A published IME surface refreshes the stored one (persists past this
        // event, survives rebuild) until a blur clears it below. `store_ime_state`
        // bumps the focus/IME edge generation only if the surface actually moved
        // (a keystroke that changes nothing observable is not an edge).
        //
        // An **inactive** publish carries the same release intent here as it does
        // in `paint` (see that method's take path): a widget that publishes
        // `active: false` is ending the session, not describing it, so it takes
        // the full release. Reachable from this pass too — every paint-time
        // producer of an inactive surface is a container/widget whose `event` arm
        // can run first — and the two passes must not disagree about what an
        // inactive surface means. No `focus_active` guard is needed (unlike
        // `paint`, which must refuse to *resurrect* a cleared surface): releasing
        // an already-released root moves nothing and fires no edge.
        //
        // Ordering: this runs before the focus match below, so a dispatch that
        // both published an inactive surface and requested focus still ends up
        // focused — the later, more specific claim wins.
        //
        // The provenance rule `paint_overlays` applies to a paint-time publish,
        // mirrored onto this pass — the two must not disagree about who is
        // allowed to speak for the session, so a change to either belongs in
        // both.
        //
        // What the root can attribute here is bounded by how the event was
        // routed. A hit-tested or focus-routed dispatch reaches a publisher
        // through the tree, and the tree's own chain is what gated it (a
        // well-behaved editable publishes only while `has_focus`, which is now
        // stamp-gated) — so the rule for those is the liveness half alone: a
        // session must exist, or be opening in this very dispatch, for a publish
        // to describe anything. An `InputEvent::Overlay` is broadcast to the
        // whole tree and reaches every floated pod whether or not it holds
        // anything, so there the addressed surface must be the recorded owner of
        // the session, or be claiming it now — the same "the publisher holds the
        // link" question `paint_overlays` answers from the pods themselves.
        //
        // The *inactive* publish is refused on the same terms and for the same
        // reason it is at paint: ending a session is a claim about it too, and a
        // branch that owns none makes neither.
        let publish_attributable = match event {
            InputEvent::Overlay(overlay) => focus_req || self.focus_surface == Some(overlay.key),
            _ => self.focus_active || focus_req,
        };
        match ime {
            Some(_) if !publish_attributable => {}
            Some(ime) if !ime.active => {
                self.release_focus_session();
            }
            Some(ime) => self.store_ime_state(Some(ime)),
            None => {}
        }

        // Root-level capture path: a captured `Down` opens a gesture; `Up`/`Cancel`
        // close it. `Move` leaves the flag untouched so it survives the drag.
        //
        // Root-level focus path (the capture mirror): a `Down` that requested
        // focus opens the focus session; a `Down` that did not is a
        // blur-on-outside-tap and closes it (the per-container `focused` flags are
        // cleared by the routing helpers). Key/Ime/Scroll only adjust focus if the
        // dispatch explicitly requested or released it.
        //
        // Every arm mutates through `set_focus_active`/`store_ime_state` or the
        // paired `release_focus_session`, the change-guarded writers that own the
        // focus/IME edge generation: a `Down` on already-blurred chrome (the
        // commonest event of all) writes the same values back and must therefore
        // NOT fire an edge.
        //
        // Whether an arm below actually *honoured* a focus claim — the one thing
        // that makes the candidate epoch opened above the real one. Deliberately
        // not `focus_req` itself: the housekeeping arm ignores a claim by
        // contract, and a claim it ignored must leave the standing session (and
        // therefore the standing epoch) exactly where it was.
        let mut focus_claim_honoured = false;
        match event {
            // A non-claimant contact delivered down a live capture (rule (c) of
            // `InputEvent::PointerContact`'s contract): not a gesture of its own,
            // so it opens, moves and releases no capture and never blurs — its
            // `Up` must not end the claimant's gesture, and its `Down` is not a
            // tap outside anything. An explicit focus request or release from
            // its handler is honoured exactly as the scroll arm below honours
            // one.
            InputEvent::Pointer(_) if secondary => {
                if focus_req {
                    focus_claim_honoured = true;
                    self.set_focus_active(true);
                }
                if focus_rel {
                    self.release_focus_session();
                }
            }
            // Never reached: the gate at the top unwrapped every contact into a
            // plain `Pointer`, and routed it by the arms around this one.
            InputEvent::PointerContact { .. } => {}
            InputEvent::Pointer(pointer) => match pointer.phase {
                PointerPhase::Down => {
                    if captured {
                        // Rule (a): the contact whose `Down` captured is the
                        // claimant, and only its release ends the gesture. The
                        // opt-in is read from the pass rather than the root
                        // context's bubble so a captor below a component
                        // boundary (which mirrors capture but not the opt-in)
                        // is still heard.
                        self.capture_claimant = Some(pointer_id);
                        self.capture_contacts = contact_pass.contacts_requested();
                        self.contacts_captor_is_root = root_opted_in;
                    }
                    if focus_req {
                        focus_claim_honoured = true;
                        self.set_focus_active(true);
                        // A hit-tested press reached the claimant through the
                        // containers, so the session is the main tree's — the one
                        // attribution the event pass can make without help, and
                        // what makes the next paint seed the tree's links again
                        // the moment focus comes back to it (see `focus_surface`).
                        self.focus_surface = None;
                    } else {
                        // Blur: no widget on the tapped path took focus. The
                        // canonical release — flag and surface drop together, as
                        // one edge (see `release_focus_session_in`).
                        self.release_focus_session();
                    }
                }
                // Only ever the claimant's (or an uncaptured contact's) release:
                // any other contact's was routed to the `secondary` arm above or
                // dropped at the gate.
                PointerPhase::Up | PointerPhase::Cancel => {
                    self.capture_claimant = None;
                    self.capture_contacts = false;
                    self.contacts_captor_is_root = false;
                }
                PointerPhase::Move => {}
            },
            // A file drop is hit-tested exactly like `Scroll`/`Scale` above —
            // no capture, no claimant bookkeeping — so it joins their bucket
            // rather than getting one of its own: a target that calls
            // `request_focus`/`release_focus` from its `FileDrop` handler is
            // honoured on the same terms a scroll's would be, even though no
            // shipped drop target does today. Its `Ended` broadcast follow-up
            // lands here too: like an overlay event it is a broadcast that
            // stands for a real user gesture, so an explicit request is
            // honoured and nothing blurs (only a pointer `Down` blurs).
            InputEvent::Scroll { .. }
            | InputEvent::Scale(_)
            | InputEvent::FileDrop(_)
            | InputEvent::Key(_)
            | InputEvent::Ime(_)
            | InputEvent::EditCommand(_) => {
                if focus_req {
                    // The candidate epoch opened above becomes this claim's, so
                    // a focus move driven from the keyboard retires the branch it
                    // supersedes on exactly the terms a press does — including a
                    // floated surface's link, which this arm could otherwise
                    // never reach. `focus_surface` is deliberately left alone:
                    // which branch a focus-routed claim came from is not
                    // something the root can attribute, so the next paint
                    // re-resolves it from the pods' own live links.
                    focus_claim_honoured = true;
                    self.set_focus_active(true);
                }
                if focus_rel {
                    self.release_focus_session();
                }
            }
            // A broadcast is not user input: it opens no gesture, claims no
            // focus, and blurs nothing. Deliberately inert here — a housekeeping
            // pass that moved the root's capture/focus bookkeeping would change
            // what the *next* real event does, which is exactly what this
            // mechanism must not do (see `InputEvent::Housekeeping`).
            //
            // The one thing a broadcast *can* still move is the IME surface, via
            // the publish handling above (which is pass-agnostic by design, and
            // was before this arm existed): a widget that publishes while
            // flushing has said something about its session either way, and an
            // inactive publish releases it. No shipped widget does — `TextInput`
            // ignores a broadcast outright, and both cleared-surface publishers
            // are paint-time — so this is a contract note, not live behavior.
            InputEvent::Housekeeping => {}
            // A floated surface's own input: a broadcast at the root, but a real
            // user gesture underneath, so it moves *some* of what a hit-tested
            // event moves and deliberately none of the rest.
            InputEvent::Overlay(overlay) => {
                // Honoured: a text field inside a popover may claim focus, and the
                // session it opens is an ordinary one.
                //
                // What it must not do is stack on top of the session it
                // supersedes. A hit-tested press has the containers' blur sweep
                // under it, which clears every focused child but the one the
                // press kept; a press inside a floated surface reaches no
                // container's hit test, so nothing retires the chain the claim
                // replaces and two branches go on believing they are focused —
                // one of them still describing its own IME surface to the shell.
                // The root retires it in the two places it has standing to: here,
                // when its record already names a *different* surface as the
                // owner, and in `paint_overlays`, which resolves that record from
                // the pods themselves and stops seeding the main tree's links the
                // pass after the session leaves it.
                //
                // The other repair — running the blur sweep for this event, with
                // the claiming surface's owner kept — was not taken: that sweep
                // belongs to the containers, which run it over the children they
                // hold when a pointer `Down` passes through them. The root has no
                // mutable route to a `focused` link below its own pod, so from
                // here it is not a sweep at all but a change to how every
                // container routes a broadcast.
                //
                // The record is deliberately not *written* here: an overlay-pass
                // focus request cannot be attributed at the root (see
                // `focus_surface`) — a field in the tree re-claiming its own
                // session through a floated toolbar raises exactly the flag an
                // editable inside the surface raises to take it away.
                if focus_req {
                    if self.focus_surface.is_some_and(|owner| owner != overlay.key) {
                        // A session another surface owns, which *is* attributable:
                        // retire it whole rather than let the new claim inherit
                        // the surface the old one published.
                        self.release_focus_session();
                    }
                    focus_claim_honoured = true;
                    self.set_focus_active(true);
                }
                // Honoured: an **explicit** `EventCtx::release_focus` from inside
                // the surface ends the session it asked to end — the same rule the
                // `Scroll`/`Key`/`Ime` arm applies.
                if focus_rel {
                    self.release_focus_session();
                }
                // NEVER the blur branch. A `Down` that claims no focus blurs the
                // tree only when it was hit-tested *in* the tree; a press inside a
                // floated surface is the one press that must not, or tapping a
                // selection toolbar would drop the very selection the toolbar acts
                // on — which is the whole reason these two are routed apart.
                //
                // Nor does it advance the hover epoch: `hover_pass`/`hover_ends`
                // above match `InputEvent::Pointer` only, so a live hover in the
                // main tree survives an overlay pass by construction rather than by
                // a check here.
                //
                // A capture requested from inside the surface IS honoured, exactly
                // as a hit-tested `Down`'s is: the gesture then owns every
                // follow-up, and because a live capture short-circuits the overlay
                // pre-pass those follow-ups arrive as ordinary `Pointer` events
                // routed by the capture path — which is what lets a drag begun
                // inside a surface continue outside it, and what closes it on the
                // `Up` through the arm above.
                //
                // Whatever phase claimed it, not `Down` alone. The pods record
                // their own `active` path on any phase, so a capture claimed on a
                // `Move` left the surface latched and the root's mirror clear:
                // the pre-pass kept hit-testing, the follow-ups kept missing the
                // latched surface, and the `Up` that would have released it never
                // arrived — leaving the surface free to divert every later
                // pointer event its owner is reached by. Mirroring the claim on
                // any phase is what routes those follow-ups, and that `Up`, back
                // down the capture path to the surface that opened it.
                //
                // The claimant is the contact this overlay event was routed for —
                // the pre-pass re-enters this method under the gesture's own
                // contact pass, so `pointer_id` is that gesture's id.
                if captured {
                    self.capture_claimant = Some(pointer_id);
                    self.capture_contacts = contact_pass.contacts_requested();
                    self.contacts_captor_is_root = root_opted_in;
                }
                // ...and released on the phase that ends the gesture, in the
                // same door it was claimed through. The claim is mirrored on any
                // phase (see above), so the release has to be too: an `Up` or
                // `Cancel` that arrives *as an overlay event* — which is what
                // happens when the surface was not holding the capture when the
                // gesture began, so the pre-pass kept hit-testing it — would
                // otherwise leave the root's latch standing on a gesture that is
                // over, and a standing latch short-circuits the pre-pass for
                // every floated surface until some unrelated pointer release
                // happens along.
                //
                // A gesture whose claim *did* short-circuit the pre-pass ends on
                // the ordinary pointer arm above instead, because that is the
                // door its follow-ups come in by; the pod's own `active` link is
                // released by whichever of the two the release arrived through
                // (see `frust-widgets`' `OverlaySlot::forward`).
                if let OverlayEventKind::Pointer(pointer) = &overlay.kind
                    && matches!(pointer.phase, PointerPhase::Up | PointerPhase::Cancel)
                    && self
                        .capture_claimant
                        .is_none_or(|claimant| claimant == pointer_id)
                {
                    self.capture_claimant = None;
                    self.capture_contacts = false;
                    self.contacts_captor_is_root = false;
                }
            }
        }

        // A takeover during the claimant's own dispatch: a container cancelled
        // the widget that opted into the gesture's other contacts and released
        // it from the active path (`EventCtx::release_captured_child`). The
        // opt-in goes with it — unless the container that took over opted in
        // itself afterwards, which the pass records — so the other contacts are
        // dropped from here on rather than routed to a captor that is gone. The
        // claimant latch is deliberately kept: the gesture is still the
        // claimant's, now held by the container that took it over (still on the
        // active path, as the released child's ancestor), and only the
        // claimant's own `Up`/`Cancel` ends it. Never on a non-claimant
        // contact's dispatch, in which no container can clear a recorded link
        // (`ChildPod::set_active`), so nothing could have left the path.
        if !secondary && self.capture_claimant.is_some() && contact_pass.capture_released() {
            self.capture_contacts = contact_pass.contacts_requested();
        }

        // Settle the session's identity — the one write that decides which
        // recorded links survive this dispatch, in one place, from what the
        // dispatch actually did.
        //
        // * A **live session that ended** strands everything: the chain it ran
        //   through, a floated surface's link, and anything this very pass
        //   stamped (a claim the same dispatch then released). Past the candidate,
        //   therefore, not merely onto it — this is the clearing sweep no
        //   container can be asked to run.
        // * A **claim the root honoured** commits the candidate, so the chain it
        //   stamped is the only one that reads live and every older link is
        //   stranded by arithmetic.
        // * Anything else leaves the session exactly where it was, which is what
        //   makes a `Move`, a housekeeping broadcast, or a press inside a surface
        //   that claimed nothing incapable of disturbing a standing link — and
        //   what strands a claim an arm declined to honour, since the candidate
        //   it was stamped with is then never published again.
        let session_ended = focus_active_before && !self.focus_active;
        self.focus_epoch = if session_ended {
            advance_focus_epoch(candidate_focus_epoch)
        } else if focus_claim_honoured {
            candidate_focus_epoch
        } else {
            live_focus_epoch
        };
        self.publish_focus_session();

        // Close the hover pass: advance the epoch (which strands every stamp this
        // pass did not renew, wherever in the tree it sits) and refresh the mirror.
        // A claim only counts on a `hover_pass` — an ineligible pass records none
        // anyway, but stating it here keeps the mirror true by construction rather
        // than by the eligibility gate alone.
        if hover_pass || hover_ends {
            self.hover_epoch = self.hover_epoch.wrapping_add(1);
            self.hover_active = hover_pass && hover_claimed;
            // Republish the link a dropping pod checks its stamp against, so a
            // rebuild that removes the claimant can report the severance the
            // epoch alone cannot strand (see `ChildPod`'s `Drop`). Epoch `0` while
            // nothing holds a link, which is what makes a stale stamp's drop —
            // the common case — cost one comparison and mark nothing. The
            // identity rides with it because epoch integers are per-root and
            // collide by construction (see `root_identity`).
            crate::event::set_live_hover_link(
                self.root_identity,
                if self.hover_active {
                    self.hover_epoch
                } else {
                    0
                },
            );
        }

        // Close the cursor pass: the last request of the pass wins, and its
        // absence resolves to `Default` — which is what makes the request
        // stateless (a widget that stops asking needs no clearing) and what a
        // pointer moving off every requesting widget resolves to. Drained
        // unconditionally so a request made on a non-cursor pass cannot survive
        // into the next one; only a cursor pass commits it.
        let requested = request_slot.take();
        if cursor_pass {
            self.cursor = requested.cursor.unwrap_or_default();
        }
        // The clipboard half of the same drain, committed on **every** pass: a copy
        // is answered from whatever event decoded it, and there is no
        // "clipboard pass" the way there is a cursor pass. Both fields are
        // one-shot (see their docs) — a write from this pass supersedes one still
        // standing undrained, and a request is raised but never lowered here, so a
        // shell that skipped a drain answers late rather than losing the paste.
        if let Some(text) = requested.clipboard_write {
            self.pending_clipboard_write = Some(text);
        }
        self.pending_paste_request |= requested.paste_request;
        // A hover that ended with *nothing* taking it needs one repaint no widget
        // can ask for: the pointer moved onto empty chrome (or a press/lift/cancel
        // cleared the link), so the old claimant's `event()` was never called and
        // the new state has no claimant to speak for it. Every other edge is the
        // consumer's own: a hover that *began* or *moved from one claimant to
        // another* is repainted by the new claimant's change-gated
        // `request_redraw`, and because a repaint is global, that one frame is also
        // what lets the widget losing the link drop its overlay from
        // `PaintCtx::is_hovered`. This mirror is identity-free, so an A→B handoff
        // is `true`→`true` here and manufactures nothing — which is exactly why the
        // consumer's internal flag is normative (see `EventCtx::claim_hover`).
        let needs_redraw = needs_redraw || (hover_was_active && !self.hover_active);

        let mut outcome = EventOutcome {
            // Merge whatever the overlay pre-pass already produced: a
            // pass-through outside-tap notification ran before this dispatch and
            // its redraw is owed just as much as the dispatch's own.
            handled: handled || carried.handled,
            needs_redraw: needs_redraw || carried.needs_redraw,
        };

        // An OS file drag that just ended (`Drop`/`Cancel`) is followed by the
        // broadcast form, handled or not, so every widget holding state the
        // drag opened — a drop target's external session — hears the drag is
        // over: one whose release resolved over a region with no drop target
        // in it, and one whose release a *different* widget handled (two
        // independent drop-target groups the drag hovered in turn). See
        // `FileDropEvent`'s *Broadcast follow-up*. Re-entered through the
        // front door for the reason `route_overlay` re-enters: the follow-up
        // gets the same request bracket and outcome folding as any dispatch.
        // Shallow by construction: `Ended` is not one of the two phases that
        // trigger it.
        if let InputEvent::FileDrop(drop) = event
            && matches!(drop.phase, FileDropPhase::Drop | FileDropPhase::Cancel)
        {
            let ended = InputEvent::FileDrop(FileDropEvent {
                phase: FileDropPhase::Ended,
                position: Point::ZERO,
                paths: Vec::new(),
            });
            let follow_up = self.event(state, &ended);
            outcome.handled |= follow_up.handled;
            outcome.needs_redraw |= follow_up.needs_redraw;
        }
        outcome
    }

    /// Decide what one pointer contact does — the root half of
    /// [`InputEvent::PointerContact`]'s multi-contact contract.
    ///
    /// `Some(false)` routes it as the gesture's own pointer (rule (a): slot `0`
    /// with nothing captured, hit-tested; or the claimant of a live capture,
    /// down the captured path). `Some(true)` routes it as a **non-claimant**
    /// contact down a live capture's path, which happens only while the captor's
    /// opt-in stands (rule (c)): the dispatch then walks the recorded active
    /// path forward-only, so only the captor's own handler sees it (see
    /// [`crate::widget::ChildPod::event_child`]). `None` drops it: an additional
    /// contact with nothing captured (rule (b)), or one the captor did not opt
    /// into — or whose opt-in ended when a container took the gesture over from
    /// the captor ([`EventCtx::release_captured_child`]) — (rule (c)).
    fn contact_route(&self, pointer_id: PointerId) -> Option<bool> {
        match self.capture_claimant {
            None if pointer_id.slot == 0 => Some(false),
            None => None,
            Some(claimant) if claimant == pointer_id => Some(false),
            Some(_) if self.capture_contacts => Some(true),
            Some(_) => None,
        }
    }

    /// Hit-test the surfaces the last paint floated, **before** the main tree
    /// sees an uncaptured pointer, scroll, or scale — the routing half of the
    /// overlay portal (see [`crate::overlay`]).
    ///
    /// # What it does
    ///
    /// Walks [`RenderRoot::overlay_hits`] topmost-first (the `Tooltip` band before
    /// `Floating`, later registration before earlier), skipping
    /// [`OverlayInput::Transparent`] entries, and on the first rect containing the
    /// event's position re-dispatches it as
    /// [`InputEvent::Overlay`] — a broadcast carrying the owner's key and a
    /// **window-space** payload — then returns
    /// [`OverlayRoute::Consumed`]. The main tree never sees the original event.
    ///
    /// If nothing was hit and the event is a primary `Down`, every `Interactive`
    /// entry registered [`OutsideTap::Notify`] is told, topmost-first, with
    /// [`OverlayEventKind::OutsideDown`]; the press is then consumed iff any of
    /// them asked to consume it, and otherwise continues into today's dispatch.
    ///
    /// # What it deliberately does not do
    ///
    /// A **live capture short-circuits it entirely**: a gesture that has captured
    /// the pointer owns every follow-up until it ends, and re-hit-testing a drag
    /// that wandered over a floated surface would hand it to the wrong widget
    /// mid-gesture. That is also what lets a drag *begun* inside a surface
    /// continue outside it — the capture the overlay `Down` opened routes the
    /// follow-ups by the ordinary captured path.
    ///
    /// Housekeeping, `Key`, `Ime`, `EditCommand` and an overlay event already
    /// being routed pass straight through: a broadcast and a focus-routed event
    /// each reach their target with no hit test, so there is nothing here to
    /// redirect.
    ///
    /// A `FileDrop` passes straight through too, but for a different reason:
    /// it is hit-tested (see [`InputEvent::FileDrop`]), yet there is no
    /// [`OverlayEventKind`] carrier for it to ride a redirect through. A
    /// native OS drag has no floated-surface concept on the platform side —
    /// winit reports it against the window, not a particular surface — so a
    /// drop target living inside a popover is unreachable by one in v1 (see
    /// `docs/LIMITATIONS.md`). Its [`FileDropPhase::Ended`] follow-up passes
    /// through as well, as the broadcast it is.
    fn route_overlay(&mut self, state: &mut State, event: &InputEvent) -> OverlayRoute {
        if self.capture_claimant.is_some() || self.overlay_hits.is_empty() {
            return OverlayRoute::Continue(EventOutcome::default());
        }
        let (position, kind) = match event {
            // A contact is routed for its position and phase exactly like a
            // plain pointer (the root's gate has already unwrapped one, so the
            // second pattern is for totality); the re-entered dispatch below
            // keeps its id through the contact pass it runs under.
            InputEvent::Pointer(pointer) | InputEvent::PointerContact { event: pointer, .. } => {
                (pointer.position, OverlayEventKind::Pointer(*pointer))
            }
            InputEvent::Scroll { position, delta } => (
                *position,
                OverlayEventKind::Scroll {
                    position: *position,
                    delta: *delta,
                },
            ),
            InputEvent::Scale(scale) => (
                scale.focal,
                OverlayEventKind::Scale {
                    focal: scale.focal,
                    phase: scale.phase,
                    scale_delta: scale.scale_delta,
                    velocity: scale.velocity,
                },
            ),
            InputEvent::FileDrop(_)
            | InputEvent::Key(_)
            | InputEvent::Ime(_)
            | InputEvent::EditCommand(_)
            | InputEvent::Housekeeping
            | InputEvent::Overlay(_) => {
                return OverlayRoute::Continue(EventOutcome::default());
            }
        };

        // `overlay_hits` is in paint order, so walking it in reverse walks from
        // the surface painted last — the one the user sees on top — downward.
        // Cloned because each dispatch below needs `&mut self`; the list is one
        // small `Copy` struct per floated surface, and the clone never happens on
        // the overwhelmingly common no-overlays path (guarded above).
        let hits = self.overlay_hits.clone();
        for hit in hits.iter().rev() {
            // An `egui`-style transparent surface is painted above the app and
            // hit-tested by nothing: the pointer passes straight through to
            // whatever the main tree has underneath.
            if hit.input == OverlayInput::Transparent {
                continue;
            }
            if hit.contains(position) {
                let routed = InputEvent::Overlay(OverlayEvent {
                    key: hit.key,
                    kind: kind.clone(),
                });
                // Re-entering this method is deliberate and shallow: the routed
                // event is a broadcast, so it takes the branch above out
                // immediately and can never recurse further. Going back through
                // the front door is what gives the overlay dispatch the same
                // request bracket, focus bookkeeping and outcome folding every
                // other event gets, with one arm's worth of difference rather
                // than a second copy of the pass.
                return OverlayRoute::Consumed(self.event(state, &routed));
            }
        }

        // Nothing floated was hit. Only a **primary** press is a light-dismiss
        // signal: a secondary press is a context gesture (see
        // `docs/CODE_STANDARDS.md`'s Interaction Semantics), and a `Move`, `Up` or
        // scroll outside a surface says nothing about dismissing it.
        let dismissing = matches!(
            event,
            InputEvent::Pointer(pointer) | InputEvent::PointerContact { event: pointer, .. }
                if pointer.phase == PointerPhase::Down
                    && pointer.button == PointerButton::Primary
        );
        if !dismissing {
            return OverlayRoute::Continue(EventOutcome::default());
        }

        let mut carried = EventOutcome::default();
        let mut consumed = false;
        for hit in hits.iter().rev() {
            // A transparent surface takes no input at all, outside-taps included:
            // it is chrome the pointer does not know about.
            if hit.input == OverlayInput::Transparent {
                continue;
            }
            let OutsideTap::Notify { consume } = hit.outside_tap else {
                continue;
            };
            let routed = InputEvent::Overlay(OverlayEvent {
                key: hit.key,
                kind: OverlayEventKind::OutsideDown,
            });
            let outcome = self.event(state, &routed);
            carried.handled |= outcome.handled;
            carried.needs_redraw |= outcome.needs_redraw;
            // Every notified surface is told before any of them consumes:
            // dismissing one menu must not hide the press from a second surface
            // that also wanted to close.
            consumed |= consume;
        }
        if consumed {
            OverlayRoute::Consumed(carried)
        } else {
            OverlayRoute::Continue(carried)
        }
    }

    /// Perform a platform accessibility action, returning the same
    /// [`EventOutcome`] the synthesized input produced.
    ///
    /// A platform `accesskit_*` adapter delivers an `ActionRequest(node_id,
    /// action)`; a shell forwards it here. v1 routes actions through the **normal
    /// event path** by synthesizing pointer events at the target node's absolute
    /// bounds center (recovered from a fresh semantics pass — the id → bounds map
    /// the pass produces), so *every* fire-on-up-inside widget is operable with
    /// **zero** widget-side changes:
    ///
    /// * [`accesskit::Action::Click`] → a `Down` then an `Up` at the center,
    ///   activating any button/switch/checkbox exactly as a real tap would.
    /// * [`accesskit::Action::Focus`] → a `Down` then a synthetic `Cancel` at
    ///   the center: the `Down` claims focus for a widget that opts in on `Down`
    ///   (the recorded-focus contract), and the `Cancel` releases the capture
    ///   that same `Down` opened without touching the recorded focus path — so
    ///   the action claims focus without leaving the widget permanently
    ///   capturing every later pointer event. A widget that does not claim focus
    ///   on `Down` is unaffected, and `Cancel` never fires an on-press callback.
    /// * any other action → ignored (a no-op [`EventOutcome`]); richer actions are
    ///   deferred.
    ///
    /// An unknown `node_id` (not in the current tree) is a benign no-op. Requires
    /// a prior [`RenderRoot::layout`] so the bounds are valid. Synthetic-pointer
    /// activation cannot drive widgets that require a real drag (e.g. a slider) —
    /// an accepted v1 limitation.
    pub fn perform_accessibility_action(
        &mut self,
        state: &mut State,
        node_id: accesskit::NodeId,
        action: accesskit::Action,
    ) -> EventOutcome {
        // Recover the node's absolute bounds from a fresh semantics pass (the
        // id → absolute-bounds map this action routing needs; recomputing keeps
        // it in step with the live tree without a stored cache).
        let update = self.semantics();
        let Some(bounds) = update
            .nodes
            .iter()
            .find(|(id, _)| *id == node_id)
            .and_then(|(_, node)| node.bounds())
        else {
            return EventOutcome::default();
        };
        let center = Point::new((bounds.x0 + bounds.x1) / 2.0, (bounds.y0 + bounds.y1) / 2.0);
        let synth = |phase| {
            InputEvent::Pointer(PointerEvent {
                phase,
                position: center,
                button: PointerButton::Primary,
            })
        };
        match action {
            accesskit::Action::Click => {
                let down = self.event(state, &synth(PointerPhase::Down));
                let up = self.event(state, &synth(PointerPhase::Up));
                EventOutcome {
                    handled: down.handled || up.handled,
                    needs_redraw: down.needs_redraw || up.needs_redraw,
                }
            }
            accesskit::Action::Focus => {
                // A `Down` claims focus for a widget that opts in on `Down`; a
                // synthetic `Cancel` then releases the capture that `Down` also
                // opened (mirroring a real gesture steal), leaving the recorded
                // focus path intact — `Cancel` clears both `active` and
                // `capture_claimant` while never touching `focused`/`focus_active`.
                // Without the `Cancel`, the `Down` alone would leave the widget
                // permanently capturing every subsequent pointer event.
                let down = self.event(state, &synth(PointerPhase::Down));
                let cancel = self.event(state, &synth(PointerPhase::Cancel));
                EventOutcome {
                    handled: down.handled || cancel.handled,
                    needs_redraw: down.needs_redraw || cancel.needs_redraw,
                }
            }
            // Other actions are not modelled in v1: ignore rather than guess.
            _ => EventOutcome::default(),
        }
    }
}

impl<State: 'static, V: View<State>> Default for RenderRoot<State, V> {
    fn default() -> Self {
        Self::new()
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::event::{ImeContentType, ScrollDelta};
    use crate::overlay::OverlayKey;

    /// Application state for the tests.
    #[derive(Default)]
    struct AppState {
        label: String,
    }

    /// The retained widget produced by `MockTextView`: stores the current text
    /// and records what it painted.
    struct TextWidget {
        text: String,
    }

    impl crate::widget::Widget for TextWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            // A crude intrinsic size: width proportional to text length.
            let intrinsic = Size::new(self.text.len() as f64 * 8.0, 16.0);
            bc.constrain(intrinsic)
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            scene.draw_text(ctx.origin(), &self.text);
        }
    }

    /// The task's `MockTextView`: a real `View` impl living in tests.
    struct MockTextView {
        text: String,
    }

    impl View<AppState> for MockTextView {
        type Element = TextWidget;

        fn build(&self, _ctx: &mut BuildCtx<'_>) -> Self::Element {
            TextWidget {
                text: self.text.clone(),
            }
        }

        fn rebuild(
            &self,
            prev: &Self,
            element: &mut Self::Element,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            if prev.text != self.text {
                element.text = self.text.clone();
                // Text change: same size model would relayout, but the intrinsic
                // width can change, so signal PAINT here and let callers decide.
                ChangeFlags::PAINT
            } else {
                ChangeFlags::NONE
            }
        }
    }

    /// A scene recorder for asserting paint output.
    #[derive(Default)]
    struct RecordingScene {
        texts: Vec<(Point, String)>,
    }
    impl PaintScene for RecordingScene {
        fn fill_rect(&mut self, _origin: Point, _size: Size, _color: peniko::Color) {}
        fn draw_text(&mut self, origin: Point, text: &str) {
            self.texts.push((origin, text.to_string()));
        }
        fn draw_scene_texture(&mut self, _id: u64, _dest: Rect) {}
    }

    fn build(state: &mut AppState) -> MockTextView {
        MockTextView {
            text: state.label.clone(),
        }
    }

    #[test]
    fn build_inserts_widget_into_arena() {
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let mut state = AppState {
            label: "hello".to_string(),
        };
        let flags = root.rebuild(&mut build, &mut state);
        // First build dirties both passes.
        assert!(flags.needs_layout());
        assert!(flags.needs_paint());
        let id = root.root_id().expect("root built");
        let pod = root.tree().pod(id).expect("pod in arena");
        assert!(pod.widget().downcast_ref_is::<TextWidget>());
    }

    #[test]
    fn rebuild_changed_data_yields_paint() {
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let mut state = AppState {
            label: "a".to_string(),
        };
        root.rebuild(&mut build, &mut state);
        state.label = "b".to_string();
        let flags = root.rebuild(&mut build, &mut state);
        assert_eq!(flags, ChangeFlags::PAINT);
    }

    #[test]
    fn rebuild_unchanged_data_yields_none() {
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let mut state = AppState {
            label: "same".to_string(),
        };
        root.rebuild(&mut build, &mut state);
        let flags = root.rebuild(&mut build, &mut state);
        assert_eq!(flags, ChangeFlags::NONE);
    }

