kui_core/

runtime.rs

//! [`Core`]: one window's runtime, the state machine a runner drives frame
//! by frame.
//!
//! A core owns everything that survives across frames and belongs to one
//! window: the per-frame tree and its builder state, interaction state
//! (hover, press, drag), focus, the scroll and edit stores, animations,
//! the glyph atlas and the finished [`DisplayList`]. What a window must
//! not duplicate — the font database, the resource registry, the audio
//! store — lives in the [`Session`] a core is constructed against;
//! [`Core::new`] makes a private one, so a single-window app never sees
//! it. The frame lifecycle, with an example, is on [`Core`].
//!
//! Builder state lives in the core itself rather than in a borrowing
//! wrapper so that flat C bindings can drive a frame through one opaque
//! pointer; the Rust [`Ui`] is a thin safe facade over it.

use std::rc::Rc;

use rustc_hash::{FxHashMap, FxHashSet};

use crate::anim::{AnimStore, Slot, Track};
use crate::atlas::GlyphAtlas;
use crate::color::Color;
use crate::depart::{At, DepartStore, Ghost, GhostContent, Place, Playback, Replay};
use crate::diag::{Diagnostics, Warning};
use crate::display::{Clip, ClipId, DisplayList, NO_CLIP_ID, Quad, QuadKind};
use crate::edit::{EditOptions, EditStore};
use crate::env::{Env, SystemEnv};
use crate::geom::{Rect, Size, Vec2};
use crate::input::{
    EditKey, HitRegion, InputEvent, Interaction, KeyCode, KeyMods, KeyPhase, KeyPress, MouseButton,
    ScrollAxis, ScrollRegion, ScrollbarRegion, UiEvent,
};
use crate::key::{Key, LabelIndex};
use crate::keyframes;
use crate::layout::{self, TextMeasure};
use crate::line::Stroke;
use crate::resources::{FontId, Resources};
use crate::scroll::ScrollStore;
use crate::session::{
    MAIN_WINDOW_NAME, Session, SharedAudio, SharedResources, WindowChange, WindowDecl,
};
use crate::spec::{NodeSpec, Sizing, TextStyle};
use crate::stats::FrameStats;
use crate::text::TextHit;
use crate::text::{Span, TextMetrics, TextSystem};
use crate::theme::{Theme, ThemeSource};
use crate::tree::{NIL, NodeContent, OriginId, Tree};
use crate::ui::Ui;
use crate::value::Value;
use crate::window::{WindowConfig, WindowId};

// `impl Core` continues in these, one concern per file (each opens with
// what it holds). Children of this module, so the fields stay private.
mod builder;
pub use builder::Content;
pub use replay::SlotFill;
pub mod cause;
mod composites;
pub mod devtools;
mod dispatch;
mod emit;
mod fills;
mod focus;
pub mod follow;
mod gesture;
pub mod inspect;
mod menu_api;
/// A slot replayed by its host (ADR 0045).
pub(crate) mod replay;
pub(crate) use menu_api::MenuSurface;
mod menubar_api;
mod resources_api;
mod scrolling;
mod select_api;
mod windows;

/// Wheel line-delta to logical px.
const SCROLL_LINE_PX: f32 = 40.0;

/// What a `Core::focus_region` call asked to enter, held until the frame
/// finishes: the main ring,
/// a region by key, or one by the label its node declares — the spelling
/// a caller has for a node the last frame did not build.
#[derive(Clone, Debug, PartialEq)]
pub(crate) enum RegionTarget {
    Main,
    Key(Key),
    Label(String),
}

/// One window's runtime: the state a runner drives frame by frame.
///
/// Construct one with [`Core::new`] (a private [`Session`]) or
/// [`Core::new_in`] (joining a session another window shares). The public
/// fields are the stores a runner or a binding reads directly:
/// `resources` and `audio` for registration and playback commands, `env`
/// for the host facts a driver pushes, `stats` for frame timing.
///
/// # One frame, in order
///
/// 1. [`Core::set_time`] with the frame clock, so transitions advance.
/// 2. [`Core::frame`] with the viewport (logical px) and the scale. It
///    begins the frame and returns the [`Ui`] builder; build the tree
///    through it.
/// 3. [`Ui::finish`] runs layout and emission. The draw data is now in
///    [`Core::output`]: the [`DisplayList`] and the [`GlyphAtlas`] a
///    renderer mirrors to a texture.
/// 4. [`Core::take_pending_events`] drains events the frame itself raised
///    (a `resize`, a hover change under a still pointer); route them like
///    any other.
/// 5. Between frames, feed input through [`Core::handle_input`]. Each
///    call returns the [`UiEvent`]s it resolved to, hit-tested against
///    the frame that finished.
/// 6. [`Core::take_warnings`] for misconfigurations the core noticed, and
///    [`Core::animating`] to decide whether to draw another frame without
///    waiting for input.
///
/// A headless core needs no window or GPU, so a test can drive it:
///
/// ```rust
/// use kui_core::{Color, Core, InputEvent, NodeSpec, QuadKind, Size, TextStyle, Vec2};
///
/// let mut core = Core::new();
///
/// // 1–3: a frame. `frame` begins it, `finish` lays it out and emits.
/// core.set_time(0.0);
/// let mut ui = core.frame(Size::new(400.0, 300.0), 1.0);
/// ui.with_keyed("toolbar", NodeSpec::row().pad(8.0).gap(8.0), |ui| {
///     // A clickable node needs an accessible name, or `take_warnings`
///     // reports `control-without-name`.
///     let button = NodeSpec::row().size(80.0, 32.0).bg(Color::hex(0x3366cc));
///     ui.leaf_keyed("save", button.on_click("save").label("Save"));
///     ui.text("Untitled", TextStyle::new(14.0));
/// });
/// ui.finish();
///
/// // 4: what the frame itself raised (nothing on a first frame).
/// assert!(core.take_pending_events().is_empty());
///
/// // The renderer's view of the frame: quads in physical pixels.