    #[test]
    fn layout_stores_size_in_pod() {
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let mut state = AppState {
            label: "hi".to_string(),
        };
        root.rebuild(&mut build, &mut state);
        let size = root.layout(Size::new(800.0, 600.0));
        // "hi" -> 2 * 8 = 16 wide, 16 tall, within the window.
        assert_eq!(size, Size::new(16.0, 16.0));
        let id = root.root_id().unwrap();
        let pod = root.tree().pod(id).unwrap();
        assert_eq!(pod.origin(), Point::ZERO);
        assert_eq!(pod.size(), Size::new(16.0, 16.0));
    }

    #[test]
    fn inspect_reports_the_laid_out_root() {
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let mut state = AppState {
            label: "hi".to_string(),
        };
        // Before the first build there is nothing to inspect.
        assert!(root.inspect().is_empty());

        root.rebuild(&mut build, &mut state);
        root.layout(Size::new(800.0, 600.0));

        let nodes = root.inspect();
        assert_eq!(nodes.len(), 1);
        let node = &nodes[0];
        assert_eq!(node.id, root.root_id().unwrap());
        assert_eq!(node.parent, None);
        assert_eq!(node.depth, 0);
        assert!(node.children.is_empty());
        // The concrete element type is captured, not the erased box.
        assert!(node.type_name.ends_with("TextWidget"), "{}", node.type_name);
        assert_eq!(node.debug_label, None);
        // Bounds match what the layout pass recorded on the pod.
        let pod = root.tree().pod(node.id).unwrap();
        assert_eq!(
            node.bounds,
            Rect::from_origin_size(pod.origin(), pod.size())
        );
        assert_eq!(node.bounds, Rect::new(0.0, 0.0, 16.0, 16.0));
    }

    #[test]
    fn layout_clamps_to_window() {
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let mut state = AppState {
            label: "wwwwwwwwwww".to_string(), // 10 chars -> 80 wide intrinsic
        };
        root.rebuild(&mut build, &mut state);
        let size = root.layout(Size::new(40.0, 40.0));
        // Intrinsic width 80 is clamped to the 40-wide window.
        assert_eq!(size.width, 40.0);
    }

    #[test]
    fn paint_emits_current_text() {
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let mut state = AppState {
            label: "one".to_string(),
        };
        root.rebuild(&mut build, &mut state);
        root.layout(Size::new(200.0, 200.0));

        let mut scene = RecordingScene::default();
        root.paint(&mut scene, FrameTime::ZERO);
        assert_eq!(scene.texts, vec![(Point::ZERO, "one".to_string())]);

        // Change data, rebuild, repaint -> new text.
        state.label = "two".to_string();
        root.rebuild(&mut build, &mut state);
        let mut scene2 = RecordingScene::default();
        root.paint(&mut scene2, FrameTime::ZERO);
        assert_eq!(scene2.texts, vec![(Point::ZERO, "two".to_string())]);
    }

    /// A leaf widget that publishes a fixed [`PlatformViewFrame`] — and reports
    /// a z-shield rect over its own bounds — on every paint, unless
    /// `should_publish` is false (the widget-level toggle that simulates a slot
    /// no longer publishing between two rebuilds). Both channels ride the same
    /// toggle so one fixture covers both replace-per-pass contracts.
    struct PlatformViewProbeWidget {
        slot_id: u64,
        should_publish: bool,
    }

    impl crate::widget::Widget for PlatformViewProbeWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            if self.should_publish {
                ctx.publish_platform_view(PlatformViewFrame {
                    slot_id: self.slot_id,
                    view_type: "dev.frust.Probe".to_string(),
                    params_json: String::new(),
                    params_generation: 0,
                    rect: kurbo::Rect::from_origin_size(ctx.origin(), ctx.size()),
                    clip: None,
                    visible: true,
                    interactive: false,
                    shields: Vec::new(),
                });
                ctx.report_input_shield(kurbo::Rect::from_origin_size(ctx.origin(), ctx.size()));
            }
        }
    }

    /// A root widget owning two independently toggleable [`ChildPod`]s (a
    /// minimal two-slot container) so a rebuild can flip either slot's
    /// `should_publish` — the fixture the "two slots in one pass" and
    /// "empty-pass clears stale frames" tests below need.
    struct PlatformViewRootWidget {
        a: crate::widget::ChildPod,
        b: crate::widget::ChildPod,
    }

    impl crate::widget::Widget for PlatformViewRootWidget {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.a.layout_child(ctx, bc);
            self.b.layout_child(ctx, bc);
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            self.a.paint_child(ctx, scene);
            self.b.paint_child(ctx, scene);
        }
    }

    /// The `View` producing [`PlatformViewRootWidget`], reconciling each
    /// slot's `should_publish` flag on rebuild like any controlled widget.
    struct PlatformViewRootView {
        publish_a: bool,
        publish_b: bool,
    }

    impl View<PvState> for PlatformViewRootView {
        type Element = PlatformViewRootWidget;

        fn build(&self, _ctx: &mut BuildCtx<'_>) -> Self::Element {
            PlatformViewRootWidget {
                a: crate::widget::ChildPod::new(Box::new(PlatformViewProbeWidget {
                    slot_id: 1,
                    should_publish: self.publish_a,
                })),
                b: crate::widget::ChildPod::new(Box::new(PlatformViewProbeWidget {
                    slot_id: 2,
                    should_publish: self.publish_b,
                })),
            }
        }

        fn rebuild(
            &self,
            prev: &Self,
            element: &mut Self::Element,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            if prev.publish_a != self.publish_a || prev.publish_b != self.publish_b {
                element
                    .a
                    .widget_mut()
                    .downcast_mut::<PlatformViewProbeWidget>()
                    .expect("slot a stays a PlatformViewProbeWidget")
                    .should_publish = self.publish_a;
                element
                    .b
                    .widget_mut()
                    .downcast_mut::<PlatformViewProbeWidget>()
                    .expect("slot b stays a PlatformViewProbeWidget")
                    .should_publish = self.publish_b;
                ChangeFlags::PAINT
            } else {
                ChangeFlags::NONE
            }
        }
    }

    /// App state for the platform-view frame-channel tests.
    #[derive(Default)]
    struct PvState {
        publish_a: bool,
        publish_b: bool,
    }

    fn platform_view_logic(state: &mut PvState) -> PlatformViewRootView {
        PlatformViewRootView {
            publish_a: state.publish_a,
            publish_b: state.publish_b,
        }
    }

    #[test]
    fn platform_view_frames_arrive_in_order_and_clear_on_empty_pass() {
        let mut root: RenderRoot<PvState, PlatformViewRootView> = RenderRoot::new();
        let mut state = PvState {
            publish_a: true,
            publish_b: true,
        };
        root.rebuild(&mut platform_view_logic, &mut state);
        root.layout(Size::new(200.0, 200.0));

        let mut scene = RecordingScene::default();
        root.paint(&mut scene, FrameTime::ZERO);

        // Two slots publishing in one pass both arrive, in paint order — the
        // regression test for the overwrite hazard (an Option-based `ime_state`
        // shape here would leave only the second slot's frame).
        let frames = root.platform_view_frames();
        assert_eq!(frames.len(), 2);
        assert_eq!(frames[0].slot_id, 1);
        assert_eq!(frames[1].slot_id, 2);

        // Next pass: neither slot publishes (simulates both going away/culled).
        // The collection is REPLACED, so the previous pass's frames must not
        // survive as stale entries.
        state.publish_a = false;
        state.publish_b = false;
        root.rebuild(&mut platform_view_logic, &mut state);
        root.layout(Size::new(200.0, 200.0));
        root.paint(&mut scene, FrameTime::ZERO);
        assert!(
            root.platform_view_frames().is_empty(),
            "a paint pass with no publishers must yield an empty slice"
        );
    }

    #[test]
    fn input_shields_arrive_in_order_and_clear_on_empty_pass() {
        // The shield channel's half of the contract above:
        // two shields reported in one pass both survive (the `Vec`
        // extend, not an `Option` overwrite), and a pass that reports none
        // replaces the collection rather than merging — a stale shield must
        // never keep stealing input from an interactive slot.
        let mut root: RenderRoot<PvState, PlatformViewRootView> = RenderRoot::new();
        let mut state = PvState {
            publish_a: true,
            publish_b: true,
        };
        root.rebuild(&mut platform_view_logic, &mut state);
        root.layout(Size::new(200.0, 200.0));

        let mut scene = RecordingScene::default();
        root.paint(&mut scene, FrameTime::ZERO);
        assert_eq!(root.input_shields().len(), 2);
        assert_eq!(
            root.input_shields()[0],
            Rect::from_origin_size(Point::ZERO, Size::new(10.0, 10.0)),
            "a shield is reported in absolute paint coordinates"
        );

        state.publish_a = false;
        state.publish_b = false;
        root.rebuild(&mut platform_view_logic, &mut state);
        root.layout(Size::new(200.0, 200.0));
        root.paint(&mut scene, FrameTime::ZERO);
        assert!(
            root.input_shields().is_empty(),
            "a paint pass reporting no shields must yield an empty slice"
        );
    }

    #[test]
    fn retired_platform_views_drain_exactly_once() {
        // The prompt-teardown channel: a reported slot
        // id is handed to the shell once and then gone, mirroring
        // `take_change_flags`. Serialized against the other test touching the
        // process-wide list (see `RETIRE_TEST_LOCK`).
        let _guard = RETIRE_TEST_LOCK.lock().unwrap_or_else(|e| e.into_inner());
        let mut root: RenderRoot<PvState, PlatformViewRootView> = RenderRoot::new();
        let _ = root.take_retired_platform_views(); // clear anything a sibling left

        crate::widget::report_retired_slot(7);
        crate::widget::report_retired_slot(9);
        assert_eq!(root.take_retired_platform_views(), vec![7, 9]);
        assert!(
            root.take_retired_platform_views().is_empty(),
            "draining is destructive — a second drain reports nothing"
        );
    }

    #[test]
    fn retired_platform_views_are_capped_dropping_the_oldest() {
        // A shell that never drains (desktop: no native compositor) must not
        // grow this list forever; past the cap the OLDEST id is dropped and the
        // differ's missing-streak backstop covers it.
        let _guard = RETIRE_TEST_LOCK.lock().unwrap_or_else(|e| e.into_inner());
        let mut root: RenderRoot<PvState, PlatformViewRootView> = RenderRoot::new();
        let _ = root.take_retired_platform_views();

        for slot_id in 0..1_000u64 {
            crate::widget::report_retired_slot(slot_id);
        }
        let drained = root.take_retired_platform_views();
        assert!(drained.len() <= 256, "the pending list stays bounded");
        assert_eq!(
            *drained.last().expect("non-empty"),
            999,
            "the newest report always survives"
        );
        assert!(
            !drained.contains(&0),
            "the oldest reports are the ones dropped"
        );
    }

    /// Serializes the two tests that drive the process-wide retire list
    /// (`crate::widget::report_retired_slot`), which `cargo test`'s parallel
    /// threads would otherwise interleave — the same shape
    /// `frust-shell-common::theme_override`'s tests use for its global slot.
    static RETIRE_TEST_LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());

    /// A root widget that advances no state but requests a continuation frame on
    /// every paint — stands in for an animating widget (e.g. a scroll fling).
    struct FrameWidget;
    impl crate::widget::Widget for FrameWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            ctx.request_frame();
        }
    }

    struct FrameView;
    impl View<AppState> for FrameView {
        type Element = FrameWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> FrameWidget {
            FrameWidget
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut FrameWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    #[test]
    fn paint_reports_needs_frame_from_animating_root() {
        // A still root reports no continuation frame.
        let mut still: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let mut state = AppState {
            label: "x".to_string(),
        };
        still.rebuild(&mut build, &mut state);
        still.layout(Size::new(100.0, 100.0));
        let mut scene = RecordingScene::default();
        assert!(!still.paint(&mut scene, FrameTime::ZERO).needs_frame);

        // An animating root bubbles request_frame out as PaintOutcome::needs_frame.
        let mut anim: RenderRoot<AppState, FrameView> = RenderRoot::new();
        anim.rebuild(&mut |_s: &mut AppState| FrameView, &mut state);
        anim.layout(Size::new(100.0, 100.0));
        let mut scene2 = RecordingScene::default();
        assert!(anim.paint(&mut scene2, FrameTime::ZERO).needs_frame);
    }

    /// A root widget whose animation changes its layout: it requests a layout
    /// re-run on every paint — stands in for an expanding accordion.
    struct LayoutFrameWidget;
    impl crate::widget::Widget for LayoutFrameWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            ctx.request_layout();
        }
    }

    struct LayoutFrameView;
    impl View<AppState> for LayoutFrameView {
        type Element = LayoutFrameWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> LayoutFrameWidget {
            LayoutFrameWidget
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut LayoutFrameWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    #[test]
    fn paint_folds_request_layout_into_pending_change_flags() {
        let mut state = AppState {
            label: "x".to_string(),
        };

        // A root calling `request_layout` in paint surfaces it on the outcome AND
        // folds LAYOUT into `pending`, so the NEXT frame's `take_change_flags`
        // reports `needs_layout()`.
        let mut anim: RenderRoot<AppState, LayoutFrameView> = RenderRoot::new();
        anim.rebuild(&mut |_s: &mut AppState| LayoutFrameView, &mut state);
        anim.layout(Size::new(100.0, 100.0));
        // Drain any rebuild/layout dirtiness so we observe only paint's fold.
        let _ = anim.take_change_flags();
        let mut scene = RecordingScene::default();
        let outcome = anim.paint(&mut scene, FrameTime::ZERO);
        assert!(outcome.needs_layout, "outcome reports needs_layout");
        // `request_layout` implies `request_frame`, so the animation still runs.
        assert!(outcome.needs_frame, "request_layout implies needs_frame");
        assert!(
            anim.has_pending_change_flags(),
            "the fold survives to the next frame"
        );
        assert!(
            anim.take_change_flags().needs_layout(),
            "next frame's take_change_flags reports needs_layout"
        );
    }

    #[test]
    fn paint_request_frame_only_does_not_fold_layout() {
        let mut state = AppState {
            label: "x".to_string(),
        };

        // A paint-only animation (request_frame, no request_layout) must NOT fold
        // LAYOUT — the mobile layout-skip win depends on this staying opt-in.
        let mut anim: RenderRoot<AppState, FrameView> = RenderRoot::new();
        anim.rebuild(&mut |_s: &mut AppState| FrameView, &mut state);
        anim.layout(Size::new(100.0, 100.0));
        let _ = anim.take_change_flags();
        let mut scene = RecordingScene::default();
        let outcome = anim.paint(&mut scene, FrameTime::ZERO);
        assert!(outcome.needs_frame);
        assert!(
            !outcome.needs_layout,
            "request_frame alone: no needs_layout"
        );
        assert!(
            !anim.has_pending_change_flags(),
            "request_frame alone must not fold LAYOUT into pending"
        );
    }

    /// A root whose paint requests a *pacable* cosmetic-loop frame — stands in
    /// for a skeleton shimmer whose cadence the mobile frame gate may throttle.
    struct PacedFrameWidget;
    impl crate::widget::Widget for PacedFrameWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            ctx.request_frame_paced();
        }
    }

    struct PacedFrameView;
    impl View<AppState> for PacedFrameView {
        type Element = PacedFrameWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> PacedFrameWidget {
            PacedFrameWidget
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut PacedFrameWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    #[test]
    fn paint_surfaces_paced_only_tick_class_on_outcome() {
        let mut state = AppState {
            label: "x".to_string(),
        };

        // A paced-only root surfaces `needs_frame_paced_only` on the outcome so
        // the mobile frame gate may throttle its cadence.
        let mut paced: RenderRoot<AppState, PacedFrameView> = RenderRoot::new();
        paced.rebuild(&mut |_s: &mut AppState| PacedFrameView, &mut state);
        paced.layout(Size::new(100.0, 100.0));
        let mut scene = RecordingScene::default();
        let outcome = paced.paint(&mut scene, FrameTime::ZERO);
        assert!(outcome.needs_frame);
        assert!(
            outcome.needs_frame_paced_only,
            "a purely-cosmetic frame surfaces as paced-only"
        );
        assert_eq!(
            outcome.paced_interval,
            Some(std::time::Duration::ZERO),
            "a bare `request_frame_paced` names no interval (the theme's own rate)"
        );

        // A Transition-class (`request_frame`) root is never paced-only, keeping
        // today's every-vsync behavior for existing callers.
        let mut anim: RenderRoot<AppState, FrameView> = RenderRoot::new();
        anim.rebuild(&mut |_s: &mut AppState| FrameView, &mut state);
        anim.layout(Size::new(100.0, 100.0));
        let mut scene2 = RecordingScene::default();
        let outcome2 = anim.paint(&mut scene2, FrameTime::ZERO);
        assert!(outcome2.needs_frame);
        assert!(
            !outcome2.needs_frame_paced_only,
            "request_frame stays unpaced (Transition)"
        );
        assert_eq!(
            outcome2.paced_interval, None,
            "an unpaced frame names no paced interval"
        );
    }

    /// A root widget whose decorative loop names its own slow cadence — the
    /// `request_frame_paced_at` counterpart of [`PacedFrameWidget`].
    struct SlowPacedFrameWidget;
    impl crate::widget::Widget for SlowPacedFrameWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            ctx.request_frame_paced_at(std::time::Duration::from_millis(500));
        }
    }
    struct SlowPacedFrameView;
    impl View<AppState> for SlowPacedFrameView {
        type Element = SlowPacedFrameWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> SlowPacedFrameWidget {
            SlowPacedFrameWidget
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut SlowPacedFrameWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    #[test]
    fn paint_surfaces_the_requested_paced_interval_on_outcome() {
        // The end-to-end core half of the per-request pacing seam: a widget's
        // `request_frame_paced_at` reaches the shell on `PaintOutcome`, which is
        // what the mobile gate latches into `FramePacing`.
        let mut state = AppState {
            label: "x".to_string(),
        };
        let mut root: RenderRoot<AppState, SlowPacedFrameView> = RenderRoot::new();
        root.rebuild(&mut |_s: &mut AppState| SlowPacedFrameView, &mut state);
        root.layout(Size::new(100.0, 100.0));
        let mut scene = RecordingScene::default();
        let outcome = root.paint(&mut scene, FrameTime::ZERO);
        assert!(outcome.needs_frame_paced_only);
        assert_eq!(
            outcome.paced_interval,
            Some(std::time::Duration::from_millis(500))
        );
    }

    /// A root widget that records the `frame_time` its paint observed, so a test
    /// can prove the shell-injected clock reaches `PaintCtx::frame_time()`.
    struct ClockWidget {
        seen: std::rc::Rc<std::cell::Cell<Option<FrameTime>>>,
    }
    impl crate::widget::Widget for ClockWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            self.seen.set(Some(ctx.frame_time()));
        }
    }

    struct ClockView {
        seen: std::rc::Rc<std::cell::Cell<Option<FrameTime>>>,
    }
    impl View<AppState> for ClockView {
        type Element = ClockWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> ClockWidget {
            ClockWidget {
                seen: self.seen.clone(),
            }
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut ClockWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    #[test]
    fn paint_threads_injected_frame_time_to_widget() {
        let seen = std::rc::Rc::new(std::cell::Cell::new(None));
        let mut root: RenderRoot<AppState, ClockView> = RenderRoot::new();
        let mut state = AppState::default();
        let seen_for_view = seen.clone();
        root.rebuild(
            &mut move |_s: &mut AppState| ClockView {
                seen: seen_for_view.clone(),
            },
            &mut state,
        );
        root.layout(Size::new(100.0, 100.0));

        // Two paints with distinct injected times: the widget observes each one,
        // proving the clock is shell-fed (not read from an ambient `Instant`).
        let mut scene = RecordingScene::default();
        root.paint(&mut scene, FrameTime::from_nanos(1_000));
        assert_eq!(seen.get(), Some(FrameTime::from_nanos(1_000)));
        root.paint(&mut scene, FrameTime::from_nanos(17_000));
        assert_eq!(seen.get(), Some(FrameTime::from_nanos(17_000)));
    }

    // --- Theme threading: a dummy theme recovered during paint/layout. ---

    /// A dummy theme type standing in for `frust_theme::Theme` — `frust-core`
    /// never names the real one, so this proves the type-erased slot works for
    /// any `'static` type.
    #[derive(Debug, Clone, PartialEq)]
    struct TestTheme {
        accent: u32,
    }

    /// A root widget recording the theme accent it recovered during paint (and
    /// during layout), or `None` when no theme was threaded in.
    struct ThemeWidget {
        seen_paint: std::rc::Rc<std::cell::Cell<Option<u32>>>,
        seen_layout: std::rc::Rc<std::cell::Cell<Option<u32>>>,
    }
    impl crate::widget::Widget for ThemeWidget {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.seen_layout
                .set(ctx.theme_as::<TestTheme>().map(|t| t.accent));
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            self.seen_paint
                .set(ctx.theme_as::<TestTheme>().map(|t| t.accent));
        }
    }

    struct ThemeView {
        seen_paint: std::rc::Rc<std::cell::Cell<Option<u32>>>,
        seen_layout: std::rc::Rc<std::cell::Cell<Option<u32>>>,
    }
    impl View<AppState> for ThemeView {
        type Element = ThemeWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> ThemeWidget {
            ThemeWidget {
                seen_paint: self.seen_paint.clone(),
                seen_layout: self.seen_layout.clone(),
            }
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut ThemeWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn drive_theme_root(theme: Option<TestTheme>) -> (Option<u32>, Option<u32>) {
        let seen_paint = std::rc::Rc::new(std::cell::Cell::new(None));
        let seen_layout = std::rc::Rc::new(std::cell::Cell::new(None));
        let mut root: RenderRoot<AppState, ThemeView> = RenderRoot::new();
        if let Some(theme) = theme {
            root.set_theme(Box::new(theme));
        }
        let mut state = AppState::default();
        let sp = seen_paint.clone();
        let sl = seen_layout.clone();
        root.rebuild(
            &mut move |_s: &mut AppState| ThemeView {
                seen_paint: sp.clone(),
                seen_layout: sl.clone(),
            },
            &mut state,
        );
        root.layout(Size::new(100.0, 100.0));
        let mut scene = RecordingScene::default();
        root.paint(&mut scene, FrameTime::ZERO);
        (seen_layout.get(), seen_paint.get())
    }

    #[test]
    fn set_theme_threads_into_layout_and_paint() {
        let (layout, paint) = drive_theme_root(Some(TestTheme { accent: 5 }));
        assert_eq!(layout, Some(5));
        assert_eq!(paint, Some(5));
    }

    #[test]
    fn no_theme_yields_none_in_layout_and_paint() {
        let (layout, paint) = drive_theme_root(None);
        assert_eq!(layout, None);
        assert_eq!(paint, None);
    }

    #[test]
    fn set_theme_replaces_the_previous_theme() {
        // A second `set_theme` (a live dark-mode flip on desktop) wins on the
        // next paint.
        let seen_paint = std::rc::Rc::new(std::cell::Cell::new(None));
        let seen_layout = std::rc::Rc::new(std::cell::Cell::new(None));
        let mut root: RenderRoot<AppState, ThemeView> = RenderRoot::new();
        root.set_theme(Box::new(TestTheme { accent: 1 }));
        let mut state = AppState::default();
        let sp = seen_paint.clone();
        let sl = seen_layout.clone();
        root.rebuild(
            &mut move |_s: &mut AppState| ThemeView {
                seen_paint: sp.clone(),
                seen_layout: sl.clone(),
            },
            &mut state,
        );
        root.layout(Size::new(100.0, 100.0));
        let mut scene = RecordingScene::default();
        root.paint(&mut scene, FrameTime::ZERO);
        assert_eq!(seen_paint.get(), Some(1));

        // Flip the theme, repaint — the new accent is observed.
        root.set_theme(Box::new(TestTheme { accent: 2 }));
        root.layout(Size::new(100.0, 100.0));
        root.paint(&mut scene, FrameTime::ZERO);
        assert_eq!(seen_paint.get(), Some(2));
    }

    // --- Event-pass fixtures: a widget that mutates state on pointer-down. ---

    #[derive(Default)]
    struct ClickState {
        clicks: u32,
    }

    struct ButtonWidget;
    impl crate::widget::Widget for ButtonWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(40.0, 20.0))
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut crate::event::EventCtx, event: &InputEvent) -> EventResult {
            if let InputEvent::Pointer(p) = event {
                match p.phase {
                    PointerPhase::Down => {
                        ctx.state_mut::<ClickState>().clicks += 1;
                        ctx.request_redraw();
                        ctx.capture_pointer();
                        return EventResult::Handled;
                    }
                    PointerPhase::Up | PointerPhase::Cancel => return EventResult::Handled,
                    PointerPhase::Move => {}
                }
            }
            EventResult::Ignored
        }
    }

    struct ButtonView;
    impl View<ClickState> for ButtonView {
        type Element = ButtonWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> ButtonWidget {
            ButtonWidget
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut ButtonWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn button_logic(_state: &mut ClickState) -> ButtonView {
        ButtonView
    }

    fn pointer(phase: PointerPhase, x: f64, y: f64) -> InputEvent {
        InputEvent::Pointer(crate::event::PointerEvent {
            phase,
            position: Point::new(x, y),
            button: crate::event::PointerButton::Primary,
        })
    }

    #[test]
    fn event_reaches_root_widget_and_mutates_state() {
        let mut root: RenderRoot<ClickState, ButtonView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut button_logic, &mut state);
        root.layout(Size::new(200.0, 200.0));

        let outcome = root.event(&mut state, &pointer(PointerPhase::Down, 5.0, 5.0));
        assert!(outcome.handled);
        assert!(outcome.needs_redraw);
        assert_eq!(state.clicks, 1);
        // A captured Down opens the root gesture.
        assert!(root.is_pointer_captured());
    }

    #[test]
    fn event_before_build_is_a_benign_no_op() {
        let mut root: RenderRoot<ClickState, ButtonView> = RenderRoot::new();
        let mut state = ClickState::default();
        let outcome = root.event(&mut state, &pointer(PointerPhase::Down, 1.0, 1.0));
        assert_eq!(outcome, EventOutcome::default());
        assert_eq!(state.clicks, 0);
    }

    #[test]
    fn capture_releases_on_pointer_up() {
        let mut root: RenderRoot<ClickState, ButtonView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut button_logic, &mut state);
        root.layout(Size::new(200.0, 200.0));

        root.event(&mut state, &pointer(PointerPhase::Down, 5.0, 5.0));
        assert!(root.is_pointer_captured());
        root.event(&mut state, &pointer(PointerPhase::Up, 5.0, 5.0));
        assert!(!root.is_pointer_captured());
    }

    #[test]
    fn take_change_flags_drains_accumulated_dirtiness() {
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let mut state = AppState {
            label: "x".to_string(),
        };
        root.rebuild(&mut build, &mut state);
        // First build accumulated LAYOUT|PAINT.
        let flags = root.take_change_flags();
        assert!(flags.needs_layout());
        // Draining leaves it empty until the next rebuild.
        assert!(root.take_change_flags().is_empty());
    }

    #[test]
    fn has_pending_change_flags_peeks_without_draining() {
        // The frame-gate peek: observe pending dirtiness without
        // clearing it, so a skipped frame preserves the flags for the next
        // frame that actually runs.
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        assert!(
            !root.has_pending_change_flags(),
            "a fresh root has nothing pending"
        );
        let mut state = AppState {
            label: "x".to_string(),
        };
        root.rebuild(&mut build, &mut state);
        // First build accumulated LAYOUT|PAINT — the peek sees it...
        assert!(root.has_pending_change_flags());
        // ...and repeated peeks do NOT drain it.
        assert!(root.has_pending_change_flags());
        // Only `take_change_flags` drains.
        assert!(!root.take_change_flags().is_empty());
        assert!(!root.has_pending_change_flags());
    }

    #[test]
    fn set_theme_marks_layout_and_paint_pending() {
        // `set_theme` alone (no rebuild) must dirty layout/paint so a shell
        // gating on `take_change_flags` doesn't skip re-resolving theme-baked
        // widget state (e.g. Text's themed glyph color) on a bare theme swap.
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        root.set_theme(Box::new(TestTheme { accent: 1 }));
        let flags = root.take_change_flags();
        assert!(flags.needs_layout());
        assert!(flags.needs_paint());

        // Draining clears it until the next `set_theme`/rebuild.
        assert!(root.take_change_flags().is_empty());
        root.set_theme(Box::new(TestTheme { accent: 2 }));
        let flags = root.take_change_flags();
        assert!(flags.needs_layout());
        assert!(flags.needs_paint());
    }

    // --- Window insets: pushed value reaches layout/paint contexts. ---

    use crate::insets::{EdgeInsets, WindowInsets};

    /// A root widget recording the `WindowInsets` it observed during layout and
    /// paint, proving the shell-pushed value threads through both contexts.
    struct InsetsWidget {
        seen_layout: std::rc::Rc<std::cell::Cell<Option<WindowInsets>>>,
        seen_paint: std::rc::Rc<std::cell::Cell<Option<WindowInsets>>>,
    }
    impl crate::widget::Widget for InsetsWidget {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.seen_layout.set(Some(ctx.window_insets()));
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            self.seen_paint.set(Some(ctx.window_insets()));
        }
    }

    struct InsetsView {
        seen_layout: std::rc::Rc<std::cell::Cell<Option<WindowInsets>>>,
        seen_paint: std::rc::Rc<std::cell::Cell<Option<WindowInsets>>>,
    }
    impl View<AppState> for InsetsView {
        type Element = InsetsWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> InsetsWidget {
            InsetsWidget {
                seen_layout: self.seen_layout.clone(),
                seen_paint: self.seen_paint.clone(),
            }
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut InsetsWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn drive_insets_root(
        insets: Option<WindowInsets>,
    ) -> (Option<WindowInsets>, Option<WindowInsets>) {
        let seen_layout = std::rc::Rc::new(std::cell::Cell::new(None));
        let seen_paint = std::rc::Rc::new(std::cell::Cell::new(None));
        let mut root: RenderRoot<AppState, InsetsView> = RenderRoot::new();
        if let Some(insets) = insets {
            root.set_insets(insets);
        }
        let mut state = AppState::default();
        let sl = seen_layout.clone();
        let sp = seen_paint.clone();
        root.rebuild(
            &mut move |_s: &mut AppState| InsetsView {
                seen_layout: sl.clone(),
                seen_paint: sp.clone(),
            },
            &mut state,
        );
        root.layout(Size::new(100.0, 100.0));
        let mut scene = RecordingScene::default();
        root.paint(&mut scene, FrameTime::ZERO);
        (seen_layout.get(), seen_paint.get())
    }

    #[test]
    fn set_insets_threads_into_layout_and_paint() {
        let insets = WindowInsets::new(
            EdgeInsets::new(0.0, 24.0, 0.0, 34.0),
            EdgeInsets::new(0.0, 0.0, 0.0, 0.0),
        );
        let (layout, paint) = drive_insets_root(Some(insets));
        assert_eq!(layout, Some(insets));
        assert_eq!(paint, Some(insets));
    }

    #[test]
    fn no_insets_yields_zero_in_layout_and_paint() {
        let (layout, paint) = drive_insets_root(None);
        assert_eq!(layout, Some(WindowInsets::default()));
        assert_eq!(paint, Some(WindowInsets::default()));
    }

    #[test]
    fn set_insets_marks_layout_and_paint_pending() {
        // Mirrors `set_theme_marks_layout_and_paint_pending`: a bare inset push
        // (no rebuild) must dirty layout/paint so a shell gating on
        // `take_change_flags` relayouts a `SafeArea` when the insets move.
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        root.set_insets(WindowInsets::new(
            EdgeInsets::new(0.0, 24.0, 0.0, 0.0),
            EdgeInsets::ZERO,
        ));
        let flags = root.take_change_flags();
        assert!(flags.needs_layout());
        assert!(flags.needs_paint());
        // Drained until the next change.
        assert!(root.take_change_flags().is_empty());
    }

    #[test]
    fn set_insets_no_op_when_unchanged_marks_nothing() {
        // The `PartialEq` no-op guard: re-pushing the current insets dirties
        // nothing, so a shell that forwards the platform insets every frame
        // never forces a needless relayout.
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let insets = WindowInsets::new(EdgeInsets::new(0.0, 24.0, 0.0, 34.0), EdgeInsets::ZERO);
        root.set_insets(insets);
        assert!(!root.take_change_flags().is_empty());
        // Same value again: no dirtiness.
        root.set_insets(insets);
        assert!(root.take_change_flags().is_empty());
        // A different value dirties again.
        root.set_insets(WindowInsets::default());
        assert!(!root.take_change_flags().is_empty());
    }

    #[test]
    fn set_insets_round_trips_corner_insets() {
        use crate::insets::{CornerInset, CornerInsets};
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let corners = CornerInsets::new(
            CornerInset::ZERO,
            CornerInset::new(72.0, 24.0),
            CornerInset::ZERO,
            CornerInset::ZERO,
        );
        let insets = WindowInsets::default().with_corner_insets(corners);
        root.set_insets(insets);
        let flags = root.take_change_flags();
        assert!(flags.needs_layout());
        assert!(flags.needs_paint());
        assert_eq!(root.insets().corner_insets, corners);
        // Identical re-push marks nothing.
        root.set_insets(insets);
        assert!(root.take_change_flags().is_empty());
        // A push differing only in corners dirties again.
        let moved = insets.with_corner_insets(CornerInsets::new(
            CornerInset::new(72.0, 24.0),
            CornerInset::ZERO,
            CornerInset::ZERO,
            CornerInset::ZERO,
        ));
        root.set_insets(moved);
        let flags = root.take_change_flags();
        assert!(flags.needs_layout());
        assert!(flags.needs_paint());
    }

    // --- Presented-frame count: pushed value reaches the paint context, unset
    //     yields `None`, and — unlike theme/insets — the setter dirties nothing. ---

    /// A root widget recording the `presented_frames` count it observed during
    /// paint, proving the shell-pushed value threads through `PaintCtx`.
    struct PresentedWidget {
        seen_paint: std::rc::Rc<std::cell::Cell<Option<Option<u64>>>>,
    }
    impl crate::widget::Widget for PresentedWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            self.seen_paint.set(Some(ctx.presented_frames()));
        }
    }

    struct PresentedView {
        seen_paint: std::rc::Rc<std::cell::Cell<Option<Option<u64>>>>,
    }
    impl View<AppState> for PresentedView {
        type Element = PresentedWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> PresentedWidget {
            PresentedWidget {
                seen_paint: self.seen_paint.clone(),
            }
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut PresentedWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn drive_presented_root(presented: Option<u64>) -> Option<u64> {
        let seen_paint = std::rc::Rc::new(std::cell::Cell::new(None));
        let mut root: RenderRoot<AppState, PresentedView> = RenderRoot::new();
        if let Some(presented) = presented {
            root.set_presented_frames(presented);
        }
        let mut state = AppState::default();
        let sp = seen_paint.clone();
        root.rebuild(
            &mut move |_s: &mut AppState| PresentedView {
                seen_paint: sp.clone(),
            },
            &mut state,
        );
        root.layout(Size::new(100.0, 100.0));
        let mut scene = RecordingScene::default();
        root.paint(&mut scene, FrameTime::ZERO);
        // Unwrap the "did paint run" outer Option; the inner is what the widget saw.
        seen_paint.get().expect("paint ran")
    }

    #[test]
    fn set_presented_frames_threads_into_paint() {
        assert_eq!(drive_presented_root(Some(12)), Some(12));
    }

    #[test]
    fn unset_presented_frames_yields_none_in_paint() {
        assert_eq!(drive_presented_root(None), None);
    }

    #[test]
    fn set_presented_frames_marks_no_change_flags() {
        // Unlike `set_theme`/`set_insets`, a presented-count push is a paint-only
        // observation — it must dirty NOTHING, so a monotonically ticking counter
        // never forces a relayout or (on mobile) keeps the frame gate perpetually
        // "Run" (the menu-idle behavior depends on this).
        let mut root: RenderRoot<AppState, MockTextView> = RenderRoot::new();
        let gen_before = root.semantics_generation();
        root.set_presented_frames(1);
        assert!(root.take_change_flags().is_empty());
        assert!(!root.has_pending_change_flags());
        // A second, changed push still dirties nothing.
        root.set_presented_frames(2);
        assert!(root.take_change_flags().is_empty());
        // And bumps no semantics generation (mirrors the no-dirty contract).
        assert_eq!(root.semantics_generation(), gen_before);
    }

    // Small test helper: does the boxed widget downcast to `W`?
    trait DowncastRefIs {
        fn downcast_ref_is<W: crate::widget::Widget>(&self) -> bool;
    }
    impl DowncastRefIs for dyn crate::widget::Widget {
        fn downcast_ref_is<W: crate::widget::Widget>(&self) -> bool {
            (self as &dyn std::any::Any).is::<W>()
        }
    }

    // --- Focus / IME surface fixtures: a root editable that focuses + publishes
    //     an IME surface on a `Down` in its left half, and blurs (no focus) on a
    //     `Down` in its right half. ---

    use std::cell::RefCell;
    use std::rc::Rc;

    use crate::event::{EditingState, ImeState};

    struct ImeWidget;
    impl crate::widget::Widget for ImeWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(100.0, 100.0))
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut crate::event::EventCtx, event: &InputEvent) -> EventResult {
            if let InputEvent::Pointer(p) = event {
                if p.phase == PointerPhase::Down && p.position.x < 50.0 {
                    ctx.request_focus();
                    ctx.publish_ime_state(ImeState {
                        active: true,
                        editing: EditingState {
                            text: "abc".to_string(),
                            selection_base: 3,
                            selection_extent: 3,
                            composing_base: -1,
                            composing_extent: -1,
                        },
                        caret: Some(kurbo::Rect::new(0.0, 0.0, 1.0, 12.0)),
                        content_type: Default::default(),
                        suppress_soft_keyboard: false,
                    });
                    return EventResult::Handled;
                }
                if p.phase == PointerPhase::Down {
                    // Right-half tap: a blur (no focus request).
                    return EventResult::Handled;
                }
            }
            EventResult::Ignored
        }
    }

    struct ImeView;
    impl View<ClickState> for ImeView {
        type Element = ImeWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> ImeWidget {
            ImeWidget
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut ImeWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn ime_logic(_state: &mut ClickState) -> ImeView {
        ImeView
    }

    #[test]
    fn focus_and_ime_state_surface_and_clear_on_blur() {
        let mut root: RenderRoot<ClickState, ImeView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut ime_logic, &mut state);
        root.layout(Size::new(100.0, 100.0));

        // No focus / no IME surface initially.
        assert!(!root.is_focus_active());
        assert!(root.ime_state().is_none());

        // A left-half Down focuses the widget and publishes an IME surface.
        root.event(&mut state, &pointer(PointerPhase::Down, 10.0, 10.0));
        assert!(root.is_focus_active());
        let ime = root
            .ime_state()
            .expect("focused widget published an IME surface");
        assert!(ime.active);
        assert_eq!(ime.editing.text, "abc");

        // The published surface survives a rebuild (shell can query it between
        // frames).
        root.rebuild(&mut ime_logic, &mut state);
        assert!(root.ime_state().is_some());

        // A right-half Down is a blur: focus and the IME surface both clear.
        root.event(&mut state, &pointer(PointerPhase::Down, 80.0, 10.0));
        assert!(!root.is_focus_active());
        assert!(root.ime_state().is_none());
    }

    // --- Session release: the focus session must die with its owner -----------
    //
    // Two routes end a session without any user input reaching the root:
    //
    //  (a) a widget publishes an INACTIVE IME surface (the navigator's post-pop
    //      `cleared_ime_state`, `PatternSwitcher`'s equivalent, a `TextInput`
    //      turned disabled under a live focus), and
    //  (b) a reconciler tears the focused pod out of the tree (any generic
    //      unmount — `frust-widgets`' `cancel_active_children`/`teardown_child`),
    //      which raises `mark_focus_orphaned` because it has no `RenderRoot` to
    //      reach from a `BuildCtx` pass.
    //
    // Both must perform the SAME full release a blur does. Leaving either half
    // standing — `focus_active` true, or `ime_state` parked at `Some(inactive)` —
    // is what stranded a popped screen: `is_focus_active()` kept lying, the
    // shell's IME poll kept seeing a surface, and the next real focus
    // interaction started from a corrupt baseline.

    /// The navigator's cleared surface, spelled out here so the fixture below
    /// publishes exactly the shape `nav::navigator::cleared_ime_state` does
    /// (`frust-core` cannot name it — `frust-widgets` sits above this crate).
    fn cleared_surface() -> ImeState {
        ImeState {
            active: false,
            editing: EditingState {
                text: String::new(),
                selection_base: -1,
                selection_extent: -1,
                composing_base: -1,
                composing_extent: -1,
            },
            caret: None,
            content_type: Default::default(),
            suppress_soft_keyboard: false,
        }
    }

    /// A focused editable that publishes its active surface from paint (like a
    /// real field), and — once `clear` is raised — publishes the *inactive*
    /// surface from paint instead: the container-after-a-pop shape.
    struct PopImeWidget {
        clear: Rc<Cell<bool>>,
    }
    impl crate::widget::Widget for PopImeWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(100.0, 100.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            if self.clear.get() {
                ctx.publish_ime_state(cleared_surface());
            } else if ctx.has_focus() {
                ctx.publish_ime_state(PaintImeWidget::surface("abc"));
            }
        }
        fn event(&mut self, ctx: &mut crate::event::EventCtx, event: &InputEvent) -> EventResult {
            match event {
                InputEvent::Pointer(p) if p.phase == PointerPhase::Down => {
                    ctx.request_focus();
                    ctx.publish_ime_state(PaintImeWidget::surface("abc"));
                    EventResult::Handled
                }
                _ => EventResult::Ignored,
            }
        }
    }

    struct PopImeView {
        clear: Rc<Cell<bool>>,
    }
    impl View<ClickState> for PopImeView {
        type Element = PopImeWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> PopImeWidget {
            PopImeWidget {
                clear: self.clear.clone(),
            }
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut PopImeWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    #[test]
    fn inactive_paint_publish_releases_the_whole_session() {
        // (a) The pop shape. Before this, the paint take stored `Some(inactive)`
        // and never touched `focus_active`, so the session outlived the page.
        let clear = Rc::new(Cell::new(false));
        let mut build = {
            let clear = clear.clone();
            move |_state: &mut ClickState| PopImeView {
                clear: clear.clone(),
            }
        };
        let mut root: RenderRoot<ClickState, PopImeView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut build, &mut state);
        root.layout(Size::new(100.0, 100.0));

        let mut scene = RecordingScene::default();
        root.event(&mut state, &pointer(PointerPhase::Down, 10.0, 10.0));
        root.paint(&mut scene, FrameTime::ZERO);
        assert!(root.is_focus_active());
        assert!(root.ime_state().is_some_and(|s| s.active));
        let focused = root.focus_ime_generation();

        // The "pop": the next paint publishes the cleared surface.
        clear.set(true);
        root.paint(&mut scene, FrameTime::ZERO);
        assert!(
            !root.is_focus_active(),
            "an inactive publish ends the session, not just the surface"
        );
        assert_eq!(
            root.ime_state(),
            None,
            "the surface is dropped, never parked at Some(inactive)"
        );
        assert_eq!(
            root.focus_ime_generation(),
            focused.wrapping_add(1),
            "one release is exactly one edge"
        );

        // The widget keeps publishing the cleared surface every frame (a real
        // one-shot flag would not, but an idle screen must survive the worst
        // case): the paint take's `focus_active` guard makes each a no-op, so
        // the released session neither resurrects nor spins the edge.
        let released = root.focus_ime_generation();
        for _ in 0..30 {
            root.paint(&mut scene, FrameTime::ZERO);
        }
        assert!(!root.is_focus_active());
        assert!(root.ime_state().is_none());
        assert_eq!(
            root.focus_ime_generation(),
            released,
            "an inactive publish against an already-released root is inert"
        );
    }

    /// A view whose rebuild raises the generic-unmount orphan mark on demand —
    /// standing in for `frust-widgets`' reconcilers, which clear a focused
    /// `ChildPod` mid-diff and raise exactly this flag (this crate has no
    /// multi-child container of its own to diff).
    struct UnmountView {
        orphan: Rc<Cell<bool>>,
    }
    impl View<ClickState> for UnmountView {
        type Element = ImeWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> ImeWidget {
            ImeWidget
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut ImeWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            if self.orphan.get() {
                crate::event::mark_focus_orphaned();
                return ChangeFlags::LAYOUT | ChangeFlags::PAINT;
            }
            ChangeFlags::NONE
        }
    }

    #[test]
    fn generic_unmount_orphan_releases_the_whole_session() {
        // (b) The child-list-diff shape: no publish, no event — the focused
        // widget simply stops existing. Nothing self-corrects this on an idle
        // screen, which is why the reconciler's mark is drained here.
        let _ = crate::event::take_focus_orphaned();
        let orphan = Rc::new(Cell::new(false));
        let mut build = {
            let orphan = orphan.clone();
            move |_state: &mut ClickState| UnmountView {
                orphan: orphan.clone(),
            }
        };
        let mut root: RenderRoot<ClickState, UnmountView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut build, &mut state);
        root.layout(Size::new(100.0, 100.0));

        root.event(&mut state, &pointer(PointerPhase::Down, 10.0, 10.0));
        assert!(root.is_focus_active());
        assert!(root.ime_state().is_some());
        let focused = root.focus_ime_generation();

        // The unmount rebuild.
        orphan.set(true);
        root.rebuild(&mut build, &mut state);
        assert!(
            !root.is_focus_active(),
            "the root's focus mirror does not outlive the widget it mirrors"
        );
        assert!(root.ime_state().is_none());
        assert_eq!(
            root.focus_ime_generation(),
            focused.wrapping_add(1),
            "one orphaned focus path is exactly one edge"
        );

        // The mark was drained, so an ordinary rebuild afterwards is inert...
        let released = root.focus_ime_generation();
        orphan.set(false);
        root.rebuild(&mut build, &mut state);
        assert_eq!(root.focus_ime_generation(), released);

        // ...and re-marking against an already-released root fires no edge
        // either (a stale `focused` flag torn down later must not spin it).
        orphan.set(true);
        root.rebuild(&mut build, &mut state);
        assert_eq!(root.focus_ime_generation(), released);
        assert!(!root.is_focus_active());
    }

    // --- The focus/IME EDGE generation ---------------------------------------
    //
    // `focus_ime_generation` is the shell-facing edge behind the mobile frame
    // gate's `FrameInputs::focus_or_ime_changed`: a shell caches the value and
    // runs a frame when it moves. Two properties make that safe, and both are
    // pinned below: EVERY real transition moves it (or a focus change strands
    // unpainted), and NO same-value write moves it (or a focused screen forces
    // a frame every vsync — the level-input behavior this replaced, measured at
    // 62–120 fps on a static focused screen).

    /// A widget that focuses on `Down`, releases focus on any `Key`, and — the
    /// point of the fixture — re-publishes an IME surface from its **paint**
    /// pass on every frame, reading the text from a shared cell so a test can
    /// make a republish genuinely change (or genuinely not).
    struct PaintImeWidget {
        published: Rc<RefCell<String>>,
    }
    impl PaintImeWidget {
        fn surface(text: &str) -> ImeState {
            ImeState {
                active: true,
                editing: EditingState {
                    text: text.to_string(),
                    selection_base: 0,
                    selection_extent: 0,
                    composing_base: -1,
                    composing_extent: -1,
                },
                caret: Some(kurbo::Rect::new(0.0, 0.0, 1.0, 12.0)),
                content_type: Default::default(),
                suppress_soft_keyboard: false,
            }
        }
    }
    impl crate::widget::Widget for PaintImeWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(100.0, 100.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            ctx.publish_ime_state(Self::surface(&self.published.borrow()));
        }
        fn event(&mut self, ctx: &mut crate::event::EventCtx, event: &InputEvent) -> EventResult {
            match event {
                InputEvent::Pointer(p) if p.phase == PointerPhase::Down => {
                    ctx.request_focus();
                    EventResult::Handled
                }
                InputEvent::Key(_) => {
                    ctx.release_focus();
                    EventResult::Handled
                }
                _ => EventResult::Ignored,
            }
        }
    }

    struct PaintImeView {
        published: Rc<RefCell<String>>,
    }
    impl View<ClickState> for PaintImeView {
        type Element = PaintImeWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> PaintImeWidget {
            PaintImeWidget {
                published: self.published.clone(),
            }
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut PaintImeWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn key_event() -> InputEvent {
        InputEvent::Key(crate::event::KeyEvent {
            key: crate::event::Key::Named(crate::event::NamedKey::Enter),
            modifiers: crate::event::Modifiers::default(),
            repeat: false,
        })
    }

    #[test]
    fn focus_ime_generation_moves_on_every_pointer_transition_only() {
        let mut root: RenderRoot<ClickState, ImeView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut ime_logic, &mut state);
        root.layout(Size::new(100.0, 100.0));

        // Idle: a rebuild/layout touches neither focus nor the IME surface.
        let idle = root.focus_ime_generation();
        root.rebuild(&mut ime_logic, &mut state);
        assert_eq!(
            root.focus_ime_generation(),
            idle,
            "a rebuild is not an edge"
        );

        // Focus gained + IME surface published (one transition for a shell,
        // however many field writes it took).
        root.event(&mut state, &pointer(PointerPhase::Down, 10.0, 10.0));
        let focused = root.focus_ime_generation();
        assert_ne!(
            focused, idle,
            "focus + IME publish must move the generation"
        );

        // The SAME tap again, on the already-focused widget publishing the
        // identical surface: no state moved, so no edge. This is the case that
        // decides whether a live text field forces a frame per vsync.
        root.event(&mut state, &pointer(PointerPhase::Down, 10.0, 10.0));
        assert_eq!(
            root.focus_ime_generation(),
            focused,
            "a same-value focus/IME write must not spin the edge"
        );

        // Blur: focus cleared and the surface dropped — a real transition.
        root.event(&mut state, &pointer(PointerPhase::Down, 80.0, 10.0));
        let blurred = root.focus_ime_generation();
        assert_ne!(blurred, focused, "a blur must move the generation");

        // Blur while already blurred (a tap on inert chrome — the commonest
        // event there is) writes `false`/`None` back over `false`/`None`.
        root.event(&mut state, &pointer(PointerPhase::Down, 80.0, 20.0));
        assert_eq!(
            root.focus_ime_generation(),
            blurred,
            "blurring an already-blurred root must not move the generation"
        );
    }

    #[test]
    fn focus_ime_generation_ignores_an_unchanged_paint_republish() {
        // The paint pass re-publishes the focused widget's IME surface on EVERY
        // frame (that is how a rebuild-applied controlled change refreshes the
        // shell-facing state). If that unconditional write moved the
        // generation, the frame gate's edge would fire every single frame for
        // the whole life of a focus session — exactly the per-vsync forcing the
        // edge exists to remove.
        let published = Rc::new(RefCell::new("abc".to_string()));
        let mut build = {
            let published = published.clone();
            move |_state: &mut ClickState| PaintImeView {
                published: published.clone(),
            }
        };
        let mut root: RenderRoot<ClickState, PaintImeView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut build, &mut state);
        root.layout(Size::new(100.0, 100.0));

        // Focus the field, then let it paint: the first paint publishes.
        root.event(&mut state, &pointer(PointerPhase::Down, 10.0, 10.0));
        let mut scene = RecordingScene::default();
        root.paint(&mut scene, FrameTime::ZERO);
        let steady = root.focus_ime_generation();
        assert_eq!(
            root.ime_state(),
            Some(PaintImeWidget::surface("abc")),
            "the paint pass published the focused widget's surface"
        );

        // 120 further frames of the same focused, unchanged field: the caret
        // blinks, nothing else moves. Not one edge.
        for _ in 0..120 {
            root.paint(&mut scene, FrameTime::ZERO);
        }
        assert_eq!(
            root.focus_ime_generation(),
            steady,
            "an unchanged paint republish must never move the generation"
        );

        // A real change (the app applied a controlled edit) publishes a
        // different surface: exactly one edge, then quiet again.
        *published.borrow_mut() = "abcd".to_string();
        root.paint(&mut scene, FrameTime::ZERO);
        let edited = root.focus_ime_generation();
        assert_ne!(edited, steady, "a changed republish IS an edge");
        for _ in 0..10 {
            root.paint(&mut scene, FrameTime::ZERO);
        }
        assert_eq!(
            root.focus_ime_generation(),
            edited,
            "the session goes quiet again at the new value"
        );
    }

    #[test]
    fn focus_ime_generation_moves_on_a_focus_release_only_once() {
        // The Key/Ime/Scroll arm of the root focus path: a dispatch that
        // RELEASES focus clears both the flag and the published surface.
        let published = Rc::new(RefCell::new("abc".to_string()));
        let mut build = {
            let published = published.clone();
            move |_state: &mut ClickState| PaintImeView {
                published: published.clone(),
            }
        };
        let mut root: RenderRoot<ClickState, PaintImeView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut build, &mut state);
        root.layout(Size::new(100.0, 100.0));
        root.event(&mut state, &pointer(PointerPhase::Down, 10.0, 10.0));
        let mut scene = RecordingScene::default();
        root.paint(&mut scene, FrameTime::ZERO);
        let focused = root.focus_ime_generation();
        assert!(root.is_focus_active());

        // A key that releases focus: one edge.
        root.event(&mut state, &key_event());
        let released = root.focus_ime_generation();
        assert!(!root.is_focus_active());
        assert!(root.ime_state().is_none());
        assert_ne!(
            released, focused,
            "a focus release must move the generation"
        );

        // A second release against an already-released root: no edge. (The
        // paint pass republishes nothing now — the paint-take arm only accepts
        // a publish while focus is active.)
        root.event(&mut state, &key_event());
        root.paint(&mut scene, FrameTime::ZERO);
        assert_eq!(
            root.focus_ime_generation(),
            released,
            "releasing an already-released focus must not move the generation"
        );
    }

    // --- The focus session's IDENTITY, beside the edge generation ------------
    //
    // `focus_epoch` answers a question `focus_ime_generation` cannot: "is this
    // still the session that asked?". A caller that binds a slow, asynchronous
    // answer to the field that asked for it needs an identity, and a counter
    // over the published surface's *value* is not one — two fields publish
    // equal surfaces, and a field is free to take focus and publish nothing at
    // all.
    //
    // Each test below asserts what BOTH counters did at the same moment. The
    // `focus_ime_generation` assertions are the point rather than decoration:
    // they are what states, in a form the compiler checks, that the edge
    // generation stands still exactly where the identity moves.

    /// The text both fields publish from a press, so neither can be told from
    /// the other by the published value alone.
    const SHARED_FIELD_TEXT: &str = "shared";

    /// One field of the two-field fixture.
    ///
    /// Takes the focus session on any press inside itself; publishes an IME
    /// surface on that press only when built to; and treats an `Ime` event as an
    /// edit — the text changes and the surface is republished, but nothing
    /// re-claims a session the field already holds.
    ///
    /// `publishes: false` is not a contrivance: the baseline text input claims
    /// focus and republishes nothing for a press that lands inside text it
    /// already had selected (such a press moves no caret and collapses no
    /// selection), and any app-authored focusable that publishes no IME surface
    /// of its own behaves the same way.
    ///
    /// A press publishes the *shared* surface, so the two fields are
    /// indistinguishable to anything reading the published value — that is the
    /// case under test. An edit publishes the field's own `name` instead, which
    /// is how a test proves which field a focus-routed event actually reached.
    struct SessionField {
        name: &'static str,
        publishes: bool,
    }

    impl SessionField {
        /// The surface a field publishes. Deliberately carries nothing that
        /// tells one field from another: `ImeState` is
        /// `{active, editing, caret, content_type}` and names no widget, so two
        /// fields holding the same text and caret publish equal values — which
        /// is the ordinary shape of two empty fields, or two overlapping ones
        /// mid-transition.
        fn surface(text: &str) -> ImeState {
            ImeState {
                active: true,
                editing: EditingState {
                    text: text.to_string(),
                    selection_base: 0,
                    selection_extent: 0,
                    composing_base: -1,
                    composing_extent: -1,
                },
                caret: Some(kurbo::Rect::new(0.0, 0.0, 1.0, 12.0)),
                content_type: Default::default(),
                suppress_soft_keyboard: false,
            }
        }
    }

    impl crate::widget::Widget for SessionField {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(100.0, 20.0))
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            match event {
                InputEvent::Pointer(p) if p.phase == PointerPhase::Down => {
                    ctx.request_focus();
                    if self.publishes {
                        ctx.publish_ime_state(Self::surface(SHARED_FIELD_TEXT));
                    }
                    EventResult::Handled
                }
                InputEvent::Ime(_) => {
                    ctx.publish_ime_state(Self::surface(self.name));
                    EventResult::Handled
                }
                _ => EventResult::Ignored,
            }
        }
    }

    /// The fixture's root: two stacked fields, with a pointer event hit-tested
    /// to the one under it and a focus-routed event forwarded down the recorded
    /// focus path without a hit test — the routing every real container does.
    struct TwoFields {
        top: crate::widget::ChildPod,
        bottom: crate::widget::ChildPod,
    }

    impl crate::widget::Widget for TwoFields {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.top.layout_child(ctx, bc);
            self.top.set_origin(Point::new(0.0, 0.0));
            self.bottom.layout_child(ctx, bc);
            self.bottom.set_origin(Point::new(0.0, 50.0));
            bc.max()
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            self.top.paint_child(ctx, scene);
            self.bottom.paint_child(ctx, scene);
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            if matches!(event, InputEvent::Pointer(_)) {
                let pos = event.position();
                if self.top.contains(pos) {
                    return self.top.event_child(ctx, event);
                }
                if self.bottom.contains(pos) {
                    return self.bottom.event_child(ctx, event);
                }
                return EventResult::Ignored;
            }
            if self.top.holds_live_focus() {
                return self.top.event_child(ctx, event);
            }
            if self.bottom.holds_live_focus() {
                return self.bottom.event_child(ctx, event);
            }
            EventResult::Ignored
        }
        fn semantics(&self, ctx: &mut SemanticsCtx) {
            self.top.semantics_child(ctx);
            self.bottom.semantics_child(ctx);
        }
    }

    struct TwoFieldsView {
        bottom_publishes: bool,
    }

    impl View<ClickState> for TwoFieldsView {
        type Element = TwoFields;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> TwoFields {
            TwoFields {
                top: crate::widget::ChildPod::new(Box::new(SessionField {
                    name: "top",
                    publishes: true,
                })),
                bottom: crate::widget::ChildPod::new(Box::new(SessionField {
                    name: "bottom",
                    publishes: self.bottom_publishes,
                })),
            }
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut TwoFields,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    /// An edit pushed down the focus path by the platform IME: it claims no
    /// focus, so only the field already holding the session sees it — which is
    /// what makes the surface it republishes name that field.
    fn edit_event() -> InputEvent {
        InputEvent::Ime(crate::event::ImeEvent::ApplyEditingState(
            SessionField::surface(SHARED_FIELD_TEXT).editing,
        ))
    }

    /// Mount the two-field fixture and press the top field, returning the root
    /// with a live session on it.
    fn two_fields_focused(
        bottom_publishes: bool,
    ) -> (RenderRoot<ClickState, TwoFieldsView>, ClickState) {
        let mut root: RenderRoot<ClickState, TwoFieldsView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut |_| TwoFieldsView { bottom_publishes }, &mut state);
        root.layout(Size::new(200.0, 200.0));
        root.event(&mut state, &pointer(PointerPhase::Down, 10.0, 10.0));
        assert!(root.is_focus_active(), "the top field opened a session");
        (root, state)
    }

    #[test]
    fn focus_epoch_moves_when_focus_crosses_two_fields_publishing_alike() {
        let (mut root, mut state) = two_fields_focused(true);
        let first_session = root.focus_epoch();
        let steady_edge = root.focus_ime_generation();
        assert_eq!(
            root.ime_state(),
            Some(SessionField::surface(SHARED_FIELD_TEXT))
        );

        // Press the bottom field. Focus really does move — and nothing
        // observable about the published surface moves with it: claiming while
        // some field is already focused writes `true` over `true`, and the
        // surface the second field publishes compares equal to the first's.
        root.event(&mut state, &pointer(PointerPhase::Down, 10.0, 60.0));
        assert!(root.is_focus_active());
        assert_eq!(
            root.focus_ime_generation(),
            steady_edge,
            "the edge generation is blind to this move, which is why a caller \
             asking 'is this the same session?' must not be built on it"
        );
        assert_ne!(
            root.focus_epoch(),
            first_session,
            "the session identity must move when focus crosses to another field"
        );

        // Not merely "some counter moved": the focus PATH is the bottom
        // field's now, which a focus-routed event proves by reaching it.
        root.event(&mut state, &edit_event());
        assert_eq!(
            root.ime_state(),
            Some(SessionField::surface("bottom")),
            "the second field is the one holding the session"
        );
    }

    #[test]
    fn focus_epoch_moves_when_the_field_taking_focus_publishes_nothing() {
        let (mut root, mut state) = two_fields_focused(false);
        let first_session = root.focus_epoch();
        let steady_edge = root.focus_ime_generation();

        // Press the bottom field, which takes the session and publishes no
        // surface of its own. A publish-nothing dispatch leaves the standing
        // surface standing, so what the shell still sees describes the field
        // the user just left — for as long as this session lasts, not merely
        // until the next frame.
        root.event(&mut state, &pointer(PointerPhase::Down, 10.0, 60.0));
        assert!(root.is_focus_active());
        assert_eq!(
            root.ime_state(),
            Some(SessionField::surface(SHARED_FIELD_TEXT)),
            "the field that lost focus is still the one the surface describes"
        );
        assert_eq!(
            root.focus_ime_generation(),
            steady_edge,
            "no published value moved, so the edge generation cannot have"
        );
        assert_ne!(
            root.focus_epoch(),
            first_session,
            "an honoured claim moves the session identity whether or not the \
             claimant publishes anything"
        );

        // And again, the move is a real one: the focus path now ends at the
        // field that published nothing.
        root.event(&mut state, &edit_event());
        assert_eq!(
            root.ime_state(),
            Some(SessionField::surface("bottom")),
            "the second field is the one holding the session"
        );
    }

    #[test]
    fn focus_epoch_ignores_an_edit_inside_one_session() {
        // The converse direction, and the reason the two counters are kept
        // apart rather than merged: a caller holding an identity can let a
        // harmless edit ride, where a caller comparing the published value has
        // to treat every keystroke as a reason to give up.
        let (mut root, mut state) = two_fields_focused(true);
        let session = root.focus_epoch();
        let before_edit = root.focus_ime_generation();

        root.event(&mut state, &edit_event());
        assert_eq!(
            root.ime_state(),
            Some(SessionField::surface("top")),
            "the focused field applied the edit and republished"
        );
        assert_ne!(
            root.focus_ime_generation(),
            before_edit,
            "a changed surface IS an edge"
        );
        assert_eq!(
            root.focus_epoch(),
            session,
            "an edit does not end or restart the session it lands in"
        );

        // A release ends the identity too, so a stale one can never come back
        // round to matching by standing still.
        root.event(&mut state, &pointer(PointerPhase::Down, 10.0, 90.0));
        assert!(!root.is_focus_active());
        assert_ne!(
            root.focus_epoch(),
            session,
            "a release retires the session's identity"
        );
    }

    // --- Semantics: stable ids + accessibility action routing ---
    //
    // Fixtures: an accessibility-visible button (fire-on-up-inside, contributes a
    // `Role::Button` node) and a checkbox variant (`Role::CheckBox`), plus a
    // labelled leaf used to prove id stability survives a pod relocation.

    use accesskit::{Action, NodeId, Role};

    /// A fire-on-up-inside button that also contributes a semantics node — the
    /// end-to-end target for `perform_accessibility_action(Click)`.
    struct A11yButtonWidget;
    impl crate::widget::Widget for A11yButtonWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(40.0, 20.0))
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            if let InputEvent::Pointer(p) = event {
                match p.phase {
                    PointerPhase::Down => {
                        ctx.capture_pointer();
                        return EventResult::Handled;
                    }
                    PointerPhase::Up => {
                        let size = ctx.size();
                        let inside = p.position.x >= 0.0
                            && p.position.y >= 0.0
                            && p.position.x <= size.width
                            && p.position.y <= size.height;
                        if inside {
                            ctx.state_mut::<ClickState>().clicks += 1;
                            ctx.request_redraw();
                        }
                        return EventResult::Handled;
                    }
                    _ => {}
                }
            }
            EventResult::Ignored
        }
        fn semantics(&self, ctx: &mut SemanticsCtx) {
            ctx.push_node(Role::Button, |n| n.set_label("Go"));
        }
    }

    struct A11yButtonView;
    impl View<ClickState> for A11yButtonView {
        type Element = A11yButtonWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> A11yButtonWidget {
            A11yButtonWidget
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _el: &mut A11yButtonWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn a11y_button_logic(_state: &mut ClickState) -> A11yButtonView {
        A11yButtonView
    }

    fn button_node_id(update: &SemanticsUpdate, role: Role) -> NodeId {
        update
            .nodes
            .iter()
            .find(|(_, n)| n.role() == role)
            .map(|(id, _)| *id)
            .unwrap_or_else(|| panic!("a {role:?} node is present"))
    }

    #[test]
    fn semantics_ids_are_stable_across_frames() {
        let mut root: RenderRoot<ClickState, A11yButtonView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut a11y_button_logic, &mut state);
        root.layout(Size::new(200.0, 200.0));

        let first = button_node_id(&root.semantics(), Role::Button);
        // Re-run rebuild+layout+semantics several times: the button keeps its id.
        for _ in 0..3 {
            root.rebuild(&mut a11y_button_logic, &mut state);
            root.layout(Size::new(200.0, 200.0));
            assert_eq!(
                button_node_id(&root.semantics(), Role::Button),
                first,
                "the same widget must keep its NodeId across frames"
            );
        }
        // The window root is the reserved constant id.
        assert_eq!(root.semantics().root, ROOT_NODE_ID);
    }

    #[test]
    fn semantics_ids_survive_a_pod_relocation() {
        // A keyed reorder relocates the whole `ChildPod` (preserving its cached
        // semantics id); simulate that here by swapping two pods in place and
        // asserting each labelled node keeps its id despite changing position.
        struct LabeledLeaf {
            label: &'static str,
        }
        impl crate::widget::Widget for LabeledLeaf {
            fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
                bc.constrain(Size::new(10.0, 10.0))
            }
            fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
            fn semantics(&self, ctx: &mut SemanticsCtx) {
                let label = self.label;
                ctx.push_node(Role::Label, |n| n.set_label(label));
            }
        }

        let mut pods = vec![
            crate::widget::ChildPod::new(Box::new(LabeledLeaf { label: "A" })),
            crate::widget::ChildPod::new(Box::new(LabeledLeaf { label: "B" })),
        ];

        // Collect (label -> id) for a given pod order. `SemanticsCtx` is
        // crate-private, so this drives the pods directly — the same allocation
        // path `RenderRoot::semantics` uses.
        let collect = |pods: &[crate::widget::ChildPod]| {
            let mut ctx = SemanticsCtx::new(Size::new(100.0, 100.0), 2);
            for pod in pods {
                pod.semantics_child(&mut ctx);
            }
            let update = ctx.finish(ROOT_NODE_ID);
            update
                .nodes
                .iter()
                .filter(|(id, _)| *id != ROOT_NODE_ID)
                .map(|(id, n)| (n.label().unwrap().to_string(), *id))
                .collect::<Vec<_>>()
        };

        let before = collect(&pods);
        // Relocate: swap the pods (the pods themselves, with their cached ids,
        // move — mirroring the keyed reconciler's `take`-and-reorder).
        pods.swap(0, 1);
        let after = collect(&pods);

        for (label, id) in &before {
            let relocated = after.iter().find(|(l, _)| l == label).unwrap().1;
            assert_eq!(
                *id, relocated,
                "widget {label:?} must keep its NodeId across the reorder"
            );
        }
        // And the reorder actually changed positions (A now second).
        assert_eq!(after[0].0, "B");
        assert_eq!(after[1].0, "A");
    }

    #[test]
    fn semantics_full_update_assembles_window_and_child() {
        let mut root: RenderRoot<ClickState, A11yButtonView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut a11y_button_logic, &mut state);
        root.layout(Size::new(200.0, 200.0));

        let update = root.semantics();
        // Window root + the button.
        assert_eq!(update.nodes.len(), 2);
        assert_eq!(update.root, ROOT_NODE_ID);
        let root_node = update
            .nodes
            .iter()
            .find(|(id, _)| *id == update.root)
            .unwrap();
        assert_eq!(root_node.1.role(), Role::Window);
        let button = button_node_id(&update, Role::Button);
        assert_eq!(
            root_node.1.children(),
            &[button],
            "the button attaches under the window root"
        );
        // Nothing focused → the adapter-facing focus id defaults to the root.
        assert!(update.focus.is_none());
        assert_eq!(update.focus_id(), ROOT_NODE_ID);
    }

    #[test]
    fn perform_click_action_activates_a_button() {
        let mut root: RenderRoot<ClickState, A11yButtonView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut a11y_button_logic, &mut state);
        root.layout(Size::new(200.0, 200.0));

        let button = button_node_id(&root.semantics(), Role::Button);
        let outcome = root.perform_accessibility_action(&mut state, button, Action::Click);
        assert!(outcome.handled, "the synthesized Down+Up was handled");
        assert!(outcome.needs_redraw);
        assert_eq!(state.clicks, 1, "Click synthesized a real up-inside tap");

        // An unknown node id is a benign no-op.
        let outcome = root.perform_accessibility_action(&mut state, NodeId(999_999), Action::Click);
        assert_eq!(outcome, EventOutcome::default());
        assert_eq!(state.clicks, 1);

        // An unmodelled action is ignored.
        let outcome = root.perform_accessibility_action(&mut state, button, Action::ScrollDown);
        assert_eq!(outcome, EventOutcome::default());
        assert_eq!(state.clicks, 1);
    }

    #[test]
    fn perform_click_action_toggles_a_checkbox() {
        // A checkbox-shaped widget (`Role::CheckBox`) reached through the same
        // synthetic-pointer path — proving Click drives any fire-on-up-inside
        // control, not just buttons.
        struct CheckboxWidget;
        impl crate::widget::Widget for CheckboxWidget {
            fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
                bc.constrain(Size::new(24.0, 24.0))
            }
            fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
            fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
                if let InputEvent::Pointer(p) = event {
                    match p.phase {
                        PointerPhase::Down => {
                            ctx.capture_pointer();
                            return EventResult::Handled;
                        }
                        PointerPhase::Up => {
                            let size = ctx.size();
                            if p.position.x >= 0.0
                                && p.position.y >= 0.0
                                && p.position.x <= size.width
                                && p.position.y <= size.height
                            {
                                ctx.state_mut::<ClickState>().clicks += 1;
                            }
                            return EventResult::Handled;
                        }
                        _ => {}
                    }
                }
                EventResult::Ignored
            }
            fn semantics(&self, ctx: &mut SemanticsCtx) {
                ctx.push_node(Role::CheckBox, |n| n.set_label("Agree"));
            }
        }
        struct CheckboxView;
        impl View<ClickState> for CheckboxView {
            type Element = CheckboxWidget;
            fn build(&self, _ctx: &mut BuildCtx<'_>) -> CheckboxWidget {
                CheckboxWidget
            }
            fn rebuild(
                &self,
                _p: &Self,
                _e: &mut CheckboxWidget,
                _c: &mut BuildCtx<'_>,
            ) -> ChangeFlags {
                ChangeFlags::NONE
            }
        }

        let mut root: RenderRoot<ClickState, CheckboxView> = RenderRoot::new();
        let mut state = ClickState::default();
        root.rebuild(&mut |_| CheckboxView, &mut state);
        root.layout(Size::new(200.0, 200.0));

        let cb = button_node_id(&root.semantics(), Role::CheckBox);
        root.perform_accessibility_action(&mut state, cb, Action::Click);
        assert_eq!(state.clicks, 1, "Click toggled the checkbox once");
    }

    #[test]
    fn perform_focus_action_claims_focus_and_clears_capture() {
        // Two focus-claiming, fire-on-up-inside buttons in a container. A11y
        // `Focus` on B must claim focus for B *and* release the capture the
        // synthesized `Down` opened — the CRITICAL leak this regresses: without
        // the trailing `Cancel`, B stayed captured and swallowed every later
        // pointer event, so a tap on A never reached A.

        #[derive(Default)]
        struct FocusState {
            a_press: u32,
            b_press: u32,
            b_move: u32,
        }

        #[derive(Clone, Copy)]
        enum Btn {
            A,
            B,
        }

        /// A button that opts into both recorded paths (capture + focus) on
        /// `Down` and fires its press only on `Up`-inside — never on `Cancel`.
        struct FocusButton {
            id: Btn,
        }
        impl crate::widget::Widget for FocusButton {
            fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
                bc.constrain(Size::new(40.0, 20.0))
            }
            fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
            fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
                let InputEvent::Pointer(p) = event else {
                    return EventResult::Ignored;
                };
                match p.phase {
                    PointerPhase::Down => {
                        ctx.capture_pointer();
                        ctx.request_focus();
                        EventResult::Handled
                    }
                    PointerPhase::Move => {
                        if let Btn::B = self.id {
                            ctx.state_mut::<FocusState>().b_move += 1;
                        }
                        EventResult::Handled
                    }
                    PointerPhase::Up => {
                        let size = ctx.size();
                        let inside = p.position.x >= 0.0
                            && p.position.y >= 0.0
                            && p.position.x <= size.width
                            && p.position.y <= size.height;
                        if inside {
                            match self.id {
                                Btn::A => ctx.state_mut::<FocusState>().a_press += 1,
                                Btn::B => ctx.state_mut::<FocusState>().b_press += 1,
                            }
                        }
                        EventResult::Handled
                    }
                    // A `Cancel` clears without firing on_press and never touches
                    // state — the contract the Focus action's trailing Cancel rides.
                    PointerPhase::Cancel => EventResult::Handled,
                }
            }
            fn semantics(&self, ctx: &mut SemanticsCtx) {
                let label = match self.id {
                    Btn::A => "A",
                    Btn::B => "B",
                };
                ctx.push_node(Role::Button, |n| n.set_label(label));
            }
        }

        /// A minimal two-child container mirroring `frust-widgets`'
        /// `route_event`: a captured gesture goes straight to the active child
        /// (auto-released on `Up`/`Cancel`), otherwise the event is hit-tested to
        /// the child under it.
        struct TwoButtons {
            a: crate::widget::ChildPod,
            b: crate::widget::ChildPod,
        }
        impl crate::widget::Widget for TwoButtons {
            fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
                self.a.layout_child(ctx, bc);
                self.a.set_origin(Point::new(0.0, 0.0));
                self.b.layout_child(ctx, bc);
                self.b.set_origin(Point::new(0.0, 30.0));
                bc.max()
            }
            fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
                self.a.paint_child(ctx, scene);
                self.b.paint_child(ctx, scene);
            }
            fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
                let releases = matches!(
                    event,
                    InputEvent::Pointer(p)
                        if matches!(p.phase, PointerPhase::Up | PointerPhase::Cancel)
                );
                // Capture fast-path: a recorded active child receives every event
                // until it releases on Up/Cancel, bypassing the hit test entirely.
                if self.a.is_active() {
                    let r = self.a.event_child(ctx, event);
                    if releases {
                        self.a.set_active(false);
                    }
                    return r;
                }
                if self.b.is_active() {
                    let r = self.b.event_child(ctx, event);
                    if releases {
                        self.b.set_active(false);
                    }
                    return r;
                }
                // Fresh event: route to the child under the point.
                let pos = event.position();
                if self.a.contains(pos) {
                    return self.a.event_child(ctx, event);
                }
                if self.b.contains(pos) {
                    return self.b.event_child(ctx, event);
                }
                EventResult::Ignored
            }
            fn semantics(&self, ctx: &mut SemanticsCtx) {
                self.a.semantics_child(ctx);
                self.b.semantics_child(ctx);
            }
        }

        struct TwoButtonsView;
        impl View<FocusState> for TwoButtonsView {
            type Element = TwoButtons;
            fn build(&self, _ctx: &mut BuildCtx<'_>) -> TwoButtons {
                TwoButtons {
                    a: crate::widget::ChildPod::new(Box::new(FocusButton { id: Btn::A })),
                    b: crate::widget::ChildPod::new(Box::new(FocusButton { id: Btn::B })),
                }
            }
            fn rebuild(
                &self,
                _p: &Self,
                _e: &mut TwoButtons,
                _c: &mut BuildCtx<'_>,
            ) -> ChangeFlags {
                ChangeFlags::NONE
            }
        }

        let mut root: RenderRoot<FocusState, TwoButtonsView> = RenderRoot::new();
        let mut state = FocusState::default();
        root.rebuild(&mut |_| TwoButtonsView, &mut state);
        root.layout(Size::new(200.0, 200.0));

        // B's semantics node (label "B") is the a11y Focus target.
        let b_id = root
            .semantics()
            .nodes
            .iter()
            .find(|(_, n)| n.label().is_some_and(|l| l == "B"))
            .map(|(id, _)| *id)
            .expect("button B contributes a semantics node");

        // A11y `Focus` on B: claims the focus session, fires no on_press, and —
        // crucially — leaves nothing captured (the Down+Cancel shape).
        root.perform_accessibility_action(&mut state, b_id, Action::Focus);
        assert!(root.is_focus_active(), "Focus opened the focus session");
        assert!(
            !root.is_pointer_captured(),
            "the trailing Cancel released the capture the Focus Down opened"
        );
        assert_eq!(
            state.b_press, 0,
            "Focus (Down+Cancel) must not fire B's on_press"
        );

        // B is not stuck-captured: a `Move` outside both buttons is ignored. Were
        // B still captured, the capture fast-path would route this to B regardless
        // of position (b_move would tick).
        let outside = InputEvent::Pointer(PointerEvent {
            phase: PointerPhase::Move,
            position: Point::new(100.0, 100.0),
            button: PointerButton::Primary,
        });
        root.event(&mut state, &outside);
        assert_eq!(state.b_move, 0, "no leaked capture: B saw no stray Move");

        // A real Down+Up on A activates A exactly once and never reaches B.
        let at_a = |phase| {
            InputEvent::Pointer(PointerEvent {
                phase,
                position: Point::new(20.0, 10.0),
                button: PointerButton::Primary,
            })
        };
        root.event(&mut state, &at_a(PointerPhase::Down));
        root.event(&mut state, &at_a(PointerPhase::Up));
        assert_eq!(state.a_press, 1, "A fired once from its own tap");
        assert_eq!(
            state.b_press, 0,
            "B never fired — its capture never leaked onto A's tap"
        );
    }

    #[test]
    fn semantics_if_changed_gates_on_generation() {
        let mut root: RenderRoot<ClickState, A11yButtonView> = RenderRoot::new();
        let mut state = ClickState::default();
        // First build bumps the generation from 0.
        root.rebuild(&mut a11y_button_logic, &mut state);
        root.layout(Size::new(200.0, 200.0));

        let generation = root.semantics_generation();
        assert!(generation > 0);
        // A shell that already pushed `generation` sees no change.
        assert!(root.semantics_if_changed(generation).is_none());
        // A stale generation triggers a fresh pull.
        assert!(root.semantics_if_changed(generation - 1).is_some());

        // A theme swap marks the tree semantics-dirty.
        root.set_theme(Box::new(0u32));
        assert!(root.semantics_generation() > generation);
        assert!(root.semantics_if_changed(generation).is_some());
    }

    #[test]
    fn orientation_from_size_is_portrait_when_taller_than_wide() {
        assert_eq!(
            Orientation::from_size(Size::new(400.0, 800.0)),
            Orientation::Portrait
        );
    }

    #[test]
    fn orientation_from_size_is_landscape_when_wider_than_tall() {
        assert_eq!(
            Orientation::from_size(Size::new(800.0, 400.0)),
            Orientation::Landscape
        );
    }

    #[test]
    fn orientation_from_size_square_reads_as_portrait() {
        // Height >= width is the derivation rule (see `Orientation::from_size`'s
        // doc); an exact square satisfies `>=` and must not panic/ambiguously
        // resolve, so this is pinned explicitly rather than left implicit.
        assert_eq!(
            Orientation::from_size(Size::new(500.0, 500.0)),
            Orientation::Portrait
        );
    }

    #[test]
    fn window_metrics_new_derives_orientation_from_size() {
        let insets = WindowInsets::default();
        let portrait = WindowMetrics::new(Size::new(390.0, 844.0), 3.0, insets);
        assert_eq!(portrait.orientation, Orientation::Portrait);
        assert_eq!(portrait.size, Size::new(390.0, 844.0));
        assert_eq!(portrait.scale, 3.0);
        assert_eq!(portrait.insets, insets);

        let landscape = WindowMetrics::new(Size::new(844.0, 390.0), 3.0, insets);
        assert_eq!(landscape.orientation, Orientation::Landscape);

        let square = WindowMetrics::new(Size::new(500.0, 500.0), 2.0, insets);
        assert_eq!(square.orientation, Orientation::Portrait);
    }

    // --- Deferred state-bearing callbacks: the `InputEvent::Housekeeping` flush
    //     `RenderRoot::rebuild` dispatches. ---

    /// App state for the flush tests.
    #[derive(Default)]
    struct FlushState {
        /// How many deferred callbacks have run.
        flushes: u32,
        /// How many more times a running callback re-queues itself — the knob the
        /// chained/capped tests turn.
        chain_left: u32,
        /// The `flushes` value each build-closure run observed, in order. This is
        /// what proves the rebuild re-runs the build closure *after* a flush rather than
        /// shipping the now-stale pre-flush view.
        observed: Vec<u32>,
    }

    /// The navigator's deferred-callback shape reduced to one leaf: a shared
    /// `Rc<Cell<u32>>` op queue (the `NavigatorController` analog) is drained
    /// during the state-free [`View::rebuild`], which can therefore only *queue*
    /// the callback and raise the flush mark; [`crate::widget::Widget::event`]
    /// runs it when the broadcast arrives, where `&mut State` finally exists.
    struct FlushView {
        ops: std::rc::Rc<Cell<u32>>,
    }

    struct FlushWidget {
        ops: std::rc::Rc<Cell<u32>>,
        queued: u32,
    }

    impl FlushWidget {
        fn drain_ops(&mut self) {
            let ops = self.ops.replace(0);
            if ops > 0 {
                self.queued += ops;
                crate::event::mark_pending_result_flush();
            }
        }
    }

    impl View<FlushState> for FlushView {
        type Element = FlushWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> FlushWidget {
            let mut widget = FlushWidget {
                ops: self.ops.clone(),
                queued: 0,
            };
            widget.drain_ops();
            widget
        }
        fn rebuild(
            &self,
            _prev: &Self,
            element: &mut FlushWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            element.ops = self.ops.clone();
            element.drain_ops();
            ChangeFlags::NONE
        }
    }

    impl crate::widget::Widget for FlushWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            if !event.is_broadcast() || self.queued == 0 {
                return EventResult::Ignored;
            }
            let queued = std::mem::take(&mut self.queued);
            let state = ctx.state_mut::<FlushState>();
            for _ in 0..queued {
                state.flushes += 1;
                if state.chain_left > 0 {
                    state.chain_left -= 1;
                    self.ops.set(self.ops.get() + 1);
                }
            }
            EventResult::Ignored
        }
    }

    fn flush_logic(ops: std::rc::Rc<Cell<u32>>) -> impl FnMut(&mut FlushState) -> FlushView {
        move |state: &mut FlushState| {
            state.observed.push(state.flushes);
            FlushView { ops: ops.clone() }
        }
    }

    #[test]
    fn a_queued_callback_flushes_and_re_diffs_inside_one_rebuild() {
        let ops = std::rc::Rc::new(Cell::new(0u32));
        let mut root: RenderRoot<FlushState, FlushView> = RenderRoot::new();
        let mut app = flush_logic(ops.clone());
        let mut state = FlushState::default();

        root.rebuild(&mut app, &mut state);
        assert_eq!(state.flushes, 0);
        assert_eq!(
            state.observed,
            vec![0],
            "nothing queued ⇒ exactly one build run, no broadcast"
        );

        // Queue one op — the `NavigatorController::pop_with_result` analog.
        state.observed.clear();
        ops.set(1);
        root.rebuild(&mut app, &mut state);
        assert_eq!(
            state.flushes, 1,
            "the queued callback ran inside this rebuild — no event was dispatched \
             by anyone but the rebuild itself"
        );
        assert_eq!(
            state.observed,
            vec![0, 1],
            "the build closure re-ran after the flush and saw the post-callback state, so \
             the view this frame ships is not the stale pre-flush one"
        );
        assert!(
            !crate::event::take_pending_result_flush(),
            "the mark was consumed; nothing is owed to a later frame"
        );
    }

    #[test]
    fn a_runaway_callback_chain_is_capped_and_deferred_to_the_next_frame() {
        let ops = std::rc::Rc::new(Cell::new(0u32));
        let mut root: RenderRoot<FlushState, FlushView> = RenderRoot::new();
        let mut app = flush_logic(ops.clone());
        // Far more chaining than the cap allows: unbounded, this rebuild would
        // never return. Reaching the assertions below at all is the no-spin proof.
        let mut state = FlushState {
            chain_left: 100,
            ..Default::default()
        };
        root.rebuild(&mut app, &mut state);

        ops.set(1);
        root.rebuild(&mut app, &mut state);
        assert_eq!(
            state.flushes, MAX_PENDING_RESULT_FLUSH_PASSES as u32,
            "exactly the cap's worth of flush passes, then stop"
        );

        // The remainder is owed, not lost: the mark still stands and the next
        // paint asks for the follow-up frame that will finish it.
        root.layout(Size::new(50.0, 50.0));
        let mut scene = RecordingScene::default();
        let outcome = root.paint(&mut scene, FrameTime::ZERO);
        assert!(
            outcome.needs_frame,
            "hitting the cap requests one more frame, so a dirty-driven shell \
             wakes instead of waiting for input"
        );
        assert!(
            !outcome.needs_frame_paced_only,
            "a deferred flush is not a cosmetic loop — the mobile frame gate must \
             not throttle it"
        );

        // That next frame picks up exactly where the capped one left off.
        root.rebuild(&mut app, &mut state);
        assert_eq!(
            state.flushes,
            2 * MAX_PENDING_RESULT_FLUSH_PASSES as u32,
            "the deferred remainder resumed on the following frame"
        );

        // Leave this thread's flag clean for anything else in the binary.
        let _ = crate::event::take_pending_result_flush();
    }

    /// The redraw-only flush shape: a widget that queues a callback exactly like
    /// [`FlushView`] above, but whose broadcast handler touches **no** state at
    /// all — it only calls [`EventCtx::request_redraw`]. `frust-widgets`' gesture
    /// long-press latch is the shipped instance (its `on_long_press` consumer may
    /// mutate nothing the view diff can see), and the widget's own
    /// `ctx.request_redraw()` after firing is then the whole wake signal.
    struct RedrawOnlyView {
        ops: std::rc::Rc<Cell<u32>>,
    }

    struct RedrawOnlyWidget {
        ops: std::rc::Rc<Cell<u32>>,
        queued: bool,
    }

    impl RedrawOnlyWidget {
        fn drain_ops(&mut self) {
            if self.ops.replace(0) > 0 {
                self.queued = true;
                crate::event::mark_pending_result_flush();
            }
        }
    }

    impl View<()> for RedrawOnlyView {
        type Element = RedrawOnlyWidget;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> RedrawOnlyWidget {
            let mut widget = RedrawOnlyWidget {
                ops: self.ops.clone(),
                queued: false,
            };
            widget.drain_ops();
            widget
        }
        fn rebuild(
            &self,
            _prev: &Self,
            element: &mut RedrawOnlyWidget,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            element.ops = self.ops.clone();
            element.drain_ops();
            // The whole point: the re-diff after the flush reports nothing, so
            // the dispatch's own outcome is the only wake signal there is.
            ChangeFlags::NONE
        }
    }

    impl crate::widget::Widget for RedrawOnlyWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(10.0, 10.0))
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            if event.is_broadcast() && std::mem::take(&mut self.queued) {
                // No `state_mut`, no signal, no view-visible change — a repaint
                // request and nothing else.
                ctx.request_redraw();
            }
            EventResult::Ignored
        }
    }

    #[test]
    fn a_redraw_only_flushed_callback_wakes_both_loop_styles() {
        let ops = std::rc::Rc::new(Cell::new(0u32));
        let mut root: RenderRoot<(), RedrawOnlyView> = RenderRoot::new();
        let mut app = |_state: &mut ()| RedrawOnlyView { ops: ops.clone() };
        let mut state = ();

        // Settle the first build so the assertions below observe only the flush.
        root.rebuild(&mut app, &mut state);
        root.layout(Size::new(50.0, 50.0));
        let mut scene = RecordingScene::default();
        let settled = root.paint(&mut scene, FrameTime::ZERO);
        assert!(!settled.needs_frame, "nothing queued ⇒ the tree is at rest");
        let _ = root.take_change_flags();

        // Queue the redraw-only callback (the gesture long-press latch analog: a
        // prior pass marks, this rebuild flushes).
        ops.set(1);
        let flags = root.rebuild(&mut app, &mut state);
        assert!(
            flags.needs_paint(),
            "the broadcast's `needs_redraw` folds into the rebuild's flags even \
             though the re-diff saw no view change"
        );
        assert!(
            root.has_pending_change_flags(),
            "PAINT reached `pending`, which is the input the mobile frame gate \
             reads to decide the next tick runs at all"
        );

        let outcome = root.paint(&mut scene, FrameTime::ZERO);
        assert!(
            outcome.needs_frame,
            "the same wake surfaces as `needs_frame`, which is how the desktop \
             `ControlFlow::Wait` loop schedules a frame with no input pending"
        );
        assert!(
            !outcome.needs_frame_paced_only,
            "a flushed callback's repaint is not a cosmetic loop — the mobile \
             frame gate must not throttle it"
        );
        assert!(
            !crate::event::take_pending_result_flush(),
            "the mark was consumed; nothing is owed to a later frame"
        );

        // And it settles: the next frame asks for nothing, so neither loop spins.
        let _ = root.take_change_flags();
        root.rebuild(&mut app, &mut state);
        let settled = root.paint(&mut scene, FrameTime::ZERO);
        assert!(
            !settled.needs_frame,
            "one wake, not a perpetual one — the flush is over"
        );
        assert!(!root.has_pending_change_flags());
    }

    // --- Hover: the claim pipeline -------------------------------------------
    //
    // Hover has no Enter/Leave phase to lean on (adding one to `PointerPhase`
    // would break every out-of-tree exhaustive match). It is instead an opt-in
    // claim a widget makes from its uncaptured `Move` arm, recorded as an epoch
    // stamp down the pod chain — so the fixture below is deliberately shaped like
    // a real container: two hit-tested children, a capture fast-path, and paint
    // recording what `PaintCtx::is_hovered` reported.

    /// Which of the fixture's two leaves an assertion is about.
    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
    enum Leaf {
        Top,
        Bottom,
    }

    /// What one hover leaf observed, shared out of the widget tree.
    #[derive(Default)]
    struct HoverProbe {
        /// `PaintCtx::is_hovered()` as of the last paint.
        painted_hovered: Cell<bool>,
        /// `EventCtx::is_hovered()` as of the last event dispatch that reached it.
        event_hovered: Cell<bool>,
    }

    /// A leaf that claims hover on any `Move` landing inside its own bounds — the
    /// canonical opt-in shape — and optionally captures the pointer on `Down` (the
    /// drag fixture: a captured pointer must never create hover).
    ///
    /// With `latches` set it follows the whole consumer contract: the same hit test
    /// updates an internal flag, `request_redraw` is gated on that flag changing,
    /// and `paint` self-corrects the flag from the authoritative
    /// `PaintCtx::is_hovered`. Clearing `latches` is a deliberate negative control —
    /// a claimant that keeps no flag — used to pin which frames the pipeline itself
    /// does and does not manufacture.
    struct HoverLeaf {
        captures: bool,
        latches: bool,
        hovered: bool,
        probe: Rc<HoverProbe>,
    }

    impl crate::widget::Widget for HoverLeaf {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(100.0, 30.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {
            self.probe.painted_hovered.set(ctx.is_hovered());
            if self.latches {
                // The self-correction half of the contract: authoritative here,
                // whatever the event arm last recorded.
                self.hovered = ctx.is_hovered();
            }
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            self.probe.event_hovered.set(ctx.is_hovered());
            let InputEvent::Pointer(p) = event else {
                return EventResult::Ignored;
            };
            match p.phase {
                PointerPhase::Move => {
                    let size = ctx.size();
                    let inside = p.position.x >= 0.0
                        && p.position.y >= 0.0
                        && p.position.x < size.width
                        && p.position.y < size.height;
                    if inside {
                        ctx.claim_hover();
                    }
                    if self.latches && self.hovered != inside {
                        self.hovered = inside;
                        ctx.request_redraw();
                    }
                    // Deliberately `Ignored`: a hovering widget does not consume a
                    // move it merely watched (the shipped `ListItem` shape).
                    EventResult::Ignored
                }
                PointerPhase::Down => {
                    if self.captures {
                        ctx.capture_pointer();
                    }
                    EventResult::Handled
                }
                _ => EventResult::Ignored,
            }
        }
    }

    /// Whether the container claims hover for itself, and when relative to routing
    /// the move into its child — the ordering the claim contract binds a container
    /// to.
    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
    enum GroupClaim {
        /// Never claims — the transparent container, hovered only via the path.
        Never,
        /// Claims *after* routing: the contract-following container, whose claim is
        /// a fallback the child's claim beats.
        AfterRouting,
        /// Claims *before* routing: the documented anti-pattern, kept as a
        /// negative control.
        BeforeRouting,
    }

    /// A container wrapping one hover leaf, recording what its **own**
    /// `PaintCtx::is_hovered`/`EventCtx::is_hovered` reported — the
    /// ancestor-on-the-claim-path case, which the two sibling leaves alone cannot
    /// show. With `claims` set it also wants hover chrome of its own, claiming
    /// either side of the route to exercise the ordering rule.
    ///
    /// The child is an `Option` so a rebuild can *remove* it — the unmount case,
    /// where the claimant stops existing between hover passes.
    struct HoverGroup {
        probe: Rc<HoverProbe>,
        claims: GroupClaim,
        child: Option<crate::widget::ChildPod>,
    }

    impl HoverGroup {
        /// Whether this event is an uncaptured-move-shaped pass landing inside the
        /// container's own bounds — the same local hit test a leaf claims on.
        fn claims_on(&self, ctx: &EventCtx, event: &InputEvent) -> bool {
            let InputEvent::Pointer(p) = event else {
                return false;
            };
            let size = ctx.size();
            matches!(p.phase, PointerPhase::Move)
                && p.position.x >= 0.0
                && p.position.y >= 0.0
                && p.position.x < size.width
                && p.position.y < size.height
        }
    }

    impl crate::widget::Widget for HoverGroup {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            match &mut self.child {
                Some(child) => {
                    let size = child.layout_child(ctx, bc);
                    child.set_origin(Point::ZERO);
                    size
                }
                // The same box with nothing in it, so removing the claimant
                // changes what is under the pointer without moving the container.
                None => bc.constrain(Size::new(100.0, 30.0)),
            }
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            self.probe.painted_hovered.set(ctx.is_hovered());
            if let Some(child) = &mut self.child {
                child.paint_child(ctx, scene);
            }
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            self.probe.event_hovered.set(ctx.is_hovered());
            let claims = self.claims != GroupClaim::Never && self.claims_on(ctx, event);
            if claims && self.claims == GroupClaim::BeforeRouting {
                ctx.claim_hover();
            }
            let result = match &mut self.child {
                Some(child) => child.event_child(ctx, event),
                None => EventResult::Ignored,
            };
            if claims && self.claims == GroupClaim::AfterRouting {
                ctx.claim_hover();
            }
            result
        }
    }

    /// Two stacked hover leaves with a hit-tested route and a capture fast-path —
    /// the minimum container that can show a claim moving between siblings.
    struct HoverPair {
        top: crate::widget::ChildPod,
        bottom: crate::widget::ChildPod,
    }

    impl crate::widget::Widget for HoverPair {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.top.layout_child(ctx, bc);
            self.top.set_origin(Point::ZERO);
            self.bottom.layout_child(ctx, bc);
            self.bottom.set_origin(Point::new(0.0, 30.0));
            bc.max()
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            self.top.paint_child(ctx, scene);
            self.bottom.paint_child(ctx, scene);
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            let releases = matches!(
                event,
                InputEvent::Pointer(p)
                    if matches!(p.phase, PointerPhase::Up | PointerPhase::Cancel)
            );
            for pod in [&mut self.top, &mut self.bottom] {
                if pod.is_active() {
                    let r = pod.event_child(ctx, event);
                    if releases {
                        pod.set_active(false);
                    }
                    return r;
                }
            }
            let pos = event.position();
            for pod in [&mut self.top, &mut self.bottom] {
                if pod.contains(pos) {
                    return pod.event_child(ctx, event);
                }
            }
            EventResult::Ignored
        }
    }

    /// How one `HoverHarness` is shaped.
    #[derive(Clone, Copy)]
    struct HoverFixture {
        /// Leaves capture the pointer on `Down` (the drag case).
        captures: bool,
        /// Leaves follow the consumer contract (latched flag + change-gated redraw
        /// + paint-time self-correction).
        latches: bool,
        /// Wrap the top leaf in a [`HoverGroup`], so the claim path has an
        /// ancestor pod between the claimant and the root.
        nested: bool,
        /// Whether (and when) that container claims hover for itself.
        group_claims: GroupClaim,
        /// Rebuild the container without its child: the unmount case, where the
        /// pod holding the hover link is dropped by the view diff.
        drop_claimant: bool,
        /// Report [`ChangeFlags::NONE`] from that removal — the hand-rolled
        /// container outside this workspace, which drops a pod while reporting
        /// whatever it likes. The in-tree reconcilers report `LAYOUT | PAINT`,
        /// which is what the release used to lean on instead of flagging its own.
        silent_reconciler: bool,
    }

    struct HoverPairView {
        top: Rc<HoverProbe>,
        bottom: Rc<HoverProbe>,
        group: Rc<HoverProbe>,
        fixture: HoverFixture,
    }

    impl HoverPairView {
        fn leaf(&self, probe: &Rc<HoverProbe>) -> Box<dyn crate::widget::Widget> {
            Box::new(HoverLeaf {
                captures: self.fixture.captures,
                latches: self.fixture.latches,
                hovered: false,
                probe: probe.clone(),
            })
        }
    }

    impl View<()> for HoverPairView {
        type Element = HoverPair;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> HoverPair {
            let top: Box<dyn crate::widget::Widget> = if self.fixture.nested {
                Box::new(HoverGroup {
                    probe: self.group.clone(),
                    claims: self.fixture.group_claims,
                    child: Some(crate::widget::ChildPod::new(self.leaf(&self.top))),
                })
            } else {
                self.leaf(&self.top)
            };
            HoverPair {
                top: crate::widget::ChildPod::new(top),
                bottom: crate::widget::ChildPod::new(self.leaf(&self.bottom)),
            }
        }
        fn rebuild(&self, p: &Self, e: &mut HoverPair, _c: &mut BuildCtx<'_>) -> ChangeFlags {
            // The only structural op this fixture performs: drop the nested
            // container's child pod, the way a real reconciler drops a truncated
            // or conditionally-removed child.
            let removes_child = self.fixture.drop_claimant && !p.fixture.drop_claimant;
            if !removes_child {
                return ChangeFlags::NONE;
            }
            let group = e
                .top
                .widget_mut()
                .downcast_mut::<HoverGroup>()
                .expect("the drop-claimant fixture is the nested one");
            group.child = None;
            if self.fixture.silent_reconciler {
                ChangeFlags::NONE
            } else {
                ChangeFlags::LAYOUT | ChangeFlags::PAINT
            }
        }
    }

    /// A `RenderRoot` over the hover fixture, plus its probes.
    struct HoverHarness {
        root: RenderRoot<(), HoverPairView>,
        top: Rc<HoverProbe>,
        bottom: Rc<HoverProbe>,
        group: Rc<HoverProbe>,
        state: (),
        /// The shape the next rebuild re-states, so
        /// [`HoverHarness::rebuild_without_claimant`] can flip one flag without
        /// restating the rest.
        fixture: HoverFixture,
    }

    impl HoverHarness {
        /// The contract-following fixture: two flat leaves that latch their own
        /// hover flag, optionally capturing on `Down`.
        fn new(captures: bool) -> Self {
            Self::build(HoverFixture {
                captures,
                latches: true,
                nested: false,
                group_claims: GroupClaim::Never,
                drop_claimant: false,
                silent_reconciler: false,
            })
        }

        /// The negative control: leaves that claim hover but keep no flag of their
        /// own, so only frames the pipeline manufactures show up.
        fn without_consumer_flag() -> Self {
            Self::build(HoverFixture {
                captures: false,
                latches: false,
                nested: false,
                group_claims: GroupClaim::Never,
                drop_claimant: false,
                silent_reconciler: false,
            })
        }

        /// The top leaf wrapped in a container pod, for the path-semantics case.
        fn nested() -> Self {
            Self::build(HoverFixture {
                captures: false,
                latches: true,
                nested: true,
                group_claims: GroupClaim::Never,
                drop_claimant: false,
                silent_reconciler: false,
            })
        }

        /// The same nesting, with the container claiming hover for itself the way
        /// the contract requires: after routing the move into its child.
        fn nested_group_claiming(claims: GroupClaim) -> Self {
            Self::build(HoverFixture {
                captures: false,
                latches: true,
                nested: true,
                group_claims: claims,
                drop_claimant: false,
                silent_reconciler: false,
            })
        }

        /// The same nesting, with a container that drops its child while
        /// reporting no flags — the hand-rolled container outside this workspace
        /// the destructor route exists to cover.
        fn nested_group_with_silent_reconciler() -> Self {
            Self::build(HoverFixture {
                captures: false,
                latches: true,
                nested: true,
                group_claims: GroupClaim::AfterRouting,
                drop_claimant: false,
                silent_reconciler: true,
            })
        }

        fn build(fixture: HoverFixture) -> Self {
            let top = Rc::new(HoverProbe::default());
            let bottom = Rc::new(HoverProbe::default());
            let group = Rc::new(HoverProbe::default());
            let mut root: RenderRoot<(), HoverPairView> = RenderRoot::new();
            let mut state = ();
            let (t, b, g) = (top.clone(), bottom.clone(), group.clone());
            root.rebuild(
                &mut move |_: &mut ()| HoverPairView {
                    top: t.clone(),
                    bottom: b.clone(),
                    group: g.clone(),
                    fixture,
                },
                &mut state,
            );
            root.layout(Size::new(100.0, 60.0));
            HoverHarness {
                root,
                top,
                bottom,
                group,
                state,
                fixture,
            }
        }

        /// Rebuild with the nested container's child removed — the claimant
        /// unmounting between hover passes — and re-lay out, returning what the
        /// diff reported.
        fn rebuild_without_claimant(&mut self) -> ChangeFlags {
            self.fixture.drop_claimant = true;
            self.rebuild_current()
        }

        /// Rebuild restating the shape already on screen: nothing of this root's
        /// own is severed, so any hover end it performs came from elsewhere.
        fn rebuild_unchanged(&mut self) -> ChangeFlags {
            self.rebuild_current()
        }

        /// Re-run the diff against the fixture as it currently stands, then
        /// re-lay out, returning what the diff reported.
        fn rebuild_current(&mut self) -> ChangeFlags {
            let fixture = self.fixture;
            let (t, b, g) = (self.top.clone(), self.bottom.clone(), self.group.clone());
            let flags = self.root.rebuild(
                &mut move |_: &mut ()| HoverPairView {
                    top: t.clone(),
                    bottom: b.clone(),
                    group: g.clone(),
                    fixture,
                },
                &mut self.state,
            );
            self.root.layout(Size::new(100.0, 60.0));
            flags
        }

        /// Dispatch a pointer event at `(x, y)` in window space.
        fn dispatch(&mut self, phase: PointerPhase, x: f64, y: f64) -> EventOutcome {
            let event = InputEvent::Pointer(PointerEvent {
                phase,
                position: Point::new(x, y),
                button: PointerButton::Primary,
            });
            self.root.event(&mut self.state, &event)
        }

        /// Move the pointer over the given leaf's middle.
        fn move_over(&mut self, leaf: Leaf) -> EventOutcome {
            match leaf {
                Leaf::Top => self.dispatch(PointerPhase::Move, 50.0, 15.0),
                Leaf::Bottom => self.dispatch(PointerPhase::Move, 50.0, 45.0),
            }
        }

        /// Paint the tree, refreshing both probes' recorded hover state.
        fn paint(&mut self) {
            let mut scene = RecordingScene::default();
            self.root.paint(&mut scene, FrameTime::ZERO);
        }

        /// `(top, bottom)` hover as the last paint reported it.
        fn painted(&mut self) -> (bool, bool) {
            self.paint();
            (
                self.top.painted_hovered.get(),
                self.bottom.painted_hovered.get(),
            )
        }
    }

    #[test]
    fn an_uncaptured_move_claims_hover_and_paint_reports_it() {
        let mut h = HoverHarness::new(false);
        assert!(!h.root.is_hover_active(), "nothing is hovered at rest");
        assert_eq!(h.painted(), (false, false));

        let outcome = h.move_over(Leaf::Top);
        assert!(h.root.is_hover_active(), "the claim reached the root");
        assert!(
            !outcome.handled,
            "a hovering widget need not consume the move"
        );
        assert_eq!(
            h.painted(),
            (true, false),
            "the claimant reads as hovered, its sibling does not"
        );

        // A second move within the same leaf keeps the link (the claim is
        // re-recorded every pass) without re-reporting a change.
        h.dispatch(PointerPhase::Move, 60.0, 20.0);
        assert_eq!(h.painted(), (true, false));
    }

    #[test]
    fn a_second_widgets_claim_clears_the_first_and_asks_for_a_repaint() {
        let mut h = HoverHarness::new(false);
        h.move_over(Leaf::Top);
        assert_eq!(h.painted(), (true, false));

        // The pointer moves onto the sibling. The container never has to clear
        // anything: the epoch advance strands the top pod's stamp.
        h.move_over(Leaf::Bottom);
        assert!(h.root.is_hover_active());
        assert_eq!(
            h.painted(),
            (false, true),
            "the previous claimant lost its link when the new one recorded"
        );

        // Both widgets need a repaint, and a repaint is global — one request
        // covers them. The *losing* side is what the root itself must guarantee:
        // moving onto a leaf that claims nothing still repaints.
        let outcome = h.dispatch(PointerPhase::Move, 50.0, 200.0);
        assert!(
            !h.root.is_hover_active(),
            "a move claiming nothing ends the hover"
        );
        assert!(
            outcome.needs_redraw,
            "the widget that lost hover cannot ask for the repaint itself"
        );
        assert_eq!(h.painted(), (false, false));

        // ...and the same move repeated is not a change any more.
        let settled = h.dispatch(PointerPhase::Move, 50.0, 200.0);
        assert!(
            !settled.needs_redraw,
            "an already-hoverless move requests nothing"
        );
    }

    #[test]
    fn a_captured_move_cannot_claim_hover() {
        let mut h = HoverHarness::new(true);
        // Press the top leaf: it captures, and the `Down` itself ends any hover.
        h.dispatch(PointerPhase::Down, 50.0, 15.0);
        assert!(h.root.is_pointer_captured());
        assert!(!h.root.is_hover_active());

        // Drag: every one of these moves routes to the captured leaf, whose `Move`
        // arm hit-tests inside and calls `claim_hover()` — and must record nothing.
        h.dispatch(PointerPhase::Move, 50.0, 16.0);
        assert!(
            !h.root.is_hover_active(),
            "a captured pointer never creates hover"
        );
        assert_eq!(h.painted(), (false, false));

        // Dragging outside the leaf keeps routing to it (capture), still no hover.
        h.dispatch(PointerPhase::Move, 50.0, 45.0);
        assert!(!h.root.is_hover_active());
        assert_eq!(h.painted(), (false, false));

        // Release, then a fresh uncaptured move: hover is claimable again.
        h.dispatch(PointerPhase::Up, 50.0, 15.0);
        h.move_over(Leaf::Top);
        assert!(h.root.is_hover_active());
        assert_eq!(h.painted(), (true, false));
    }

    #[test]
    fn a_down_up_or_cancel_ends_the_hover() {
        // Every pointer phase other than an uncaptured `Move` ends the link. `Up`
        // is in here for touch: a lifted finger sends no further move, so a tint
        // claimed during an uncaptured touch drag would otherwise stand for good.
        for ending in [PointerPhase::Down, PointerPhase::Up, PointerPhase::Cancel] {
            let mut h = HoverHarness::new(false);
            h.move_over(Leaf::Top);
            assert!(h.root.is_hover_active());

            let outcome = h.dispatch(ending, 50.0, 15.0);
            assert!(
                !h.root.is_hover_active(),
                "{ending:?} ends the hover link outright"
            );
            assert!(
                outcome.needs_redraw,
                "{ending:?} that dropped a hover asks for the repaint"
            );
            assert_eq!(h.painted(), (false, false));
        }
    }

    #[test]
    fn a_non_pointer_pass_leaves_a_live_hover_standing() {
        let mut h = HoverHarness::new(false);
        h.move_over(Leaf::Top);
        assert_eq!(h.painted(), (true, false));

        // Neither a scroll, a key, nor the housekeeping broadcast is a hover pass:
        // the pointer has not moved, so the link must survive them untouched.
        h.root.event(
            &mut h.state,
            &InputEvent::Scroll {
                position: Point::new(50.0, 15.0),
                delta: crate::event::ScrollDelta::Lines(0.0, 1.0),
            },
        );
        assert!(h.root.is_hover_active());
        h.root.event(&mut h.state, &InputEvent::Housekeeping);
        assert!(h.root.is_hover_active());
        assert_eq!(h.painted(), (true, false));
    }

    #[test]
    fn a_container_on_the_claim_path_reads_hovered_and_a_sibling_does_not() {
        // The recorded thing is a path, so hover is `:hover`-shaped: the claimant
        // and every ancestor enclosing it read hovered, nothing off the path does.
        let mut h = HoverHarness::nested();
        h.move_over(Leaf::Top);
        h.paint();
        assert!(h.top.painted_hovered.get(), "the claimant itself");
        assert!(
            h.group.painted_hovered.get(),
            "the container enclosing the claimant is on the path too"
        );
        assert!(
            !h.bottom.painted_hovered.get(),
            "a sibling leaf is off the path"
        );

        // Move onto the sibling: the container goes unhovered with its child, and
        // both reads agree about it on the next pass.
        h.move_over(Leaf::Bottom);
        h.paint();
        assert!(!h.group.painted_hovered.get());
        assert!(!h.top.painted_hovered.get());
        assert!(h.bottom.painted_hovered.get());
        h.move_over(Leaf::Top);
        assert!(
            !h.group.event_hovered.get(),
            "the event read reports the previous pass, like the leaf's"
        );
        h.move_over(Leaf::Top);
        assert!(
            h.group.event_hovered.get(),
            "the container observes the link its child holds"
        );
    }

    #[test]
    fn an_ancestor_claiming_after_routing_loses_to_its_child_and_still_reads_hovered() {
        // A container that wants hover chrome of its own claims after routing the
        // move into its child. Only one claim per pass is recorded and the first one
        // recorded wins, so the child's claim is the one that lands; the container's
        // own late call is a silent no-op, and it reads hovered through the stamped
        // path anyway — which is what makes this ordering correct in every case.
        let mut h = HoverHarness::nested_group_claiming(GroupClaim::AfterRouting);
        let gain = h.move_over(Leaf::Top);
        h.paint();
        assert!(
            h.top.painted_hovered.get(),
            "the child under the pointer holds the link"
        );
        assert!(
            h.group.painted_hovered.get(),
            "the container is on that path, so it reads hovered too"
        );
        assert!(
            !h.bottom.painted_hovered.get(),
            "a sibling leaf is off the path"
        );
        assert!(gain.needs_redraw, "hover gain repaints");

        // The child's latched flag now agrees with the authoritative paint read, so
        // wandering on within the same widget settles instead of repainting.
        let settled = h.dispatch(PointerPhase::Move, 60.0, 20.0);
        assert!(!settled.needs_redraw, "an unchanged flag asks for nothing");
        h.paint();
        assert!(h.top.painted_hovered.get());
        assert!(h.group.painted_hovered.get());
    }

    #[test]
    fn an_ancestor_claiming_before_routing_starves_its_subtree() {
        // The negative control for the ordering rule above, pinning the trap it
        // exists to prevent: a container that claims *before* forwarding is recorded
        // first, which closes the pass to every descendant. The child under the
        // pointer can never read hovered, so its hover chrome never appears — and
        // because its latched flag is corrected back to `false` at paint time, it
        // flips and asks for a frame again on every single move.
        let mut h = HoverHarness::nested_group_claiming(GroupClaim::BeforeRouting);
        h.move_over(Leaf::Top);
        h.paint();
        assert!(
            !h.top.painted_hovered.get(),
            "the ancestor's earlier claim made its child ineligible"
        );
        assert!(
            h.group.painted_hovered.get(),
            "the outermost claimant is the one holding the link here"
        );
        assert!(!h.bottom.painted_hovered.get());

        // Repaint-per-move: the flag never converges, because the event arm and the
        // authoritative paint read permanently disagree.
        let again = h.dispatch(PointerPhase::Move, 60.0, 20.0);
        assert!(
            again.needs_redraw,
            "the starved child re-flips its flag on every move"
        );
        h.paint();
        assert!(!h.top.painted_hovered.get(), "and still paints no chrome");
    }

    #[test]
    fn hover_gain_is_repainted_by_the_consumers_own_flag() {
        let mut h = HoverHarness::new(false);
        // Entering a widget: the root manufactures nothing here (its mirror went
        // `false` → `true`, and it cannot know which widget cares), so the frame
        // comes from the claimant's own change-gated request.
        let gain = h.move_over(Leaf::Top);
        assert!(gain.needs_redraw, "hover gain repaints");
        assert_eq!(h.painted(), (true, false));

        // Wandering within the same widget claims again but changes nothing, so it
        // must not repaint per event.
        let settled = h.dispatch(PointerPhase::Move, 60.0, 20.0);
        assert!(!settled.needs_redraw, "an unchanged flag asks for nothing");

        // A handoff repaints both sides at once: the arriving leaf's flag changed
        // (it asks), and because a repaint is global that same frame is what lets
        // the departing leaf drop its chrome from the authoritative paint read.
        let handoff = h.move_over(Leaf::Bottom);
        assert!(handoff.needs_redraw, "a claimant handoff repaints");
        assert_eq!(
            h.painted(),
            (false, true),
            "one frame settles both the loss and the gain"
        );
    }

    #[test]
    fn a_claimant_without_its_own_flag_gets_only_the_loss_frame() {
        // The negative control for the contract above: leaves that claim hover but
        // keep no flag of their own. Gain and handoff are invisible to the root
        // (its hover mirror is identity-free — `false` → `true` and `true` →
        // `true`), so nothing repaints for them, which is exactly why the
        // consumer's latched flag is normative rather than an optimization.
        let mut h = HoverHarness::without_consumer_flag();
        let gain = h.move_over(Leaf::Top);
        assert!(h.root.is_hover_active());
        assert!(!gain.needs_redraw, "no widget asked, and the root cannot");

        let handoff = h.move_over(Leaf::Bottom);
        assert!(h.root.is_hover_active());
        assert!(!handoff.needs_redraw, "a handoff is `true` → `true` here");

        // Loss is the one edge the root does cover, since no widget can see it.
        let loss = h.dispatch(PointerPhase::Move, 50.0, 200.0);
        assert!(!h.root.is_hover_active());
        assert!(loss.needs_redraw, "the root manufactures the loss frame");
    }

    #[test]
    fn a_rebuild_that_removes_the_claimant_ends_the_hover() {
        // The one severance the epoch cannot strand: the claimant is dropped by a
        // view diff, so it will never see the `Move` that would have re-derived
        // the link. Its pod reports the drop and `rebuild` ends the hover.
        let mut h = HoverHarness::nested_group_claiming(GroupClaim::AfterRouting);
        h.move_over(Leaf::Top);
        h.paint();
        assert!(h.root.is_hover_active());
        assert!(h.top.painted_hovered.get(), "the claimant holds the link");
        assert!(
            h.group.painted_hovered.get(),
            "its container is on the path"
        );

        let flags = h.rebuild_without_claimant();
        assert!(
            !h.root.is_hover_active(),
            "the mirror cannot outlive the widget it described"
        );
        assert!(
            flags.contains(ChangeFlags::PAINT),
            "the correction states its own need for a frame, whatever the \
             reconciler that dropped the claimant reported"
        );

        // The survivor is the ancestor that was on the claim path: its own stamp
        // still names the epoch the claim recorded, so nothing but the epoch
        // advance keeps it from painting hover chrome for a child that is gone.
        h.paint();
        assert!(
            !h.group.painted_hovered.get(),
            "the surviving ancestor lost the link with its child"
        );
        assert!(
            !h.bottom.painted_hovered.get(),
            "and the sibling never had it"
        );

        // And the pipeline is not wedged: the next move over the same spot claims
        // cleanly, now for the container itself. (No frame is manufactured for
        // that gain — this container keeps no latched flag of its own, which is
        // the documented consumer-side half of the contract, not a pipeline job.)
        h.move_over(Leaf::Top);
        assert!(h.root.is_hover_active(), "the next move re-claims");
        h.paint();
        assert!(
            h.group.painted_hovered.get(),
            "the container is the claimant now"
        );
    }

    #[test]
    fn a_silent_reconcilers_removal_still_carries_its_own_repaint() {
        // The reason the release flags `PAINT` itself rather than trusting the
        // diff to have reported one. The destructor route deliberately reaches
        // containers this workspace never sees, and such a container can drop the
        // claimant while reporting nothing — leaving the hover correctly ended but
        // the frame that shows it unrequested, on a pointer the user has stopped
        // moving.
        let mut h = HoverHarness::nested_group_with_silent_reconciler();
        h.move_over(Leaf::Top);
        assert!(h.root.is_hover_active());

        let flags = h.rebuild_without_claimant();
        assert!(!h.root.is_hover_active(), "the link ends either way");
        assert_eq!(
            flags,
            ChangeFlags::PAINT,
            "and the frame it needs comes from the release, not from the diff"
        );
    }

    #[test]
    fn a_rebuild_that_keeps_the_claimant_leaves_the_hover_standing() {
        // The negative control for the release above, and the reason the mark is
        // gated on the *live* epoch rather than on "some pod with a stamp died":
        // an ordinary rebuild — including one that drops pods carrying stale
        // stamps — must not touch a link the pointer still rests on.
        let mut h = HoverHarness::nested_group_claiming(GroupClaim::AfterRouting);
        // Hover the top leaf, then hand the link to the sibling. The top pod chain
        // keeps its (now stale) stamp, which is what the next rebuild drops.
        h.move_over(Leaf::Top);
        h.move_over(Leaf::Bottom);
        assert!(h.root.is_hover_active());

        h.rebuild_without_claimant();
        assert!(
            h.root.is_hover_active(),
            "dropping a stale stamp is not a severance"
        );
        h.paint();
        assert!(
            h.bottom.painted_hovered.get(),
            "the widget actually under the pointer keeps its chrome"
        );
    }

    #[test]
    fn another_roots_dying_claimant_cannot_end_this_roots_hover() {
        // Two roots on one thread. Each has run exactly one hover pass, so their
        // epoch counters hold the identical integer — the collision the root
        // identity exists to break. Without it, the first root's pods dying (its
        // window closing, a page tearing down) raise a mark the second root's
        // next rebuild drains, ending a hover the pointer is still resting on.
        let mut first = HoverHarness::nested_group_claiming(GroupClaim::AfterRouting);
        let mut second = HoverHarness::nested_group_claiming(GroupClaim::AfterRouting);
        first.move_over(Leaf::Top);
        second.move_over(Leaf::Top);
        assert_eq!(
            first.root.hover_epoch, second.root.hover_epoch,
            "the two roots' epochs collide, which is the whole premise"
        );
        assert!(first.root.is_hover_active());
        assert!(second.root.is_hover_active());

        // Drop the first root outright: every pod it owns runs the destructor
        // that reports a severed hover link, including the claimant's.
        drop(first);

        second.rebuild_unchanged();
        assert!(
            second.root.is_hover_active(),
            "a mark another root raised is none of this root's business"
        );
        second.paint();
        assert!(
            second.top.painted_hovered.get(),
            "the widget under the pointer keeps its chrome"
        );
    }

    #[test]
    fn event_ctx_hover_reports_the_previous_pass_not_this_ones_claim() {
        let mut h = HoverHarness::new(false);
        // First move over the top leaf: it was not hovered when its handler ran.
        h.move_over(Leaf::Top);
        assert!(
            !h.top.event_hovered.get(),
            "a fresh claim does not retroactively flip `is_hovered`"
        );
        // Second move over the same leaf: now it observes the link it holds.
        h.move_over(Leaf::Top);
        assert!(
            h.top.event_hovered.get(),
            "the link recorded last pass is visible to this pass's handler"
        );
    }

    // --- Cursor: the per-pass request channel ---------------------------------
    //
    // The cursor is hover's sibling and is deliberately not derived from it (the
    // root's hover mirror is identity-free), so it gets its own fixture: two
    // leaves that ask for different shapes, a container that can speak before or
    // after routing (the last-writer case), and a capture fast-path (the drag
    // case, where the shape must survive the pointer leaving the widget).

    /// How the cursor fixture's container speaks relative to its children.
    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
    enum ContainerCursor {
        /// Says nothing at all — the ordinary container.
        Silent,
        /// Asks *before* routing, so whatever the child asks for comes later.
        BeforeRouting(CursorIcon),
        /// Asks *after* routing, deliberately overriding its child.
        AfterRouting(CursorIcon),
    }

    /// A leaf that asks for one cursor while the pointer is over it and another
    /// while it holds the capture — the two shapes a real draggable control wants.
    struct CursorLeaf {
        hover_icon: Option<CursorIcon>,
        drag_icon: Option<CursorIcon>,
        captures: bool,
        captured: bool,
    }

    impl crate::widget::Widget for CursorLeaf {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(100.0, 30.0))
        }
        // A cursor is resolved entirely in the event pass — this fixture never
        // paints, unlike the hover one (whose authoritative read is at paint time).
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            let InputEvent::Pointer(p) = event else {
                return EventResult::Ignored;
            };
            match p.phase {
                PointerPhase::Move => {
                    if self.captured {
                        // The captured pass belongs to this widget wherever the
                        // pointer has gone — re-asking here is what keeps the drag
                        // shape alive outside its own bounds.
                        if let Some(icon) = self.drag_icon {
                            ctx.set_cursor(icon);
                        }
                        return EventResult::Handled;
                    }
                    let size = ctx.size();
                    let inside = p.position.x >= 0.0
                        && p.position.y >= 0.0
                        && p.position.x < size.width
                        && p.position.y < size.height;
                    if inside && let Some(icon) = self.hover_icon {
                        ctx.set_cursor(icon);
                    }
                    EventResult::Ignored
                }
                PointerPhase::Down => {
                    if self.captures {
                        ctx.capture_pointer();
                        self.captured = true;
                    }
                    EventResult::Handled
                }
                PointerPhase::Up | PointerPhase::Cancel => {
                    self.captured = false;
                    EventResult::Ignored
                }
            }
        }
    }

    /// Two stacked cursor leaves routed exactly like [`HoverPair`], plus the
    /// container's own optional request.
    struct CursorPair {
        top: crate::widget::ChildPod,
        bottom: crate::widget::ChildPod,
        own: ContainerCursor,
    }

    impl crate::widget::Widget for CursorPair {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.top.layout_child(ctx, bc);
            self.top.set_origin(Point::ZERO);
            self.bottom.layout_child(ctx, bc);
            self.bottom.set_origin(Point::new(0.0, 30.0));
            bc.max()
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            if let ContainerCursor::BeforeRouting(icon) = self.own {
                ctx.set_cursor(icon);
            }
            let result = self.route(ctx, event);
            if let ContainerCursor::AfterRouting(icon) = self.own {
                ctx.set_cursor(icon);
            }
            result
        }
    }

    impl CursorPair {
        /// The capture-first, then hit-test routing every real container does.
        fn route(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            let releases = matches!(
                event,
                InputEvent::Pointer(p)
                    if matches!(p.phase, PointerPhase::Up | PointerPhase::Cancel)
            );
            for pod in [&mut self.top, &mut self.bottom] {
                if pod.is_active() {
                    let r = pod.event_child(ctx, event);
                    if releases {
                        pod.set_active(false);
                    }
                    return r;
                }
            }
            let pos = event.position();
            for pod in [&mut self.top, &mut self.bottom] {
                if pod.contains(pos) {
                    return pod.event_child(ctx, event);
                }
            }
            EventResult::Ignored
        }
    }

    #[derive(Clone, Copy)]
    struct CursorPairView {
        own: ContainerCursor,
        captures: bool,
    }

    impl View<()> for CursorPairView {
        type Element = CursorPair;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> CursorPair {
            CursorPair {
                top: crate::widget::ChildPod::new(Box::new(CursorLeaf {
                    hover_icon: Some(CursorIcon::Pointer),
                    drag_icon: Some(CursorIcon::Grabbing),
                    captures: self.captures,
                    captured: false,
                })),
                bottom: crate::widget::ChildPod::new(Box::new(CursorLeaf {
                    hover_icon: Some(CursorIcon::Text),
                    drag_icon: None,
                    captures: false,
                    captured: false,
                })),
                own: self.own,
            }
        }
        fn rebuild(&self, _p: &Self, _e: &mut CursorPair, _c: &mut BuildCtx<'_>) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    /// A `RenderRoot` over the cursor fixture.
    struct CursorHarness {
        root: RenderRoot<(), CursorPairView>,
        state: (),
    }

    impl CursorHarness {
        fn new(own: ContainerCursor, captures: bool) -> Self {
            let mut root: RenderRoot<(), CursorPairView> = RenderRoot::new();
            let mut state = ();
            root.rebuild(
                &mut move |_: &mut ()| CursorPairView { own, captures },
                &mut state,
            );
            root.layout(Size::new(100.0, 60.0));
            CursorHarness { root, state }
        }

        fn dispatch(&mut self, phase: PointerPhase, x: f64, y: f64) -> EventOutcome {
            let event = InputEvent::Pointer(PointerEvent {
                phase,
                position: Point::new(x, y),
                button: PointerButton::Primary,
            });
            self.root.event(&mut self.state, &event)
        }

        fn move_over(&mut self, leaf: Leaf) {
            match leaf {
                Leaf::Top => self.dispatch(PointerPhase::Move, 50.0, 15.0),
                Leaf::Bottom => self.dispatch(PointerPhase::Move, 50.0, 45.0),
            };
        }

        fn cursor(&self) -> CursorIcon {
            self.root.cursor()
        }
    }

    #[test]
    fn a_move_resolves_the_requested_cursor_and_absence_resolves_default() {
        let mut h = CursorHarness::new(ContainerCursor::Silent, false);
        assert_eq!(
            h.cursor(),
            CursorIcon::Default,
            "nothing has asked for anything yet"
        );

        h.move_over(Leaf::Top);
        assert_eq!(
            h.cursor(),
            CursorIcon::Pointer,
            "the request reached the root"
        );

        // Moving onto the sibling re-resolves to *its* shape with nothing cleared:
        // the pass simply has a different last writer.
        h.move_over(Leaf::Bottom);
        assert_eq!(h.cursor(), CursorIcon::Text);

        // Moving off both: the next pass has no writer at all, and absence is the
        // default rather than a stale value — the whole point of a stateless
        // request.
        h.dispatch(PointerPhase::Move, 50.0, 200.0);
        assert_eq!(
            h.cursor(),
            CursorIcon::Default,
            "a widget that stops asking falls back with nothing to clear"
        );
    }

    #[test]
    fn a_cursor_change_does_not_ask_for_a_repaint() {
        // Applying a cursor is a platform call the shell makes after the pass, with
        // no frame involved; folding it into `needs_redraw` would repaint the whole
        // tree on every hover move.
        let mut h = CursorHarness::new(ContainerCursor::Silent, false);
        let outcome = h.dispatch(PointerPhase::Move, 50.0, 15.0);
        assert_eq!(h.cursor(), CursorIcon::Pointer);
        assert!(
            !outcome.needs_redraw,
            "a cursor request alone never schedules a frame"
        );
    }

    #[test]
    fn the_last_writer_on_the_routed_path_wins() {
        // A container that asks before routing loses to its child: the child's
        // handler runs later in the same pass, which is what makes a specific
        // control override the generic surface behind it.
        let mut h =
            CursorHarness::new(ContainerCursor::BeforeRouting(CursorIcon::ColResize), false);
        h.move_over(Leaf::Top);
        assert_eq!(
            h.cursor(),
            CursorIcon::Pointer,
            "the innermost widget the route reached spoke last"
        );

        // ...and the container's own request still resolves where no child asks
        // (moving off both leaves leaves the container as the only writer).
        h.dispatch(PointerPhase::Move, 50.0, 200.0);
        assert_eq!(h.cursor(), CursorIcon::ColResize);

        // The deliberate override is the mirror case: asking *after* routing beats
        // the child.
        let mut h =
            CursorHarness::new(ContainerCursor::AfterRouting(CursorIcon::NotAllowed), false);
        h.move_over(Leaf::Top);
        assert_eq!(
            h.cursor(),
            CursorIcon::NotAllowed,
            "a container overriding its children asks after routing"
        );
    }

    #[test]
    fn only_a_pointer_move_re_resolves_the_cursor() {
        let mut h = CursorHarness::new(ContainerCursor::Silent, false);
        h.move_over(Leaf::Top);
        assert_eq!(h.cursor(), CursorIcon::Pointer);

        // A press/release says nothing about the cursor, and must not blink it back
        // to `Default` for the duration of a click.
        h.dispatch(PointerPhase::Down, 50.0, 15.0);
        assert_eq!(h.cursor(), CursorIcon::Pointer, "a Down leaves it standing");
        h.dispatch(PointerPhase::Up, 50.0, 15.0);
        assert_eq!(h.cursor(), CursorIcon::Pointer, "an Up leaves it standing");

        // Neither does a scroll, a key, or the housekeeping broadcast.
        h.root.event(
            &mut h.state,
            &InputEvent::Scroll {
                position: Point::new(50.0, 15.0),
                delta: crate::event::ScrollDelta::Lines(0.0, 1.0),
            },
        );
        assert_eq!(h.cursor(), CursorIcon::Pointer);
        h.root.event(&mut h.state, &InputEvent::Housekeeping);
        assert_eq!(h.cursor(), CursorIcon::Pointer);
    }

    #[test]
    fn a_captured_drag_keeps_the_capturing_widgets_cursor() {
        let mut h = CursorHarness::new(ContainerCursor::Silent, true);
        h.move_over(Leaf::Top);
        assert_eq!(h.cursor(), CursorIcon::Pointer);

        // Press the top leaf: it captures. The `Down` itself resolves nothing.
        h.dispatch(PointerPhase::Down, 50.0, 15.0);
        assert!(h.root.is_pointer_captured());
        assert_eq!(h.cursor(), CursorIcon::Pointer);

        // Drag inside, then well outside its own bounds and over the sibling: every
        // one of these moves routes to the captured leaf alone, so its drag shape is
        // the only request in the pass — the sibling's `Text` never gets a say.
        h.dispatch(PointerPhase::Move, 50.0, 16.0);
        assert_eq!(h.cursor(), CursorIcon::Grabbing);
        h.dispatch(PointerPhase::Move, 50.0, 45.0);
        assert_eq!(
            h.cursor(),
            CursorIcon::Grabbing,
            "a captured drag keeps its own cursor outside its bounds"
        );
        h.dispatch(PointerPhase::Move, 50.0, 500.0);
        assert_eq!(h.cursor(), CursorIcon::Grabbing);

        // Release, then a fresh uncaptured move: the ordinary hit-tested resolution
        // is back, and the drag shape is gone with nothing cleared.
        h.dispatch(PointerPhase::Up, 50.0, 45.0);
        h.move_over(Leaf::Bottom);
        assert_eq!(h.cursor(), CursorIcon::Text);
    }

    use crate::event::{EditCommand, Key, KeyEvent, Modifiers, NamedKey};

    // --- Clipboard: the per-pass write / paste-request channels ---------------
    //
    // The cursor's two siblings, resolved by the same bracket but committed on
    // every pass rather than on a pointer `Move` alone, and drained one-shot
    // rather than read as a standing level. The fixture is the cursor pair's
    // shape with focus in place of hit testing, because a clipboard verb is
    // focus-routed: two stacked leaves, a container that can speak before or
    // after routing (the last-writer case) or ask for a paste of its own (the
    // two-channels-in-one-pass case).

    /// How the clipboard fixture's container speaks relative to its children.
    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
    enum ContainerClipboard {
        /// Says nothing at all — the ordinary container.
        Silent,
        /// Writes *before* routing, so whatever the child writes comes later.
        WritesBeforeRouting(&'static str),
        /// Writes *after* routing, deliberately overriding its child.
        WritesAfterRouting(&'static str),
        /// Asks for a paste before routing — a toolbar refreshing whether its
        /// paste button should be enabled, which is how a write and a request
        /// legitimately ride one pass.
        AsksForPaste,
    }

    /// A focusable editable stand-in: claims focus on a `Down`, answers a
    /// copy/cut by writing its "selection", and asks for the clipboard when it
    /// sees the hardware [`NamedKey::Paste`] key it decoded itself.
    struct ClipboardLeaf {
        /// What this leaf would copy.
        text: &'static str,
        /// Every [`EditCommand`] this leaf was handed, in order — the proof of
        /// who the focus routing actually reached.
        seen: Rc<RefCell<Vec<EditCommand>>>,
    }

    impl crate::widget::Widget for ClipboardLeaf {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(100.0, 30.0))
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            match event {
                InputEvent::Pointer(p) if p.phase == PointerPhase::Down => {
                    ctx.request_focus();
                    EventResult::Handled
                }
                InputEvent::EditCommand(cmd) => {
                    self.seen.borrow_mut().push(cmd.clone());
                    match cmd {
                        // The widget owns the selection, so it is the only thing
                        // that can say what "copy" means.
                        EditCommand::Copy | EditCommand::Cut => {
                            ctx.write_clipboard(self.text.to_string());
                        }
                        // A paste arrives with its text already read by the shell,
                        // and select-all touches no clipboard at all.
                        EditCommand::Paste(_) | EditCommand::SelectAll => {}
                    }
                    EventResult::Handled
                }
                // The hardware clipboard key, decoded by the widget rather than by
                // the shell: it carries no text, so the widget asks for some.
                InputEvent::Key(k) if k.key == Key::Named(NamedKey::Paste) => {
                    ctx.request_paste();
                    EventResult::Handled
                }
                _ => EventResult::Ignored,
            }
        }
    }

    /// Two stacked clipboard leaves, routed by focus for a focus-routed event and
    /// by hit test for a pointer one, plus the container's own optional request.
    struct ClipboardPair {
        top: crate::widget::ChildPod,
        bottom: crate::widget::ChildPod,
        own: ContainerClipboard,
    }

    impl crate::widget::Widget for ClipboardPair {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.top.layout_child(ctx, bc);
            self.top.set_origin(Point::ZERO);
            self.bottom.layout_child(ctx, bc);
            self.bottom.set_origin(Point::new(0.0, 30.0));
            bc.max()
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            match self.own {
                ContainerClipboard::WritesBeforeRouting(text) => {
                    ctx.write_clipboard(text.to_string())
                }
                ContainerClipboard::AsksForPaste => ctx.request_paste(),
                _ => {}
            }
            let result = self.route(ctx, event);
            if let ContainerClipboard::WritesAfterRouting(text) = self.own {
                ctx.write_clipboard(text.to_string());
            }
            result
        }
    }

    impl ClipboardPair {
        /// Focus routing for a focus-routed event, hit testing for the rest —
        /// the two branches every real container has.
        fn route(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            if event.is_focus_routed() {
                for pod in [&mut self.top, &mut self.bottom] {
                    if pod.is_focused() {
                        return pod.event_child(ctx, event);
                    }
                }
                return EventResult::Ignored;
            }
            let pos = event.position();
            let blurs = matches!(
                event,
                InputEvent::Pointer(p) if p.phase == PointerPhase::Down
            );
            let mut result = EventResult::Ignored;
            let mut hit = false;
            for pod in [&mut self.top, &mut self.bottom] {
                if !hit && pod.contains(pos) {
                    hit = true;
                    result = pod.event_child(ctx, event);
                } else if blurs {
                    // A `Down` that lands elsewhere blurs the chain, exactly as
                    // `frust-widgets`' routers do.
                    pod.set_focused(false);
                }
            }
            result
        }
    }

    /// The fixture's view. It carries the two `seen` logs rather than letting the
    /// harness reach into the built tree for them: `Widget` has no downcast, and
    /// a shared handle is the same way the hover fixture's probe is watched.
    #[derive(Clone)]
    struct ClipboardPairView {
        own: ContainerClipboard,
        top_seen: Rc<RefCell<Vec<EditCommand>>>,
        bottom_seen: Rc<RefCell<Vec<EditCommand>>>,
    }

    impl View<()> for ClipboardPairView {
        type Element = ClipboardPair;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> ClipboardPair {
            ClipboardPair {
                top: crate::widget::ChildPod::new(Box::new(ClipboardLeaf {
                    text: "top selection",
                    seen: self.top_seen.clone(),
                })),
                bottom: crate::widget::ChildPod::new(Box::new(ClipboardLeaf {
                    text: "bottom selection",
                    seen: self.bottom_seen.clone(),
                })),
                own: self.own,
            }
        }
        fn rebuild(&self, _p: &Self, _e: &mut ClipboardPair, _c: &mut BuildCtx<'_>) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    /// A `RenderRoot` over the clipboard fixture, plus the two leaves' `seen`
    /// logs (cloned out of the built widgets, which the root owns).
    struct ClipboardHarness {
        root: RenderRoot<(), ClipboardPairView>,
        state: (),
        top_seen: Rc<RefCell<Vec<EditCommand>>>,
        bottom_seen: Rc<RefCell<Vec<EditCommand>>>,
    }

    impl ClipboardHarness {
        fn new(own: ContainerClipboard) -> Self {
            let mut root: RenderRoot<(), ClipboardPairView> = RenderRoot::new();
            let mut state = ();
            let view = ClipboardPairView {
                own,
                top_seen: Rc::new(RefCell::new(Vec::new())),
                bottom_seen: Rc::new(RefCell::new(Vec::new())),
            };
            let (top_seen, bottom_seen) = (view.top_seen.clone(), view.bottom_seen.clone());
            root.rebuild(&mut move |_: &mut ()| view.clone(), &mut state);
            root.layout(Size::new(100.0, 60.0));
            ClipboardHarness {
                root,
                state,
                top_seen,
                bottom_seen,
            }
        }

        /// Focus a leaf the way a user does: a `Down` inside its bounds.
        fn focus(&mut self, leaf: Leaf) {
            let y = match leaf {
                Leaf::Top => 15.0,
                Leaf::Bottom => 45.0,
            };
            self.root
                .event(&mut self.state, &pointer(PointerPhase::Down, 50.0, y));
        }

        fn dispatch(&mut self, event: &InputEvent) -> EventOutcome {
            self.root.event(&mut self.state, event)
        }
    }

    #[test]
    fn a_copy_reaches_the_focused_leaf_alone_and_its_text_drains_once() {
        let mut h = ClipboardHarness::new(ContainerClipboard::Silent);
        h.focus(Leaf::Top);
        assert!(
            h.root.take_clipboard_write().is_none(),
            "a focusing tap writes nothing"
        );

        h.dispatch(&InputEvent::EditCommand(EditCommand::Copy));
        assert_eq!(
            h.top_seen.borrow().as_slice(),
            &[EditCommand::Copy],
            "the focused leaf received the command"
        );
        assert!(
            h.bottom_seen.borrow().is_empty(),
            "and the unfocused sibling never saw it — focus routing, not hit testing"
        );
        assert_eq!(
            h.root.take_clipboard_write().as_deref(),
            Some("top selection")
        );
        assert_eq!(
            h.root.take_clipboard_write(),
            None,
            "the drain is one-shot: a clipboard write is an edge, not a level"
        );
    }

    #[test]
    fn a_copy_follows_the_focus_when_it_moves() {
        let mut h = ClipboardHarness::new(ContainerClipboard::Silent);
        h.focus(Leaf::Top);
        h.focus(Leaf::Bottom);
        h.dispatch(&InputEvent::EditCommand(EditCommand::Cut));
        assert!(
            h.top_seen.borrow().is_empty(),
            "the blurred leaf is out of the routed path"
        );
        assert_eq!(h.bottom_seen.borrow().as_slice(), &[EditCommand::Cut]);
        assert_eq!(
            h.root.take_clipboard_write().as_deref(),
            Some("bottom selection")
        );
    }

    #[test]
    fn a_paste_request_drains_once_and_its_answer_is_an_ordinary_dispatch() {
        let mut h = ClipboardHarness::new(ContainerClipboard::Silent);
        h.focus(Leaf::Top);
        assert!(
            !h.root.take_paste_request(),
            "a focusing tap asks for nothing"
        );

        // The widget decoded the hardware Paste key itself and asked the shell.
        h.dispatch(&InputEvent::Key(KeyEvent {
            key: Key::Named(NamedKey::Paste),
            modifiers: Modifiers::default(),
            repeat: false,
        }));
        assert!(h.root.take_paste_request());
        assert!(
            !h.root.take_paste_request(),
            "the flag is one-shot, so a shell answers a request once"
        );

        // The shell's answer is a new, focus-routed dispatch carrying the text.
        h.dispatch(&InputEvent::EditCommand(EditCommand::Paste(
            "from the host".to_string(),
        )));
        assert_eq!(
            h.top_seen.borrow().as_slice(),
            &[EditCommand::Paste("from the host".to_string())]
        );
        assert!(
            !h.root.take_paste_request(),
            "answering a request does not raise a new one"
        );
        assert!(
            h.root.take_clipboard_write().is_none(),
            "and a paste writes nothing back to the host clipboard"
        );
    }

    #[test]
    fn a_paste_answered_after_a_blur_reaches_nobody() {
        let mut h = ClipboardHarness::new(ContainerClipboard::Silent);
        h.focus(Leaf::Top);
        h.dispatch(&InputEvent::Key(KeyEvent {
            key: Key::Named(NamedKey::Paste),
            modifiers: Modifiers::default(),
            repeat: false,
        }));
        assert!(h.root.take_paste_request());

        // Focus is released while the shell's clipboard read is in flight: a
        // `Down` on empty chrome past both leaves blurs the chain.
        h.dispatch(&pointer(PointerPhase::Down, 50.0, 100.0));
        assert!(!h.root.is_focus_active(), "the tap blurred the field");

        // The shell answers anyway — it never has to track who asked, because the
        // answer is focus-routed and simply reaches no widget.
        h.dispatch(&InputEvent::EditCommand(EditCommand::Paste(
            "from the host".to_string(),
        )));
        assert!(h.top_seen.borrow().is_empty());
        assert!(h.bottom_seen.borrow().is_empty());
    }

    #[test]
    fn the_last_clipboard_write_of_a_pass_wins_at_the_root() {
        // A container that writes BEFORE routing yields to its child, exactly as
        // `set_cursor` does: the innermost widget the route reaches speaks last.
        let mut h = ClipboardHarness::new(ContainerClipboard::WritesBeforeRouting("container"));
        h.focus(Leaf::Top);
        h.dispatch(&InputEvent::EditCommand(EditCommand::Copy));
        assert_eq!(
            h.root.take_clipboard_write().as_deref(),
            Some("top selection")
        );

        // A container that writes AFTER routing deliberately overrides it.
        let mut h = ClipboardHarness::new(ContainerClipboard::WritesAfterRouting("container"));
        h.focus(Leaf::Top);
        h.dispatch(&InputEvent::EditCommand(EditCommand::Copy));
        assert_eq!(h.root.take_clipboard_write().as_deref(), Some("container"));
    }

    #[test]
    fn a_write_and_a_paste_request_ride_one_pass_independently() {
        let mut h = ClipboardHarness::new(ContainerClipboard::AsksForPaste);
        h.focus(Leaf::Top);
        // The focusing tap already carried the container's ask; drain it so the
        // assertion below is about the copy pass alone.
        assert!(h.root.take_paste_request());

        h.dispatch(&InputEvent::EditCommand(EditCommand::Copy));
        assert_eq!(
            h.root.take_clipboard_write().as_deref(),
            Some("top selection"),
            "the leaf's write landed"
        );
        assert!(
            h.root.take_paste_request(),
            "and the container's request landed in the same pass"
        );
    }

    #[test]
    fn a_pass_that_asks_for_neither_leaves_both_empty() {
        let mut h = ClipboardHarness::new(ContainerClipboard::Silent);
        h.focus(Leaf::Top);
        // A select-all is a real clipboard verb that touches no clipboard, and a
        // pointer move touches nothing at all.
        h.dispatch(&InputEvent::EditCommand(EditCommand::SelectAll));
        h.dispatch(&pointer(PointerPhase::Move, 50.0, 15.0));
        assert_eq!(h.top_seen.borrow().as_slice(), &[EditCommand::SelectAll]);
        assert_eq!(h.root.take_clipboard_write(), None);
        assert!(!h.root.take_paste_request());
    }

    #[test]
    fn an_edit_command_moves_focus_only_when_the_dispatch_asks() {
        // The bookkeeping arm `EditCommand` shares with `Key`/`Ime`: unlike a
        // `Down`, it neither claims nor blurs by itself.
        let mut h = ClipboardHarness::new(ContainerClipboard::Silent);
        h.focus(Leaf::Top);
        assert!(h.root.is_focus_active());
        let gen_before = h.root.focus_ime_generation();
        h.dispatch(&InputEvent::EditCommand(EditCommand::Copy));
        assert!(
            h.root.is_focus_active(),
            "a clipboard verb leaves the focus session exactly as it found it"
        );
        assert_eq!(h.root.focus_ime_generation(), gen_before);
    }

    // ---------------------------------------------------------------------
    // Overlay portal: an owner that floats a pod, a sibling painted after it,
    // and a root that routes like any hand-written container.
    // ---------------------------------------------------------------------

    /// What the owner does when a `Down` arrives inside one of its surfaces.
    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
    enum OwnerReaction {
        /// Nothing — the commonest shape (a menu item acts on `Up`).
        Nothing,
        /// Capture the pointer, the drag-from-inside-a-surface shape.
        Capture,
        /// Claim focus, the text-field-inside-a-popover shape.
        Focus,
    }

    /// One floated surface the fixture's owner registers.
    #[derive(Clone, Debug, PartialEq)]
    struct SurfaceSpec {
        key: OverlayKey,
        label: &'static str,
        rect: Rect,
        band: crate::overlay::OverlayBand,
        input: OverlayInput,
        outside_tap: OutsideTap,
        /// Whether the owner registers it at all this frame — the fixture for
        /// "stop registering and the surface stops existing".
        register: bool,
        on_down: OwnerReaction,
        /// The same, for a `Move` inside the surface — a drag threshold latching
        /// a capture mid-gesture, the shape that claims one on no `Down` at all.
        on_move: OwnerReaction,
        /// The same again, for an `Up` — the phase on which a capture claim has
        /// nothing left to own, and the one that reaches the surface as an
        /// overlay event precisely because no capture was standing to divert it.
        on_up: OwnerReaction,
        /// Whether the leaf INSIDE the pod claims focus on a `Down` of its own —
        /// the editable-in-a-popover shape, distinct from `on_down`'s claim,
        /// which the owner makes on the surface's behalf.
        pod_claims_focus: bool,
        /// Whether that leaf republishes an active IME surface on every paint.
        /// Stated unconditionally, so the fixture can publish from a pod holding
        /// no focus link at all — the provenance case.
        pod_publishes_ime: bool,
        /// The same publish, from the leaf's `event` handler instead of its
        /// paint — the event-route half of the provenance case, and equally
        /// ungated.
        pod_publishes_ime_on_event: bool,
    }

    impl SurfaceSpec {
        fn floating(label: &'static str, rect: Rect) -> Self {
            Self {
                key: OverlayKey::next(),
                label,
                rect,
                band: crate::overlay::OverlayBand::Floating,
                input: OverlayInput::Interactive,
                outside_tap: OutsideTap::Ignore,
                register: true,
                on_down: OwnerReaction::Nothing,
                on_move: OwnerReaction::Nothing,
                on_up: OwnerReaction::Nothing,
                pod_claims_focus: false,
                pod_publishes_ime: false,
                pod_publishes_ime_on_event: false,
            }
        }
    }

    /// Everything the overlay fixture recorded, shared between the widgets and
    /// the test.
    #[derive(Default)]
    struct OverlayLog {
        /// Events the owner received, in the order they arrived.
        owner: Vec<InputEvent>,
        /// `(surface label, event)` for everything a floated pod's content saw —
        /// positions here are the pod's own local space.
        pod: Vec<(&'static str, InputEvent)>,
        /// Events the main-tree sibling received.
        sibling: Vec<InputEvent>,
        /// What the sibling read from `EventCtx::has_focus` on each event.
        sibling_focus: Vec<bool>,
        /// What the pod's leaf read from `PaintCtx::has_focus` on each paint.
        pod_paint_focus: Vec<bool>,
        /// The same read for the main-tree sibling. The two together answer
        /// "which branches believe they are focused".
        sibling_paint_focus: Vec<bool>,
        /// The window size the owner observed in its own (child) layout.
        child_window_size: Option<Size>,
    }

    type Log = std::rc::Rc<std::cell::RefCell<OverlayLog>>;

    /// The leaf inside a floated pod: paints its label at the pod's absolute
    /// origin and records what reaches it.
    ///
    /// Optionally an editable — it claims focus on a `Down` of its own and
    /// republishes an IME surface on every paint. The publish is deliberately
    /// ungated: a pod describing a session it holds no focus link for is
    /// precisely what the root has to answer for.
    struct OverlayContentWidget {
        label: &'static str,
        log: Log,
        claims_focus: bool,
        publishes_ime: bool,
        publishes_ime_on_event: bool,
    }

    impl OverlayContentWidget {
        /// What such a pod publishes: an ordinary field's surface, carrying no
        /// secure-text configuration of its own.
        fn surface() -> ImeState {
            ImeState {
                active: true,
                content_type: ImeContentType::Normal,
                ..Default::default()
            }
        }
    }

    impl crate::widget::Widget for OverlayContentWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(80.0, 30.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            scene.draw_text(ctx.origin(), self.label);
            self.log.borrow_mut().pod_paint_focus.push(ctx.has_focus());
            if self.publishes_ime {
                ctx.publish_ime_state(Self::surface());
            }
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            self.log.borrow_mut().pod.push((self.label, event.clone()));
            if self.publishes_ime_on_event {
                ctx.publish_ime_state(Self::surface());
            }
            if self.claims_focus
                && let InputEvent::Pointer(pointer) = event
                && pointer.phase == PointerPhase::Down
            {
                ctx.request_focus();
            }
            EventResult::Handled
        }
    }

    /// One live surface: its spec plus the pod the owner keeps.
    struct OwnedSurface {
        spec: SurfaceSpec,
        pod: crate::overlay::OverlayPod,
    }

    /// The overlay owner: hosts the pods, registers them from `paint`, and
    /// forwards the broadcasts the root routes back to it into the right pod.
    struct OverlayOwnerWidget {
        surfaces: Vec<OwnedSurface>,
        log: Log,
    }

    impl OverlayOwnerWidget {
        fn new(specs: &[SurfaceSpec], log: Log) -> Self {
            let mut owner = Self {
                surfaces: Vec::new(),
                log,
            };
            owner.sync(specs);
            owner
        }

        /// Reconcile the live surfaces against `specs`, keeping each pod alive
        /// across a rebuild (a real owner keeps its popover's widget state).
        fn sync(&mut self, specs: &[SurfaceSpec]) {
            self.surfaces
                .retain(|s| specs.iter().any(|n| n.key == s.spec.key));
            for spec in specs {
                match self.surfaces.iter_mut().find(|s| s.spec.key == spec.key) {
                    Some(live) => live.spec = spec.clone(),
                    None => {
                        let log = std::rc::Rc::clone(&self.log);
                        self.surfaces.push(OwnedSurface {
                            spec: spec.clone(),
                            pod: std::rc::Rc::new(std::cell::RefCell::new(
                                crate::widget::ChildPod::new(Box::new(OverlayContentWidget {
                                    label: spec.label,
                                    log,
                                    claims_focus: spec.pod_claims_focus,
                                    publishes_ime: spec.pod_publishes_ime,
                                    publishes_ime_on_event: spec.pod_publishes_ime_on_event,
                                })),
                            )),
                        });
                    }
                }
            }
        }
    }

    impl crate::widget::Widget for OverlayOwnerWidget {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            // The window size reaches a CHILD's layout unchanged — this widget is
            // a `ChildPod` of the fixture's root.
            self.log.borrow_mut().child_window_size = Some(ctx.window_size());
            // A floated pod is laid out against the WINDOW, never against the
            // owner's own constraints: it escapes the owner's box entirely.
            let window = BoxConstraints::loose(ctx.window_size());
            for surface in &mut self.surfaces {
                surface.pod.borrow_mut().layout_child(ctx, &window);
            }
            bc.constrain(Size::new(40.0, 20.0))
        }

        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            scene.draw_text(ctx.origin(), "owner");
            for surface in &self.surfaces {
                if !surface.spec.register {
                    continue;
                }
                // Registered, never painted here: the root paints it last.
                ctx.register_overlay(OverlayEntry {
                    key: surface.spec.key,
                    band: surface.spec.band,
                    input: surface.spec.input,
                    outside_tap: surface.spec.outside_tap,
                    window_rect: surface.spec.rect,
                    pod: std::rc::Rc::clone(&surface.pod),
                    insets: ctx.window_insets(),
                });
            }
        }

        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            let InputEvent::Overlay(overlay) = event else {
                self.log.borrow_mut().owner.push(event.clone());
                return EventResult::Ignored;
            };
            let Some(surface) = self.surfaces.iter_mut().find(|s| s.spec.key == overlay.key) else {
                // Another owner's surface: the broadcast reached us, and we
                // ignore it. This fall-through is the whole addressing rule.
                return EventResult::Ignored;
            };
            self.log.borrow_mut().owner.push(event.clone());
            match &overlay.kind {
                OverlayEventKind::Pointer(pointer) => {
                    let reaction = match pointer.phase {
                        PointerPhase::Down => Some(surface.spec.on_down),
                        PointerPhase::Move => Some(surface.spec.on_move),
                        PointerPhase::Up => Some(surface.spec.on_up),
                        _ => None,
                    };
                    match reaction {
                        Some(OwnerReaction::Capture) => ctx.capture_pointer(),
                        Some(OwnerReaction::Focus) => ctx.request_focus(),
                        Some(OwnerReaction::Nothing) | None => {}
                    }
                    // Window space → the pod's own space is one subtraction: the
                    // registered rect's origin.
                    let local = InputEvent::Pointer(PointerEvent {
                        position: pointer.position - surface.spec.rect.origin().to_vec2(),
                        ..*pointer
                    });
                    surface.pod.borrow_mut().event_child(ctx, &local);
                }
                OverlayEventKind::Scroll { position, delta } => {
                    let local = InputEvent::Scroll {
                        position: *position - surface.spec.rect.origin().to_vec2(),
                        delta: *delta,
                    };
                    surface.pod.borrow_mut().event_child(ctx, &local);
                }
                OverlayEventKind::Scale {
                    focal,
                    phase,
                    scale_delta,
                    velocity,
                } => {
                    let local = InputEvent::Scale(crate::event::ScaleEvent {
                        phase: *phase,
                        scale_delta: *scale_delta,
                        focal: *focal - surface.spec.rect.origin().to_vec2(),
                        velocity: *velocity,
                    });
                    surface.pod.borrow_mut().event_child(ctx, &local);
                }
                OverlayEventKind::OutsideDown => {}
            }
            // A broadcast is never consumed, whatever the pod returned.
            EventResult::Ignored
        }
    }

    /// The main-tree sibling: painted AFTER the owner, claims focus on a `Down`
    /// inside it, and records what it sees.
    ///
    /// It stands in for a secure text field: the surface it publishes carries
    /// [`ImeContentType::Password`], and it republishes that surface on every
    /// paint for as long as the pass seeds it focused — the shipped field's
    /// republish-and-self-correct shape, in miniature.
    struct SiblingLeafWidget {
        log: Log,
        /// The selection-toolbar request to publish each paint, if any.
        publish: Option<crate::selection_toolbar::SelectionToolbarRequest>,
    }

    impl SiblingLeafWidget {
        /// The secure surface this field describes its session with.
        fn surface() -> ImeState {
            ImeState {
                active: true,
                content_type: ImeContentType::Password,
                ..Default::default()
            }
        }
    }

    impl crate::widget::Widget for SiblingLeafWidget {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(60.0, 40.0))
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            scene.draw_text(ctx.origin(), "sibling");
            self.log
                .borrow_mut()
                .sibling_paint_focus
                .push(ctx.has_focus());
            if ctx.has_focus() {
                ctx.publish_ime_state(Self::surface());
            }
            if let Some(request) = self.publish {
                ctx.publish_selection_toolbar(request);
            }
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            {
                let mut log = self.log.borrow_mut();
                log.sibling.push(event.clone());
                log.sibling_focus.push(ctx.has_focus());
            }
            match event {
                InputEvent::Pointer(pointer) if pointer.phase == PointerPhase::Down => {
                    ctx.request_focus();
                    ctx.publish_ime_state(Self::surface());
                    EventResult::Handled
                }
                // A scroll is hit-tested like a press but travels the root's
                // keyboard-class focus arm (`Scroll | Key | Ime | EditCommand`),
                // so claiming focus here is how the fixture expresses a focus
                // move that is not a pointer `Down`.
                InputEvent::Scroll { .. } => {
                    ctx.request_focus();
                    ctx.publish_ime_state(Self::surface());
                    EventResult::Handled
                }
                _ => EventResult::Ignored,
            }
        }
    }

    /// The fixture's root: a hand-written two-child container routing exactly
    /// like `frust-widgets`' helpers — broadcast first, then capture, then focus,
    /// then a topmost-first hit test.
    struct OverlayRootWidget {
        owner: crate::widget::ChildPod,
        sibling: crate::widget::ChildPod,
    }

    impl crate::widget::Widget for OverlayRootWidget {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.owner.layout_child(ctx, bc);
            self.owner.set_origin(Point::new(0.0, 0.0));
            self.sibling.layout_child(ctx, bc);
            self.sibling.set_origin(Point::new(0.0, 100.0));
            bc.constrain(Size::new(200.0, 200.0))
        }

        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            // The owner paints FIRST, the sibling after it — so an overlay pod
            // landing after both proves the root's post-pass really is last.
            self.owner.paint_child(ctx, scene);
            self.sibling.paint_child(ctx, scene);
        }

        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            if event.is_broadcast() {
                self.owner.event_child(ctx, event);
                self.sibling.event_child(ctx, event);
                return EventResult::Ignored;
            }
            let ends = matches!(
                event,
                InputEvent::Pointer(p)
                    if matches!(p.phase, PointerPhase::Up | PointerPhase::Cancel)
            );
            for (pod, _) in [(&mut self.owner, 0), (&mut self.sibling, 1)] {
                if pod.is_active() {
                    let result = pod.event_child(ctx, event);
                    if ends {
                        pod.set_active(false);
                    }
                    return result;
                }
            }
            if event.is_focus_routed() {
                // `holds_live_focus`, not `is_focused` — the same read
                // `frust-widgets`' `route_event` makes, and the reason this
                // fixture can stand in for it: both children can carry a
                // recorded link at once, and only one of them can carry the live
                // session's.
                if self.sibling.holds_live_focus() {
                    return self.sibling.event_child(ctx, event);
                }
                if self.owner.holds_live_focus() {
                    return self.owner.event_child(ctx, event);
                }
                return EventResult::Ignored;
            }
            // Topmost-first: the sibling paints last, so it hit-tests first.
            let position = event.position();
            if self.sibling.contains(position) {
                return self.sibling.event_child(ctx, event);
            }
            if self.owner.contains(position) {
                return self.owner.event_child(ctx, event);
            }
            EventResult::Ignored
        }
    }

    /// The view producing the fixture, reconciling the surface specs in place.
    struct OverlayRootView {
        specs: Vec<SurfaceSpec>,
        publish: Option<crate::selection_toolbar::SelectionToolbarRequest>,
        log: Log,
    }

    impl View<OverlayState> for OverlayRootView {
        type Element = OverlayRootWidget;

        fn build(&self, _ctx: &mut BuildCtx<'_>) -> Self::Element {
            OverlayRootWidget {
                owner: crate::widget::ChildPod::new(Box::new(OverlayOwnerWidget::new(
                    &self.specs,
                    std::rc::Rc::clone(&self.log),
                ))),
                sibling: crate::widget::ChildPod::new(Box::new(SiblingLeafWidget {
                    log: std::rc::Rc::clone(&self.log),
                    publish: self.publish,
                })),
            }
        }

        fn rebuild(
            &self,
            prev: &Self,
            element: &mut Self::Element,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            element
                .owner
                .widget_mut()
                .downcast_mut::<OverlayOwnerWidget>()
                .expect("the owner keeps its type")
                .sync(&self.specs);
            element
                .sibling
                .widget_mut()
                .downcast_mut::<SiblingLeafWidget>()
                .expect("the sibling keeps its type")
                .publish = self.publish;
            if prev.specs != self.specs || prev.publish != self.publish {
                ChangeFlags::PAINT
            } else {
                ChangeFlags::NONE
            }
        }
    }

    /// App state for the overlay fixture: the surface specs the next rebuild
    /// applies, plus the shared log.
    struct OverlayState {
        specs: Vec<SurfaceSpec>,
        publish: Option<crate::selection_toolbar::SelectionToolbarRequest>,
        log: Log,
    }

    fn overlay_logic(state: &mut OverlayState) -> OverlayRootView {
        OverlayRootView {
            specs: state.specs.clone(),
            publish: state.publish,
            log: std::rc::Rc::clone(&state.log),
        }
    }

    /// Drives the overlay fixture the way a shell does: rebuild, layout, paint,
    /// then dispatch.
    struct OverlayHarness {
        root: RenderRoot<OverlayState, OverlayRootView>,
        state: OverlayState,
        log: Log,
    }

    impl OverlayHarness {
        fn new(specs: Vec<SurfaceSpec>) -> Self {
            let log: Log = std::rc::Rc::new(std::cell::RefCell::new(OverlayLog::default()));
            let mut harness = Self {
                root: RenderRoot::new(),
                state: OverlayState {
                    specs,
                    publish: None,
                    log: std::rc::Rc::clone(&log),
                },
                log,
            };
            harness.frame();
            harness
        }

        /// One full frame: rebuild, layout at a 200x200 window, paint.
        fn frame(&mut self) -> RecordingScene {
            self.root.rebuild(&mut overlay_logic, &mut self.state);
            self.root.layout(Size::new(200.0, 200.0));
            let mut scene = RecordingScene::default();
            self.root.paint(&mut scene, FrameTime::ZERO);
            scene
        }

        fn dispatch(&mut self, event: &InputEvent) -> EventOutcome {
            self.root.event(&mut self.state, event)
        }

        fn down(&mut self, x: f64, y: f64) -> EventOutcome {
            self.dispatch(&pointer(PointerPhase::Down, x, y))
        }

        fn clear_log(&mut self) {
            let mut log = self.log.borrow_mut();
            log.owner.clear();
            log.pod.clear();
            log.sibling.clear();
            log.sibling_focus.clear();
            log.pod_paint_focus.clear();
            log.sibling_paint_focus.clear();
        }

        /// What the pod's leaf and the main-tree sibling each read from
        /// `PaintCtx::has_focus` on the most recent frame — "which branches
        /// believe they are focused", read from the branches themselves.
        fn branch_focus(&self) -> (bool, bool) {
            let log = self.log.borrow();
            (
                *log.pod_paint_focus
                    .last()
                    .expect("the pod painted at least once"),
                *log.sibling_paint_focus
                    .last()
                    .expect("the sibling painted at least once"),
            )
        }

        /// The overlay events the owner received, as `(key, kind)`.
        fn owner_overlays(&self) -> Vec<(OverlayKey, OverlayEventKind)> {
            self.log
                .borrow()
                .owner
                .iter()
                .filter_map(|event| match event {
                    InputEvent::Overlay(o) => Some((o.key, o.kind.clone())),
                    _ => None,
                })
                .collect()
        }

        /// Pointer events the sibling saw — the "did the main tree get it?" read.
        fn sibling_pointers(&self) -> Vec<PointerEvent> {
            self.log
                .borrow()
                .sibling
                .iter()
                .filter_map(|event| match event {
                    InputEvent::Pointer(p) => Some(*p),
                    _ => None,
                })
                .collect()
        }
    }

    /// A rect well clear of the sibling (which sits at y >= 100).
    fn floating_rect() -> Rect {
        Rect::new(120.0, 10.0, 200.0, 60.0)
    }

    #[test]
    fn a_registered_pod_paints_after_a_later_sibling_at_its_window_rect() {
        let spec = SurfaceSpec::floating("popover", floating_rect());
        let mut h = OverlayHarness::new(vec![spec.clone()]);
        let scene = h.frame();

        assert_eq!(
            scene
                .texts
                .iter()
                .map(|(_, text)| text.as_str())
                .collect::<Vec<_>>(),
            vec!["owner", "sibling", "popover"],
            "the floated pod paints after the owner AND after the sibling painted \
             later than the owner — escaping paint order is the whole point"
        );
        assert_eq!(
            scene.texts[2].0,
            floating_rect().origin(),
            "and it paints at its registered window rect, not at its owner's origin"
        );
        assert_eq!(h.root.overlay_hits.len(), 1);
        assert_eq!(h.root.overlay_hits[0].key, spec.key);
        assert_eq!(h.root.overlay_hits[0].window_rect, floating_rect());
    }

    #[test]
    fn two_bands_paint_floating_then_tooltip_whatever_the_registration_order() {
        // Registered tooltip-first, so registration order and band order
        // disagree: the band must win.
        let mut tooltip = SurfaceSpec::floating("tooltip", Rect::new(0.0, 0.0, 40.0, 20.0));
        tooltip.band = crate::overlay::OverlayBand::Tooltip;
        tooltip.input = OverlayInput::Transparent;
        let floating = SurfaceSpec::floating("floating", floating_rect());
        let mut h = OverlayHarness::new(vec![tooltip, floating]);
        let scene = h.frame();

        assert_eq!(
            scene
                .texts
                .iter()
                .map(|(_, text)| text.as_str())
                .collect::<Vec<_>>(),
            vec!["owner", "sibling", "floating", "tooltip"],
            "Floating paints below Tooltip regardless of who registered first"
        );
    }

    #[test]
    fn the_registry_is_empty_at_the_start_of_every_paint() {
        let spec = SurfaceSpec::floating("popover", floating_rect());
        let mut h = OverlayHarness::new(vec![spec.clone()]);
        h.frame();
        assert_eq!(h.root.overlay_hits.len(), 1);

        // Paint again with nothing changed: the entry is re-registered, not
        // accumulated — an owner registering every frame must not grow the table.
        let scene = h.frame();
        assert_eq!(h.root.overlay_hits.len(), 1);
        assert_eq!(scene.texts.len(), 3);

        // The owner stops registering (its popover closed). There is nothing to
        // unregister: the next paint simply does not see it.
        h.state.specs[0].register = false;
        let scene = h.frame();
        assert!(
            h.root.overlay_hits.is_empty(),
            "a surface nobody registers stops existing after the next paint"
        );
        assert_eq!(
            scene
                .texts
                .iter()
                .map(|(_, text)| text.as_str())
                .collect::<Vec<_>>(),
            vec!["owner", "sibling"],
            "and stops painting"
        );

        // A `Down` inside where it used to be now reaches the main tree.
        h.clear_log();
        h.down(150.0, 30.0);
        assert!(h.owner_overlays().is_empty());
    }

    #[test]
    fn the_window_size_reaches_a_childs_layout() {
        let h = OverlayHarness::new(vec![SurfaceSpec::floating("popover", floating_rect())]);
        assert_eq!(
            h.log.borrow().child_window_size,
            Some(Size::new(200.0, 200.0)),
            "a child lays out knowing the window, which is what an overlay pod is \
             sized against"
        );
    }

    #[test]
    fn a_down_inside_a_floating_surface_reaches_only_its_owner_and_never_blurs() {
        let spec = SurfaceSpec::floating("popover", floating_rect());
        let mut h = OverlayHarness::new(vec![spec.clone()]);

        // A field elsewhere in the main tree takes focus first.
        h.down(30.0, 120.0);
        assert!(
            h.root.is_focus_active(),
            "the sibling holds a focus session"
        );
        let ime_before = h.root.ime_state();
        assert!(ime_before.is_some());
        let focus_gen_before = h.root.focus_ime_generation();
        h.clear_log();

        // Now press inside the floated surface.
        h.down(150.0, 30.0);

        assert_eq!(
            h.owner_overlays(),
            vec![(
                spec.key,
                OverlayEventKind::Pointer(PointerEvent {
                    phase: PointerPhase::Down,
                    position: Point::new(150.0, 30.0),
                    button: PointerButton::Primary,
                })
            )],
            "the owner is reached by key, with a WINDOW-space payload"
        );
        assert_eq!(
            h.log.borrow().pod,
            vec![(
                "popover",
                InputEvent::Pointer(PointerEvent {
                    phase: PointerPhase::Down,
                    // 150-120, 30-10: the owner's one subtraction, the rect origin.
                    position: Point::new(30.0, 20.0),
                    button: PointerButton::Primary,
                })
            )],
            "and the owner forwards it into the pod in the pod's own space"
        );
        assert!(
            h.sibling_pointers().is_empty(),
            "the main tree never saw the press"
        );
        assert!(
            h.root.is_focus_active(),
            "and the press did NOT blur the field the surface belongs to"
        );
        assert_eq!(
            h.root.ime_state(),
            ime_before,
            "nor disturb its IME surface"
        );
        assert_eq!(
            h.root.focus_ime_generation(),
            focus_gen_before,
            "so no focus/IME edge fires at the shell either"
        );
        assert_eq!(
            h.log.borrow().sibling_focus.last(),
            Some(&true),
            "the focused leaf still reads as focused when the broadcast reaches it"
        );
    }

    #[test]
    fn a_down_inside_a_transparent_surface_reaches_the_main_tree_normally() {
        // The transparent surface covers the sibling exactly.
        let mut spec = SurfaceSpec::floating("tooltip", Rect::new(0.0, 100.0, 60.0, 140.0));
        spec.input = OverlayInput::Transparent;
        spec.outside_tap = OutsideTap::Notify { consume: true };
        let mut h = OverlayHarness::new(vec![spec]);
        h.frame();
        h.clear_log();

        h.down(30.0, 120.0);

        assert!(
            h.owner_overlays().is_empty(),
            "a transparent surface is never hit-tested — not even for OutsideDown"
        );
        assert_eq!(
            h.sibling_pointers().len(),
            1,
            "the pointer passed straight through to the widget underneath"
        );
        assert!(h.root.is_focus_active(), "which claimed focus as usual");
    }

    #[test]
    fn a_scroll_inside_a_surface_routes_to_its_owner_in_window_space() {
        let spec = SurfaceSpec::floating("popover", floating_rect());
        let mut h = OverlayHarness::new(vec![spec.clone()]);
        h.clear_log();

        h.dispatch(&InputEvent::Scroll {
            position: Point::new(150.0, 30.0),
            delta: ScrollDelta::Lines(0.0, 3.0),
        });

        assert_eq!(
            h.owner_overlays(),
            vec![(
                spec.key,
                OverlayEventKind::Scroll {
                    position: Point::new(150.0, 30.0),
                    delta: ScrollDelta::Lines(0.0, 3.0),
                }
            )]
        );
        assert_eq!(
            h.log.borrow().pod,
            vec![(
                "popover",
                InputEvent::Scroll {
                    position: Point::new(30.0, 20.0),
                    delta: ScrollDelta::Lines(0.0, 3.0),
                }
            )]
        );
    }

    #[test]
    fn a_capture_from_inside_a_surface_routes_the_next_move_by_the_capture_path() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.on_down = OwnerReaction::Capture;
        let mut h = OverlayHarness::new(vec![spec.clone()]);
        h.clear_log();

        h.down(150.0, 30.0);
        assert!(
            h.root.is_pointer_captured(),
            "a capture bubbled from an overlay Down opens a gesture exactly like a \
             hit-tested one"
        );
        h.clear_log();

        // Still INSIDE the surface's own rect — which is the case that
        // discriminates: the pre-pass would happily hit-test this one and route it
        // as another broadcast, and it must not, because the gesture is captured.
        h.dispatch(&pointer(PointerPhase::Move, 160.0, 45.0));
        assert!(
            h.owner_overlays().is_empty(),
            "a live capture short-circuits the overlay pre-pass even inside the \
             surface's own rect"
        );
        assert_eq!(
            h.log.borrow().owner,
            vec![InputEvent::Pointer(PointerEvent {
                phase: PointerPhase::Move,
                position: Point::new(160.0, 45.0),
                button: PointerButton::Primary,
            })],
            "the move reaches the owner by the ordinary captured path instead"
        );
        h.clear_log();

        // And the drag may wander far outside the surface — over the sibling, in
        // fact — without the sibling ever hearing about it.
        h.dispatch(&pointer(PointerPhase::Move, 30.0, 120.0));

        assert!(h.owner_overlays().is_empty());
        assert_eq!(
            h.log.borrow().owner,
            vec![InputEvent::Pointer(PointerEvent {
                phase: PointerPhase::Move,
                position: Point::new(30.0, 120.0),
                button: PointerButton::Primary,
            })],
            "which is what lets a drag begun inside a floated surface continue \
             outside it"
        );
        assert!(
            h.sibling_pointers().is_empty(),
            "and never reaches the widget it passed over"
        );

        // The `Up` closes the gesture through the ordinary pointer arm.
        h.dispatch(&pointer(PointerPhase::Up, 30.0, 120.0));
        assert!(!h.root.is_pointer_captured());
    }

    /// A capture claimed on a phase other than `Down` is a capture all the
    /// same. The pods record their own active path on any phase, so a root that
    /// mirrored the `Down` alone left the surface latched with no `Up` able to
    /// reach it — and a latched surface diverts every later pointer event its
    /// owner is reached by.
    #[test]
    fn a_capture_claimed_on_a_move_inside_a_surface_reaches_the_root() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.on_move = OwnerReaction::Capture;
        let mut h = OverlayHarness::new(vec![spec]);

        h.dispatch(&pointer(PointerPhase::Move, 150.0, 30.0));
        assert!(
            h.root.is_pointer_captured(),
            "the root mirrors a claim the surface made mid-gesture"
        );

        // Which is what ends it: the mirrored capture short-circuits the overlay
        // pre-pass, so the `Up` routes down the capture path — outside the
        // surface's own rect, where a hit test would never have delivered it.
        h.dispatch(&pointer(PointerPhase::Up, 30.0, 120.0));
        assert!(
            !h.root.is_pointer_captured(),
            "and the gesture closes on the ordinary pointer arm"
        );
    }

    /// The release side of the root's overlay capture mirror. The claim is
    /// mirrored on any phase, so the release has to be bounded on any phase too:
    /// an `Up` reaches a surface as an overlay event exactly when no capture was
    /// standing to divert it, and a claim made there has no gesture left to own.
    /// A latch left standing short-circuits the overlay pre-pass, which stops
    /// every floated surface taking input until some unrelated pointer release
    /// happens along.
    #[test]
    fn a_capture_claimed_on_an_overlay_up_does_not_outlive_the_gesture() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.on_up = OwnerReaction::Capture;
        let mut h = OverlayHarness::new(vec![spec]);

        h.down(150.0, 30.0);
        h.dispatch(&pointer(PointerPhase::Up, 150.0, 30.0));

        assert!(
            !h.root.is_pointer_captured(),
            "the phase that ends a gesture releases the mirror it just set"
        );

        // Which is what keeps the surfaces routable: a press inside one still
        // reaches its owner rather than being diverted down a capture path.
        h.clear_log();
        h.down(150.0, 30.0);
        assert_eq!(
            h.owner_overlays().len(),
            1,
            "the overlay pre-pass is still hit-testing"
        );
    }

    #[test]
    fn a_focus_request_from_inside_a_surface_opens_a_session() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.on_down = OwnerReaction::Focus;
        let mut h = OverlayHarness::new(vec![spec]);
        assert!(!h.root.is_focus_active());

        h.down(150.0, 30.0);

        assert!(
            h.root.is_focus_active(),
            "a text field inside a popover may claim focus — the overlay arm \
             honours the request even though it refuses the blur"
        );
    }

    /// The case the test above cannot express: the claim arrives while ANOTHER
    /// branch already holds the session. Honouring it without retiring what it
    /// supersedes leaves two branches believing they are focused — and leaves
    /// the root describing, to the shell, a field that no longer owns anything.
    #[test]
    fn a_pod_focus_claim_retires_the_branch_it_supersedes() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.pod_claims_focus = true;
        let mut h = OverlayHarness::new(vec![spec]);

        // A secure field in the main tree takes the session first.
        h.down(30.0, 120.0);
        h.frame();
        assert_eq!(
            h.branch_focus(),
            (false, true),
            "only the field's own branch reads focused while it holds the session"
        );
        assert_eq!(
            h.root.ime_state().map(|ime| ime.content_type),
            Some(ImeContentType::Password),
            "and the surface the shell configures its keyboard from is the \
             secure one that field published"
        );

        // Now the editable inside the floated surface claims focus.
        h.down(150.0, 30.0);
        assert!(h.root.is_focus_active(), "the claim is honoured");

        // The claim stamped its own chain with a session identity nothing older
        // carries, so the field's link stops counting on the very next pass —
        // no convergence frame, and nothing had to visit the branch the session
        // left in order to clear it.
        h.frame();
        assert_eq!(
            h.branch_focus(),
            (true, false),
            "exactly one branch believes it is focused: the one that claimed it"
        );
        assert_eq!(
            h.root.ime_state(),
            None,
            "and the surface that field published for the session it just lost \
             does NOT stand: the branch that took the session described none of \
             its own, so the shell is left configuring nothing rather than a \
             secure field nobody is in"
        );

        h.frame();
        assert_eq!(
            h.branch_focus(),
            (true, false),
            "which is a settled state, not a frame of transition"
        );
        assert!(
            h.root.is_focus_active(),
            "and the session the pod opened is still the live one"
        );
    }

    /// The return direction, which the test above never exercises: the session
    /// goes to a floated surface and the user then presses the field in the main
    /// tree again.
    ///
    /// A press outside every floated rect is hit-tested through the containers
    /// like any other, so the claim it produces is the main tree's by
    /// construction — and the surface's own recorded link has to stop counting
    /// the moment that claim lands, or the tree is de-seeded for the rest of the
    /// pod's life and the secure surface the field publishes is thrown away
    /// every frame.
    #[test]
    fn a_hit_tested_claim_takes_the_session_back_from_a_floated_surface() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.pod_claims_focus = true;
        let mut h = OverlayHarness::new(vec![spec]);

        // The field takes the session, then the editable inside the surface
        // takes it away.
        h.down(30.0, 120.0);
        h.frame();
        h.down(150.0, 30.0);
        h.frame();
        assert_eq!(
            h.branch_focus(),
            (true, false),
            "the surface holds the session and the field's link no longer counts"
        );

        // The user presses the field again, outside `floating_rect()`, so the
        // press routes through the containers exactly as the first one did.
        h.down(30.0, 120.0);
        h.frame();

        assert_eq!(
            h.branch_focus(),
            (false, true),
            "the session is the field's again and the surface's link is retired"
        );
        assert_eq!(
            h.root.ime_state().map(|ime| ime.content_type),
            Some(ImeContentType::Password),
            "and the secure surface that field publishes reaches the shell"
        );
    }

    /// The same move, driven from the root's keyboard-class arm
    /// (`Scroll | Key | Ime | EditCommand`) rather than a pointer `Down`. That
    /// arm honours the claim and nothing else, so a mechanism that only retires
    /// a surface's link on a press leaves the session split here.
    #[test]
    fn a_keyboard_class_claim_in_the_tree_retires_a_surfaces_link() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.pod_claims_focus = true;
        let mut h = OverlayHarness::new(vec![spec]);

        h.down(150.0, 30.0);
        h.frame();
        assert!(h.branch_focus().0, "the surface holds the session");

        // A scroll over the field, outside every floated rect: hit-tested into
        // the main tree, but routed through the root's non-pointer focus arm.
        h.dispatch(&InputEvent::Scroll {
            position: Point::new(30.0, 120.0),
            delta: ScrollDelta::Lines(0.0, 3.0),
        });
        h.frame();

        assert_eq!(
            h.branch_focus(),
            (false, true),
            "a claim that arrives without a pointer `Down` retires the surface's \
             link just the same"
        );
        assert_eq!(
            h.root.ime_state().map(|ime| ime.content_type),
            Some(ImeContentType::Password),
        );
    }

    /// A pod dropped while it holds the link takes the session with it. Nothing
    /// else can end that session: the widget that owned it no longer exists, so
    /// no later pass can reach it to release it, and the root would otherwise go
    /// on reporting a live focus session to the shell for a surface nobody can
    /// see.
    #[test]
    fn a_pod_dropped_while_it_holds_the_link_ends_the_session_it_owned() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.pod_claims_focus = true;
        let mut h = OverlayHarness::new(vec![spec]);

        h.down(150.0, 30.0);
        h.frame();
        assert!(h.root.is_focus_active(), "the surface holds the session");

        // The owner stops owning the surface: the spec goes, the pod is dropped
        // by the next rebuild.
        h.state.specs.clear();
        h.frame();

        assert!(
            !h.root.is_focus_active(),
            "the session dies with the widget that held it"
        );
        assert_eq!(h.root.ime_state(), None);

        // And the tree is not left de-seeded: the next press focuses normally.
        h.down(30.0, 120.0);
        h.frame();
        assert!(
            h.branch_focus().1,
            "a field in the tree takes the session as if no surface had existed"
        );
        assert_eq!(
            h.root.ime_state().map(|ime| ime.content_type),
            Some(ImeContentType::Password),
        );
    }

    /// The event-route half of the provenance rule. `paint_overlays` refuses a
    /// paint-time publish from a pod holding no link; the dispatch that carries
    /// an overlay broadcast has to refuse the same publish, or a surface the
    /// user merely touched reconfigures the platform keyboard for a secure field
    /// it has nothing to do with.
    #[test]
    fn a_pod_publish_from_an_event_cannot_overwrite_a_field_it_does_not_own() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        // Publishes from its `event`, and never claims focus.
        spec.pod_publishes_ime_on_event = true;
        let mut h = OverlayHarness::new(vec![spec]);

        h.down(30.0, 120.0);
        assert_eq!(
            h.root.ime_state().map(|ime| ime.content_type),
            Some(ImeContentType::Password),
            "the focused field's own surface"
        );

        // A press inside the surface. The pod publishes, holds no focus link,
        // and claims none.
        h.down(150.0, 30.0);

        assert_eq!(
            h.root.ime_state().map(|ime| ime.content_type),
            Some(ImeContentType::Password),
            "a publish from a branch that owns no session describes nobody's"
        );
        assert!(
            h.root.is_focus_active(),
            "and the field it did not own still holds the session"
        );
    }

    /// A pod publishing an IME surface it holds no focus link for describes
    /// nobody's session. Accepting it would let a surface overwrite a focused
    /// field's — including the content type that configures the platform
    /// keyboard as a secure one.
    #[test]
    fn a_pod_publish_cannot_overwrite_the_surface_of_a_field_it_does_not_own() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        // Publishes on every paint, and never claims focus.
        spec.pod_publishes_ime = true;
        let mut h = OverlayHarness::new(vec![spec]);

        h.down(30.0, 120.0);
        assert_eq!(
            h.root.ime_state().map(|ime| ime.content_type),
            Some(ImeContentType::Password),
            "the focused field's own surface"
        );

        h.frame();

        assert!(
            h.root.is_focus_active(),
            "the field still holds the session"
        );
        assert_eq!(
            h.root.ime_state().map(|ime| ime.content_type),
            Some(ImeContentType::Password),
            "and it still describes it: the publishing pod holds no focus link, \
             so its surface is not the session's"
        );
    }

    /// The provenance rule is a check, not a refusal: a pod that DOES hold the
    /// recorded focus path publishes exactly like a field in the tree.
    #[test]
    fn a_pod_that_holds_the_focus_path_publishes_its_own_surface() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.pod_claims_focus = true;
        spec.pod_publishes_ime = true;
        let mut h = OverlayHarness::new(vec![spec]);

        h.down(30.0, 120.0);
        h.down(150.0, 30.0);
        h.frame();

        assert!(h.root.is_focus_active());
        assert_eq!(
            h.root.ime_state().map(|ime| ime.content_type),
            Some(ImeContentType::Normal),
            "the surface published by the branch that owns the session"
        );
    }

    #[test]
    fn an_outside_press_notifies_a_consuming_surface_and_stops_there() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.outside_tap = OutsideTap::Notify { consume: true };
        let mut h = OverlayHarness::new(vec![spec.clone()]);
        h.clear_log();

        // Inside the sibling, outside every floated rect.
        h.down(30.0, 120.0);

        assert_eq!(
            h.owner_overlays(),
            vec![(spec.key, OverlayEventKind::OutsideDown)],
            "the light-dismiss notification carries no position"
        );
        assert!(
            h.sibling_pointers().is_empty(),
            "and the press that dismissed the menu did not also activate what was \
             underneath it"
        );
        assert!(
            !h.root.is_focus_active(),
            "the main tree saw no Down at all, so nothing claimed focus"
        );
    }

    #[test]
    fn a_pass_through_outside_press_notifies_and_still_reaches_the_main_tree() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.outside_tap = OutsideTap::Notify { consume: false };
        let mut h = OverlayHarness::new(vec![spec.clone()]);
        h.clear_log();

        let outcome = h.down(30.0, 120.0);

        assert_eq!(
            h.owner_overlays(),
            vec![(spec.key, OverlayEventKind::OutsideDown)]
        );
        assert_eq!(
            h.sibling_pointers().len(),
            1,
            "consume: false means both — the owner hears, and the press continues"
        );
        assert!(h.root.is_focus_active(), "so the tapped field took focus");
        assert!(
            outcome.handled,
            "and the main dispatch's own outcome survives"
        );
    }

    #[test]
    fn an_ignoring_surface_hears_nothing_about_an_outside_press() {
        // `Ignore` is the default; state it explicitly.
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.outside_tap = OutsideTap::Ignore;
        let mut h = OverlayHarness::new(vec![spec]);
        h.clear_log();

        h.down(30.0, 120.0);

        assert!(
            h.owner_overlays().is_empty(),
            "a surface that dismisses some other way is never told"
        );
        assert_eq!(h.sibling_pointers().len(), 1);
    }

    #[test]
    fn only_a_primary_press_dismisses() {
        let mut spec = SurfaceSpec::floating("popover", floating_rect());
        spec.outside_tap = OutsideTap::Notify { consume: true };
        let mut h = OverlayHarness::new(vec![spec]);
        h.clear_log();

        // A secondary press is a context gesture, not a dismissal.
        h.dispatch(&InputEvent::Pointer(PointerEvent {
            phase: PointerPhase::Down,
            position: Point::new(30.0, 120.0),
            button: PointerButton::Secondary,
        }));
        assert!(h.owner_overlays().is_empty());
        assert_eq!(h.sibling_pointers().len(), 1, "and it reaches the tree");

        // Nor does a move or a lift outside the surface.
        h.clear_log();
        h.dispatch(&pointer(PointerPhase::Move, 30.0, 120.0));
        h.dispatch(&pointer(PointerPhase::Up, 30.0, 120.0));
        assert!(h.owner_overlays().is_empty());
    }

    #[test]
    fn an_overlay_press_leaves_the_main_trees_hover_standing() {
        let spec = SurfaceSpec::floating("popover", floating_rect());
        let mut h = OverlayHarness::new(vec![spec]);
        let epoch_before = h.root.hover_epoch;

        h.down(150.0, 30.0);

        assert_eq!(
            h.root.hover_epoch, epoch_before,
            "an overlay pass advances no hover epoch, so a live hover in the main \
             tree is not stranded by a press on a floated surface"
        );
    }

    // ---------------------------------------------------------------------
    // Selection toolbar: publish, generation, clear.
    // ---------------------------------------------------------------------

    /// A request with the menu up, anchored at `x` — the shape a field
    /// publishes while its bar stands.
    fn toolbar_request(x: f64) -> crate::selection_toolbar::SelectionToolbarRequest {
        crate::selection_toolbar::SelectionToolbarRequest {
            anchor: Rect::new(x, 100.0, x + 50.0, 120.0),
            actions: crate::selection_toolbar::SelectionToolbarActions {
                copy: true,
                cut: true,
                paste: false,
                select_all: true,
            },
            present_menu: true,
        }
    }

    /// The same request with no menu wanted — what a focused field publishes
    /// with nothing on screen, so the platform can still answer "may I offer
    /// Copy?" for a hardware shortcut.
    fn toolbar_level(x: f64) -> crate::selection_toolbar::SelectionToolbarRequest {
        crate::selection_toolbar::SelectionToolbarRequest {
            present_menu: false,
            ..toolbar_request(x)
        }
    }

    #[test]
    fn a_selection_toolbar_publish_resolves_and_only_a_menu_edge_moves_the_generation() {
        let mut h = OverlayHarness::new(vec![]);
        // A toolbar describes the FOCUSED field's selection, so open a session
        // first — a publish with nothing focused describes nothing.
        h.down(30.0, 120.0);
        assert!(h.root.is_focus_active());

        h.state.publish = Some(toolbar_request(10.0));
        h.frame();
        assert_eq!(h.root.selection_toolbar(), Some(toolbar_request(10.0)));
        let first_gen = h.root.selection_toolbar_generation();
        assert!(first_gen > 0, "appearing is an edge");

        // The field republishes the same request every frame its selection
        // stands: that must not ask the shell to re-present the menu per vsync.
        h.frame();
        h.frame();
        assert_eq!(h.root.selection_toolbar(), Some(toolbar_request(10.0)));
        assert_eq!(h.root.selection_toolbar_generation(), first_gen);

        // A moved anchor is stored — a shell re-reads it to place a menu it
        // already has on screen — but it is NOT a menu edge. This assertion used
        // to read `first_gen + 1`: the anchor is recomputed every painted frame
        // and a drag that widens a selection moves it on every touch sample, so
        // bumping here asked the platform to re-present its menu per sample.
        h.state.publish = Some(toolbar_request(60.0));
        h.frame();
        assert_eq!(h.root.selection_toolbar(), Some(toolbar_request(60.0)));
        assert_eq!(
            h.root.selection_toolbar_generation(),
            first_gen,
            "an anchor following the selection is not a reason to re-present"
        );

        // A verb changing IS: the menu's own contents just changed.
        let mut fewer_verbs = toolbar_request(60.0);
        fewer_verbs.actions.select_all = false;
        h.state.publish = Some(fewer_verbs);
        h.frame();
        assert_eq!(h.root.selection_toolbar_generation(), first_gen + 1);

        // The field blurs: it stops publishing, and the menu goes away with
        // nothing retracted.
        h.state.publish = None;
        h.frame();
        assert_eq!(h.root.selection_toolbar(), None);
        assert_eq!(
            h.root.selection_toolbar_generation(),
            first_gen + 2,
            "going away is an edge too, or a shell never learns to dismiss"
        );

        // ...and staying away is not.
        h.frame();
        assert_eq!(h.root.selection_toolbar_generation(), first_gen + 2);
    }

    #[test]
    fn only_the_menu_flag_going_up_asks_a_shell_to_present() {
        let mut h = OverlayHarness::new(vec![]);
        h.down(30.0, 120.0);
        assert!(h.root.is_focus_active());

        // A focused field with no bar up publishes all the same: the verbs are
        // the answer a platform responder chain needs for a hardware shortcut
        // that arrives with nothing on screen.
        h.state.publish = Some(toolbar_level(10.0));
        h.frame();
        assert_eq!(h.root.selection_toolbar(), Some(toolbar_level(10.0)));
        let level_gen = h.root.selection_toolbar_generation();

        // Republishing the same level is not an edge, however many frames it
        // stands for.
        h.frame();
        h.frame();
        assert_eq!(h.root.selection_toolbar_generation(), level_gen);

        // The gesture fires and the field asks for a menu: THAT is the edge.
        h.state.publish = Some(toolbar_request(10.0));
        h.frame();
        assert_eq!(
            h.root.selection_toolbar_generation(),
            level_gen + 1,
            "the flag going false to true is what presents the menu"
        );

        // And dropping it again is the dismiss edge, with the field still
        // focused and still publishing its verbs.
        h.state.publish = Some(toolbar_level(10.0));
        h.frame();
        assert!(h.root.selection_toolbar().is_some());
        assert_eq!(h.root.selection_toolbar_generation(), level_gen + 2);
    }

    #[test]
    fn a_pass_that_publishes_nothing_still_moves_the_generation() {
        // `RenderRoot::paint` resolves a pass nobody published in to `None`,
        // which reads as "no menu, no verbs" and must differ from whatever
        // stood — otherwise a shell holding a presented menu never learns to
        // put it away.
        let mut h = OverlayHarness::new(vec![]);
        h.down(30.0, 120.0);
        h.state.publish = Some(toolbar_request(10.0));
        h.frame();
        let standing = h.root.selection_toolbar_generation();

        h.state.publish = None;
        h.frame();
        assert_eq!(h.root.selection_toolbar(), None);
        assert_eq!(h.root.selection_toolbar_generation(), standing + 1);
    }

    #[test]
    fn a_blur_clears_the_selection_toolbar_on_the_event_pass() {
        let mut h = OverlayHarness::new(vec![]);
        h.down(30.0, 120.0);
        h.state.publish = Some(toolbar_request(10.0));
        h.frame();
        assert!(h.root.selection_toolbar().is_some());
        let gen_before = h.root.selection_toolbar_generation();

        // A press on chrome that claims no focus ends the session — and the menu
        // must go with it immediately, not a frame later.
        h.down(150.0, 30.0);
        assert!(!h.root.is_focus_active());
        assert_eq!(
            h.root.selection_toolbar(),
            None,
            "a toolbar cannot outlive the focus session its selection belonged to"
        );
        assert_eq!(h.root.selection_toolbar_generation(), gen_before + 1);

        // And a field still publishing into the blurred session cannot resurrect
        // it — the same refusal the IME republish makes.
        h.frame();
        assert_eq!(h.root.selection_toolbar(), None);
    }
}

/// The root half of the multi-contact contract (`InputEvent::PointerContact`):
/// how a contact's id, the claimant latch and the `capture_contacts` opt-in
/// decide where an event goes.
#[cfg(test)]
mod contact_tests {
    use super::*;

    /// What every probe saw: which probe, which contact, which phase.
    #[derive(Default)]
    struct Log {
        seen: Vec<(char, PointerId, PointerPhase)>,
    }

    /// A leaf that logs every pointer event it receives with the contact id its
    /// context reports, and on a `Down` captures (optionally opting into the
    /// gesture's other contacts) and claims focus.
    struct Probe {
        tag: char,
        opt_in: bool,
    }
    impl crate::widget::Widget for Probe {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(40.0, 20.0))
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            let InputEvent::Pointer(p) = event else {
                return EventResult::Ignored;
            };
            let id = ctx.pointer_id();
            ctx.state_mut::<Log>().seen.push((self.tag, id, p.phase));
            if p.phase == PointerPhase::Down {
                ctx.capture_pointer();
                if self.opt_in {
                    ctx.capture_contacts();
                }
                ctx.request_focus();
            }
            EventResult::Handled
        }
    }

    /// Two probes stacked vertically (A at y 0..20, B at y 30..50) behind the
    /// standard recorded-path routing: a captured gesture goes straight to the
    /// active child and releases it on `Up`/`Cancel`; anything else is
    /// hit-tested, and a `Down` that hits nothing blurs both.
    struct Pair {
        a: crate::widget::ChildPod,
        b: crate::widget::ChildPod,
    }
    impl crate::widget::Widget for Pair {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.a.layout_child(ctx, bc);
            self.a.set_origin(Point::new(0.0, 0.0));
            self.b.layout_child(ctx, bc);
            self.b.set_origin(Point::new(0.0, 30.0));
            bc.max()
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            self.a.paint_child(ctx, scene);
            self.b.paint_child(ctx, scene);
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            let releases = matches!(
                event,
                InputEvent::Pointer(p) if matches!(p.phase, PointerPhase::Up | PointerPhase::Cancel)
            );
            for pod in [&mut self.a, &mut self.b] {
                if pod.is_active() {
                    let result = pod.event_child(ctx, event);
                    if releases {
                        pod.set_active(false);
                    }
                    return result;
                }
            }
            let pos = event.position();
            let down = matches!(event, InputEvent::Pointer(p) if p.phase == PointerPhase::Down);
            for pod in [&mut self.a, &mut self.b] {
                if pod.contains(pos) {
                    return pod.event_child(ctx, event);
                }
            }
            if down {
                self.a.set_focused(false);
                self.b.set_focused(false);
            }
            EventResult::Ignored
        }
    }

    struct PairView {
        opt_in: bool,
    }
    impl View<Log> for PairView {
        type Element = Pair;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> Pair {
            Pair {
                a: crate::widget::ChildPod::new(Box::new(Probe {
                    tag: 'A',
                    opt_in: self.opt_in,
                })),
                b: crate::widget::ChildPod::new(Box::new(Probe {
                    tag: 'B',
                    opt_in: self.opt_in,
                })),
            }
        }
        fn rebuild(&self, _p: &Self, _e: &mut Pair, _c: &mut BuildCtx<'_>) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn root_with(opt_in: bool) -> (RenderRoot<Log, PairView>, Log) {
        let mut root: RenderRoot<Log, PairView> = RenderRoot::new();
        let mut log = Log::default();
        root.rebuild(&mut |_| PairView { opt_in }, &mut log);
        root.layout(Size::new(200.0, 200.0));
        (root, log)
    }

    fn ev(phase: PointerPhase, x: f64, y: f64) -> PointerEvent {
        PointerEvent {
            phase,
            position: Point::new(x, y),
            button: PointerButton::Primary,
        }
    }

    fn mouse(phase: PointerPhase, x: f64, y: f64) -> InputEvent {
        InputEvent::Pointer(ev(phase, x, y))
    }

    fn touch(slot: u32, phase: PointerPhase, x: f64, y: f64) -> InputEvent {
        InputEvent::PointerContact {
            pointer_id: PointerId::touch(slot),
            event: ev(phase, x, y),
        }
    }

    /// Positions: over A, over B, and over neither.
    const A: (f64, f64) = (10.0, 10.0);
    const B: (f64, f64) = (10.0, 40.0);
    const NOWHERE: (f64, f64) = (150.0, 150.0);

    #[test]
    fn slot_zero_touch_takes_the_same_path_as_a_plain_pointer() {
        use PointerPhase::{Down, Move, Up};
        let script = [
            (Down, A),
            (Move, NOWHERE),
            (Up, NOWHERE),
            (Down, NOWHERE),
            (Move, B),
            (Down, B),
            (Up, B),
        ];
        let (mut via_pointer, mut pointer_log) = root_with(false);
        let (mut via_contact, mut contact_log) = root_with(false);
        for (phase, (x, y)) in script {
            let a = via_pointer.event(&mut pointer_log, &mouse(phase, x, y));
            let b = via_contact.event(&mut contact_log, &touch(0, phase, x, y));
            assert_eq!(a, b, "{phase:?} at ({x}, {y}): same outcome");
            assert_eq!(
                via_pointer.is_pointer_captured(),
                via_contact.is_pointer_captured()
            );
            assert_eq!(via_pointer.is_focus_active(), via_contact.is_focus_active());
            assert_eq!(via_pointer.is_hover_active(), via_contact.is_hover_active());
            assert_eq!(
                via_pointer.focus_ime_generation(),
                via_contact.focus_ime_generation()
            );
        }
        // Same widgets, same phases; only the reported id differs.
        let strip = |log: &Log| -> Vec<(char, PointerPhase)> {
            log.seen.iter().map(|(t, _, p)| (*t, *p)).collect()
        };
        assert_eq!(strip(&pointer_log), strip(&contact_log));
        assert!(
            pointer_log
                .seen
                .iter()
                .all(|(_, id, _)| *id == PointerId::MOUSE)
        );
        assert!(
            contact_log
                .seen
                .iter()
                .all(|(_, id, _)| *id == PointerId::touch(0))
        );
        // The capture the contact's Down took named it as the claimant.
        via_contact.event(&mut contact_log, &touch(0, Down, A.0, A.1));
        assert_eq!(
            via_contact.pointer_capture_claimant(),
            Some(PointerId::touch(0))
        );
    }

    #[test]
    fn an_additional_contact_with_nothing_captured_is_dropped() {
        let (mut root, mut log) = root_with(true);
        let outcome = root.event(&mut log, &touch(1, PointerPhase::Down, A.0, A.1));
        assert_eq!(outcome, EventOutcome::default());
        assert!(log.seen.is_empty(), "no widget saw the slot-1 contact");
        assert!(!root.is_pointer_captured());
        assert!(!root.is_focus_active(), "a dropped contact claims nothing");
    }

    #[test]
    fn an_opted_in_captor_receives_the_other_contacts_on_the_captured_path() {
        use PointerPhase::{Down, Move, Up};
        let (mut root, mut log) = root_with(true);
        root.event(&mut log, &touch(0, Down, A.0, A.1));
        assert_eq!(root.pointer_capture_claimant(), Some(PointerId::touch(0)));

        // The second finger lands over B, yet travels the captured path to A.
        let t1 = PointerId::touch(1);
        assert!(root.event(&mut log, &touch(1, Down, B.0, B.1)).handled);
        root.event(&mut log, &touch(1, Move, NOWHERE.0, NOWHERE.1));
        root.event(&mut log, &touch(1, Up, NOWHERE.0, NOWHERE.1));
        assert_eq!(
            log.seen[1..],
            [('A', t1, Down), ('A', t1, Move), ('A', t1, Up)],
            "A saw touch(1)'s whole contact, B saw nothing"
        );
        assert_eq!(root.pointer_capture_claimant(), Some(PointerId::touch(0)));
    }

    #[test]
    fn a_captor_that_did_not_opt_in_never_sees_the_other_contacts() {
        use PointerPhase::{Down, Move, Up};
        let (mut root, mut log) = root_with(false);
        root.event(&mut log, &touch(0, Down, A.0, A.1));
        for phase in [Down, Move, Up] {
            let outcome = root.event(&mut log, &touch(1, phase, B.0, B.1));
            assert_eq!(
                outcome,
                EventOutcome::default(),
                "touch(1) {phase:?} dropped"
            );
        }
        assert_eq!(log.seen, [('A', PointerId::touch(0), Down)]);
        assert_eq!(root.pointer_capture_claimant(), Some(PointerId::touch(0)));
    }

    #[test]
    fn a_touch_release_never_ends_a_mouse_capture_and_vice_versa() {
        use PointerPhase::{Cancel, Down, Move, Up};
        // A mouse drag holds A; a finger tapping elsewhere must not break it.
        let (mut root, mut log) = root_with(false);
        root.event(&mut log, &mouse(Down, A.0, A.1));
        root.event(&mut log, &touch(0, Down, B.0, B.1));
        root.event(&mut log, &touch(0, Up, B.0, B.1));
        assert_eq!(root.pointer_capture_claimant(), Some(PointerId::MOUSE));
        root.event(&mut log, &mouse(Move, NOWHERE.0, NOWHERE.1));
        assert_eq!(
            log.seen.last(),
            Some(&('A', PointerId::MOUSE, Move)),
            "the drag still reaches its captor"
        );
        assert!(log.seen.iter().all(|(tag, _, _)| *tag == 'A'));
        root.event(&mut log, &mouse(Up, NOWHERE.0, NOWHERE.1));
        assert!(!root.is_pointer_captured());

        // The mirror image: a touch drag holds B; the mouse cannot end it.
        let (mut root, mut log) = root_with(false);
        root.event(&mut log, &touch(0, Down, B.0, B.1));
        root.event(&mut log, &mouse(Up, A.0, A.1));
        root.event(&mut log, &mouse(Cancel, A.0, A.1));
        assert_eq!(root.pointer_capture_claimant(), Some(PointerId::touch(0)));
        root.event(&mut log, &touch(0, Move, NOWHERE.0, NOWHERE.1));
        assert_eq!(log.seen.last(), Some(&('B', PointerId::touch(0), Move)));
        root.event(&mut log, &touch(0, Up, NOWHERE.0, NOWHERE.1));
        assert!(!root.is_pointer_captured());
    }

    #[test]
    fn only_the_claimants_release_clears_the_latch() {
        use PointerPhase::{Cancel, Down, Move, Up};
        let (mut root, mut log) = root_with(true);
        root.event(&mut log, &touch(0, Down, A.0, A.1));
        // Another contact ends twice over — delivered, yet nothing releases:
        // not the root latch, and not the container's active link either.
        root.event(&mut log, &touch(1, Down, A.0, A.1));
        root.event(&mut log, &touch(1, Up, A.0, A.1));
        root.event(&mut log, &touch(2, Down, A.0, A.1));
        root.event(&mut log, &touch(2, Cancel, A.0, A.1));
        assert_eq!(root.pointer_capture_claimant(), Some(PointerId::touch(0)));
        // Still on the captured path: a claimant move far outside A reaches A.
        root.event(&mut log, &touch(0, Move, NOWHERE.0, NOWHERE.1));
        assert_eq!(log.seen.last(), Some(&('A', PointerId::touch(0), Move)));

        // The claimant's own release ends it; later contacts fall to rule (b).
        root.event(&mut log, &touch(0, Up, NOWHERE.0, NOWHERE.1));
        assert!(!root.is_pointer_captured());
        let before = log.seen.len();
        root.event(&mut log, &touch(1, Move, A.0, A.1));
        assert_eq!(
            log.seen.len(),
            before,
            "touch(1) dropped once the gesture ended"
        );
        // And the container's link went with it: a fresh contact is hit-tested.
        root.event(&mut log, &touch(0, Down, B.0, B.1));
        assert_eq!(log.seen.last(), Some(&('B', PointerId::touch(0), Down)));
    }

    #[test]
    fn a_non_claimant_contact_never_blurs_or_moves_hover() {
        use PointerPhase::{Down, Up};
        let (mut root, mut log) = root_with(true);
        root.event(&mut log, &touch(0, Down, A.0, A.1));
        assert!(root.is_focus_active());
        let generation = root.focus_ime_generation();
        root.event(&mut log, &touch(1, Down, NOWHERE.0, NOWHERE.1));
        root.event(&mut log, &touch(1, Up, NOWHERE.0, NOWHERE.1));
        assert!(
            root.is_focus_active(),
            "a second finger is not a tap outside"
        );
        assert_eq!(root.focus_ime_generation(), generation);
    }

    /// A container in front of [`Pair`] with pointer handling of its own that
    /// another contact must never reach: it logs every pointer event it routes
    /// (tag `'G'`) and treats a `Down` outside its child as a tap outside — an
    /// explicit focus release. With `forwards_broadcasts` unset it drops every
    /// broadcast instead of forwarding it (a container the forward-only walk
    /// cannot pass).
    struct Guard {
        inner: crate::widget::ChildPod,
        forwards_broadcasts: bool,
    }
    impl crate::widget::Widget for Guard {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.inner
                .layout_child(ctx, &BoxConstraints::tight(Size::new(100.0, 60.0)));
            self.inner.set_origin(Point::ZERO);
            bc.max()
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            self.inner.paint_child(ctx, scene);
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            if event.is_broadcast() {
                if self.forwards_broadcasts {
                    self.inner.event_child(ctx, event);
                }
                return EventResult::Ignored;
            }
            let InputEvent::Pointer(p) = event else {
                return EventResult::Ignored;
            };
            let id = ctx.pointer_id();
            ctx.state_mut::<Log>().seen.push(('G', id, p.phase));
            let inside = self.inner.contains(p.position);
            let result = if self.inner.is_active() || inside {
                self.inner.event_child(ctx, event)
            } else {
                EventResult::Ignored
            };
            if matches!(p.phase, PointerPhase::Up | PointerPhase::Cancel) {
                self.inner.set_active(false);
            }
            if p.phase == PointerPhase::Down && !inside {
                ctx.release_focus();
            }
            result
        }
    }

    struct GuardView {
        forwards_broadcasts: bool,
    }
    impl View<Log> for GuardView {
        type Element = Guard;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> Guard {
            Guard {
                inner: crate::widget::ChildPod::new(Box::new(Pair {
                    a: crate::widget::ChildPod::new(Box::new(Probe {
                        tag: 'A',
                        opt_in: true,
                    })),
                    b: crate::widget::ChildPod::new(Box::new(Probe {
                        tag: 'B',
                        opt_in: true,
                    })),
                })),
                forwards_broadcasts: self.forwards_broadcasts,
            }
        }
        fn rebuild(&self, _p: &Self, _e: &mut Guard, _c: &mut BuildCtx<'_>) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn guarded_root(forwards_broadcasts: bool) -> (RenderRoot<Log, GuardView>, Log) {
        let mut root: RenderRoot<Log, GuardView> = RenderRoot::new();
        let mut log = Log::default();
        root.rebuild(
            &mut |_| GuardView {
                forwards_broadcasts,
            },
            &mut log,
        );
        root.layout(Size::new(200.0, 200.0));
        (root, log)
    }

    #[test]
    fn another_contact_reaches_only_the_captor_and_moves_no_hover_or_focus() {
        use PointerPhase::{Down, Move, Up};
        let (mut root, mut log) = guarded_root(true);
        let (t0, t1) = (PointerId::touch(0), PointerId::touch(1));
        root.event(&mut log, &touch(0, Down, A.0, A.1));
        assert!(root.is_focus_active());
        assert_eq!(root.pointer_capture_claimant(), Some(t0));
        assert!(root.pointer_capture_contacts());
        let (focus_generation, hover) = (root.focus_ime_generation(), root.is_hover_active());

        // A second finger landing outside the guard's child: delivered to A down
        // the captured path, while the guard — whose own `Down` handling would
        // release focus on a tap outside — never runs on it.
        assert!(
            root.event(&mut log, &touch(1, Down, NOWHERE.0, NOWHERE.1))
                .handled
        );
        root.event(&mut log, &touch(1, Move, B.0, B.1));
        root.event(&mut log, &touch(1, Up, B.0, B.1));
        assert_eq!(
            log.seen,
            [
                ('G', t0, Down),
                ('A', t0, Down),
                ('A', t1, Down),
                ('A', t1, Move),
                ('A', t1, Up),
            ],
            "only the captor saw touch(1)"
        );
        assert!(root.is_focus_active(), "no tap outside was seen");
        assert_eq!(root.focus_ime_generation(), focus_generation);
        assert_eq!(root.is_hover_active(), hover);
        assert_eq!(root.pointer_capture_claimant(), Some(t0));
        assert!(root.pointer_capture_contacts());

        // The claimant itself still takes the ordinary path through the guard.
        root.event(&mut log, &touch(0, Move, NOWHERE.0, NOWHERE.1));
        assert_eq!(
            log.seen[5..],
            [('G', t0, Move), ('A', t0, Move)],
            "the claimant's own move runs every handler on the path"
        );
        root.event(&mut log, &touch(0, Up, NOWHERE.0, NOWHERE.1));
        assert!(!root.is_pointer_captured());
        assert!(!root.pointer_capture_contacts());
    }

    #[test]
    fn a_walk_a_container_cannot_pass_falls_back_to_the_ordinary_delivery() {
        use PointerPhase::Down;
        let (mut root, mut log) = guarded_root(false);
        let (t0, t1) = (PointerId::touch(0), PointerId::touch(1));
        root.event(&mut log, &touch(0, Down, A.0, A.1));
        // The guard drops the carrier, so it is handed the real event instead —
        // and the captor still receives it, down the captured path.
        assert!(root.event(&mut log, &touch(1, Down, B.0, B.1)).handled);
        assert_eq!(
            log.seen,
            [
                ('G', t0, Down),
                ('A', t0, Down),
                ('G', t1, Down),
                ('A', t1, Down),
            ]
        );
        assert_eq!(root.pointer_capture_claimant(), Some(t0));
    }

    /// A root container that takes a gesture over from its child once the
    /// claimant moves more than 10 px: it cancels the child and releases it
    /// through [`EventCtx::release_captured_child`]. Logs every pointer event it
    /// sees (tag `'T'`); with `opt_in` it also opts into the gesture's other
    /// contacts itself, ahead of its child, on the `Down`.
    struct TakeOver {
        inner: crate::widget::ChildPod,
        opt_in: bool,
        down_at: Option<Point>,
        released_seen: bool,
    }
    impl crate::widget::Widget for TakeOver {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.inner.layout_child(ctx, bc);
            self.inner.set_origin(Point::ZERO);
            bc.max()
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            self.inner.paint_child(ctx, scene);
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            if event.is_broadcast() {
                self.inner.event_child(ctx, event);
                return EventResult::Ignored;
            }
            let InputEvent::Pointer(p) = event else {
                return EventResult::Ignored;
            };
            let id = ctx.pointer_id();
            ctx.state_mut::<Log>().seen.push(('T', id, p.phase));
            match p.phase {
                PointerPhase::Down if self.down_at.is_none() => {
                    self.down_at = Some(p.position);
                    ctx.capture_pointer();
                    if self.opt_in {
                        ctx.capture_contacts();
                    }
                    if self.inner.contains(p.position) {
                        self.inner.event_child(ctx, event);
                    }
                }
                PointerPhase::Move => {
                    let moved = self.down_at.map(|at| (p.position - at).hypot());
                    if self.inner.is_active() && moved.is_some_and(|d| d > 10.0) {
                        let cancel = InputEvent::Pointer(PointerEvent {
                            phase: PointerPhase::Cancel,
                            ..*p
                        });
                        self.inner.event_child(ctx, &cancel);
                        ctx.release_captured_child(&mut self.inner);
                        self.released_seen = ctx.is_capture_released();
                    } else if self.inner.is_active() {
                        self.inner.event_child(ctx, event);
                    }
                }
                PointerPhase::Up | PointerPhase::Cancel => {
                    if self.inner.is_active() {
                        self.inner.event_child(ctx, event);
                        self.inner.set_active(false);
                    }
                    self.down_at = None;
                }
                PointerPhase::Down => {}
            }
            EventResult::Handled
        }
    }

    struct TakeOverView {
        opt_in: bool,
    }
    impl View<Log> for TakeOverView {
        type Element = TakeOver;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> TakeOver {
            TakeOver {
                inner: crate::widget::ChildPod::new(Box::new(Probe {
                    tag: 'A',
                    opt_in: true,
                })),
                opt_in: self.opt_in,
                down_at: None,
                released_seen: false,
            }
        }
        fn rebuild(&self, _p: &Self, _e: &mut TakeOver, _c: &mut BuildCtx<'_>) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn take_over_root(opt_in: bool) -> (RenderRoot<Log, TakeOverView>, Log) {
        let mut root: RenderRoot<Log, TakeOverView> = RenderRoot::new();
        let mut log = Log::default();
        root.rebuild(&mut |_| TakeOverView { opt_in }, &mut log);
        root.layout(Size::new(200.0, 200.0));
        (root, log)
    }

    fn take_over_widget(root: &mut RenderRoot<Log, TakeOverView>) -> &mut TakeOver {
        let id = root.root_id().expect("root built");
        root.tree
            .pod_mut(id)
            .expect("root pod")
            .widget_mut()
            .downcast_mut::<TakeOver>()
            .expect("root is a TakeOver")
    }

    #[test]
    fn a_takeover_ends_the_contact_opt_in_but_keeps_the_claimant() {
        use PointerPhase::{Cancel, Down, Move, Up};
        let (mut root, mut log) = take_over_root(false);
        let t0 = PointerId::touch(0);
        root.event(&mut log, &touch(0, Down, A.0, A.1));
        assert_eq!(root.pointer_capture_claimant(), Some(t0));
        assert!(root.pointer_capture_contacts(), "A opted in");

        // The claimant moves past the container's threshold: it cancels A and
        // releases it, and the release reaches the root.
        root.event(&mut log, &touch(0, Move, A.0, A.1 + 30.0));
        assert!(take_over_widget(&mut root).released_seen);
        assert_eq!(log.seen[3], ('A', t0, Cancel));
        assert_eq!(
            root.pointer_capture_claimant(),
            Some(t0),
            "the gesture is still the claimant's, now held by the container"
        );
        assert!(
            !root.pointer_capture_contacts(),
            "the widget that asked for the other contacts is gone"
        );

        // Another finger is dropped from here on — neither the container nor
        // the cancelled captor hears it.
        let before = log.seen.len();
        let outcome = root.event(&mut log, &touch(1, Down, A.0, A.1));
        assert_eq!(outcome, EventOutcome::default());
        assert_eq!(log.seen.len(), before);

        // The claimant keeps driving the container, and its release ends it.
        root.event(&mut log, &touch(0, Move, A.0, A.1 + 60.0));
        assert_eq!(log.seen.last(), Some(&('T', t0, Move)));
        root.event(&mut log, &touch(0, Up, A.0, A.1 + 60.0));
        assert!(!root.is_pointer_captured());
    }

    #[test]
    fn a_takeover_by_the_widget_holding_the_opt_in_keeps_it() {
        use PointerPhase::{Down, Move};
        let (mut root, mut log) = take_over_root(true);
        let (t0, t1) = (PointerId::touch(0), PointerId::touch(1));
        root.event(&mut log, &touch(0, Down, A.0, A.1));
        root.event(&mut log, &touch(0, Move, A.0, A.1 + 30.0));
        assert!(
            !take_over_widget(&mut root).released_seen,
            "the container itself holds the opt-in, so releasing its child ends nothing"
        );
        assert!(root.pointer_capture_contacts());
        // The container is the captor: another finger reaches it directly.
        assert!(root.event(&mut log, &touch(1, Down, A.0, A.1)).handled);
        assert_eq!(log.seen.last(), Some(&('T', t1, Down)));
        assert_eq!(root.pointer_capture_claimant(), Some(t0));
    }
}

/// [`InputEvent::Scale`]: hit-tested by [`ScaleEvent::focal`] exactly like
/// [`InputEvent::Scroll`], translated down the tree the same way, and bubbles
/// topmost-first until a widget reports [`EventResult::Handled`].
#[cfg(test)]
mod scale_tests {
    use super::*;
    use crate::event::{ScaleEvent, ScalePhase};

    /// What every leaf saw: which leaf, which event.
    #[derive(Default)]
    struct Log {
        seen: Vec<(char, ScaleEvent)>,
    }

    /// Logs every [`InputEvent::Scale`] it receives, in its own local space,
    /// and reports `Handled` only when `handles` is set — everything else
    /// (including a non-`Scale` event) is ignored.
    struct ScaleLeaf {
        tag: char,
        handles: bool,
    }
    impl crate::widget::Widget for ScaleLeaf {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(40.0, 40.0))
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            let InputEvent::Scale(scale) = event else {
                return EventResult::Ignored;
            };
            ctx.state_mut::<Log>().seen.push((self.tag, *scale));
            if self.handles {
                EventResult::Handled
            } else {
                EventResult::Ignored
            }
        }
    }

    /// Two leaves at identical bounds `(0, 0)..(40, 40)` — `top` painted (and
    /// hit-tested) before `bottom`, mirroring `frust-widgets::route_event`'s
    /// topmost-first, fall-through-on-`Ignored` hit test: a `Scale` landing in
    /// the shared rect reaches `top` first, and only reaches `bottom` if `top`
    /// ignores it.
    struct Overlapping {
        top: crate::widget::ChildPod,
        bottom: crate::widget::ChildPod,
    }
    impl crate::widget::Widget for Overlapping {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.top.layout_child(ctx, bc);
            self.top.set_origin(Point::ZERO);
            self.bottom.layout_child(ctx, bc);
            self.bottom.set_origin(Point::ZERO);
            bc.max()
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            self.bottom.paint_child(ctx, scene);
            self.top.paint_child(ctx, scene);
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            let pos = event.position();
            for pod in [&mut self.top, &mut self.bottom] {
                if pod.contains(pos) && pod.event_child(ctx, event) == EventResult::Handled {
                    return EventResult::Handled;
                }
            }
            EventResult::Ignored
        }
    }

    /// Wraps [`Overlapping`] one container deeper, offset by `(10, 20)` — so
    /// the fixture also proves [`InputEvent::translated`] shifts a `Scale`
    /// event's focal point correctly across a container boundary.
    struct Offset {
        inner: crate::widget::ChildPod,
    }
    impl crate::widget::Widget for Offset {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.inner.layout_child(ctx, bc);
            self.inner.set_origin(Point::new(10.0, 20.0));
            bc.max()
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            self.inner.paint_child(ctx, scene);
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            let pos = event.position();
            if self.inner.contains(pos) {
                return self.inner.event_child(ctx, event);
            }
            EventResult::Ignored
        }
    }

    struct OffsetView {
        top_handles: bool,
    }
    impl View<Log> for OffsetView {
        type Element = Offset;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> Offset {
            Offset {
                inner: crate::widget::ChildPod::new(Box::new(Overlapping {
                    top: crate::widget::ChildPod::new(Box::new(ScaleLeaf {
                        tag: 'T',
                        handles: self.top_handles,
                    })),
                    bottom: crate::widget::ChildPod::new(Box::new(ScaleLeaf {
                        tag: 'B',
                        handles: true,
                    })),
                })),
            }
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut Offset,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn root_with(top_handles: bool) -> (RenderRoot<Log, OffsetView>, Log) {
        let mut root: RenderRoot<Log, OffsetView> = RenderRoot::new();
        let mut log = Log::default();
        root.rebuild(&mut |_| OffsetView { top_handles }, &mut log);
        root.layout(Size::new(200.0, 200.0));
        (root, log)
    }

    fn scale(phase: ScalePhase, scale_delta: f64, x: f64, y: f64) -> InputEvent {
        InputEvent::Scale(ScaleEvent {
            phase,
            scale_delta,
            focal: Point::new(x, y),
            velocity: 0.0,
        })
    }

    #[test]
    fn scale_hit_tests_by_focal_point_and_translates_through_a_container() {
        let (mut root, mut log) = root_with(true);

        // Window (5, 5): outside the offset container entirely (it starts at
        // (10, 20)) — nothing is hit, nothing logged.
        let outside = root.event(&mut log, &scale(ScalePhase::Begin, 1.1, 5.0, 5.0));
        assert!(
            log.seen.is_empty(),
            "a focal point outside every pod hits nothing"
        );
        assert!(!outside.handled);

        // Window (30, 40): inside the container, local (20, 20) once the
        // Offset container's (10, 20) origin is subtracted by
        // `InputEvent::translated` — squarely inside both overlapping 40x40
        // leaves, so the topmost one (`top`) is the one that sees it.
        let inside = root.event(&mut log, &scale(ScalePhase::Update, 1.2, 30.0, 40.0));
        assert!(inside.handled);
        assert_eq!(log.seen.len(), 1, "the topmost leaf alone handled it");
        let (tag, seen) = log.seen[0];
        assert_eq!(tag, 'T');
        assert_eq!(
            seen.focal,
            Point::new(20.0, 20.0),
            "translated() shifted the focal point into the container's local space"
        );
        assert_eq!(seen.scale_delta, 1.2);
        assert_eq!(seen.phase, ScalePhase::Update);
    }

    #[test]
    fn an_ignored_scale_bubbles_to_the_next_hit_widget() {
        // `top` ignores every `Scale` it sees; `bottom`, at the identical
        // bounds, still handles it — proving the hit test falls through to
        // the next topmost-first candidate instead of stopping (and
        // swallowing the event) at the first hit.
        let (mut root, mut log) = root_with(false);
        let outcome = root.event(&mut log, &scale(ScalePhase::Begin, 0.9, 30.0, 40.0));
        assert!(outcome.handled, "the bottom leaf still handled it");
        assert_eq!(
            log.seen.iter().map(|(tag, _)| *tag).collect::<Vec<_>>(),
            vec!['T', 'B'],
            "the ignoring top leaf saw it first, and bottom is what it bubbled to"
        );
    }
}

/// The [`RenderRoot::event`] file-drag follow-up: every `Drop`/`Cancel`,
/// handled or not, is followed by an `Ended` broadcast.
#[cfg(test)]
mod file_drop_tests {
    use super::*;

    /// Every `FileDrop` phase a leaf saw, in order, and whether the leaf
    /// handles the hit-tested phases.
    #[derive(Default)]
    struct DropLog {
        handles: bool,
        seen: Vec<(char, FileDropPhase)>,
    }

    struct DropLeaf {
        tag: char,
    }
    impl crate::widget::Widget for DropLeaf {
        fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            bc.constrain(Size::new(40.0, 40.0))
        }
        fn paint(&mut self, _ctx: &mut PaintCtx, _scene: &mut dyn PaintScene) {}
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            let InputEvent::FileDrop(drop) = event else {
                return EventResult::Ignored;
            };
            let log = ctx.state_mut::<DropLog>();
            log.seen.push((self.tag, drop.phase));
            if log.handles && !event.is_broadcast() {
                EventResult::Handled
            } else {
                EventResult::Ignored
            }
        }
    }

    /// One 40x40 leaf at the origin of a 200x200 root that routes like
    /// `frust-widgets`' helpers: a broadcast to the child unconditionally,
    /// anything else only on a hit.
    struct DropRoot {
        leaf: crate::widget::ChildPod,
    }
    impl crate::widget::Widget for DropRoot {
        fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
            self.leaf.layout_child(ctx, bc);
            self.leaf.set_origin(Point::ZERO);
            bc.max()
        }
        fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
            self.leaf.paint_child(ctx, scene);
        }
        fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
            if event.is_broadcast() {
                self.leaf.event_child(ctx, event);
                return EventResult::Ignored;
            }
            if self.leaf.contains(event.position()) {
                return self.leaf.event_child(ctx, event);
            }
            EventResult::Ignored
        }
    }

    struct DropRootView;
    impl View<DropLog> for DropRootView {
        type Element = DropRoot;
        fn build(&self, _ctx: &mut BuildCtx<'_>) -> DropRoot {
            DropRoot {
                leaf: crate::widget::ChildPod::new(Box::new(DropLeaf { tag: 'L' })),
            }
        }
        fn rebuild(
            &self,
            _prev: &Self,
            _element: &mut DropRoot,
            _ctx: &mut BuildCtx<'_>,
        ) -> ChangeFlags {
            ChangeFlags::NONE
        }
    }

    fn drop_root(handles: bool) -> (RenderRoot<DropLog, DropRootView>, DropLog) {
        let mut root: RenderRoot<DropLog, DropRootView> = RenderRoot::new();
        let mut log = DropLog {
            handles,
            seen: Vec::new(),
        };
        root.rebuild(&mut |_| DropRootView, &mut log);
        root.layout(Size::new(200.0, 200.0));
        (root, log)
    }

    fn file_drop(phase: FileDropPhase, x: f64, y: f64) -> InputEvent {
        InputEvent::FileDrop(FileDropEvent {
            phase,
            position: Point::new(x, y),
            paths: Vec::new(),
        })
    }

    #[test]
    fn an_unhandled_file_drop_end_is_re_broadcast_to_the_whole_tree() {
        for phase in [FileDropPhase::Drop, FileDropPhase::Cancel] {
            let (mut root, mut log) = drop_root(false);
            // Far outside the leaf: the hit-tested dispatch reaches nothing.
            let outcome = root.event(&mut log, &file_drop(phase, 150.0, 150.0));
            assert!(!outcome.handled, "a broadcast is never consumed");
            assert_eq!(
                log.seen,
                vec![('L', FileDropPhase::Ended)],
                "the {phase:?} that missed the leaf is followed by an Ended broadcast it hears"
            );

            // Over the leaf but ignored by it: the leaf hears the hit-tested
            // phase, then Ended.
            log.seen.clear();
            root.event(&mut log, &file_drop(phase, 10.0, 10.0));
            assert_eq!(log.seen, vec![('L', phase), ('L', FileDropPhase::Ended)]);
        }
    }

    #[test]
    fn a_handled_file_drop_end_is_still_re_broadcast_to_the_whole_tree() {
        let (mut root, mut log) = drop_root(true);
        for phase in [FileDropPhase::Drop, FileDropPhase::Cancel] {
            log.seen.clear();
            let outcome = root.event(&mut log, &file_drop(phase, 10.0, 10.0));
            assert!(outcome.handled, "the hit-tested {phase:?} was handled");
            assert_eq!(
                log.seen,
                vec![('L', phase), ('L', FileDropPhase::Ended)],
                "and is still followed by the Ended broadcast"
            );
        }
    }

    #[test]
    fn a_hover_and_a_direct_ended_get_no_follow_up() {
        let (mut root, mut log) = drop_root(false);
        root.event(&mut log, &file_drop(FileDropPhase::Hover, 150.0, 150.0));
        assert!(log.seen.is_empty(), "a Hover is never re-broadcast");

        root.event(&mut log, &file_drop(FileDropPhase::Ended, 0.0, 0.0));
        assert_eq!(
            log.seen,
            vec![('L', FileDropPhase::Ended)],
            "an Ended dispatched directly is delivered once, not followed again"
        );
    }
}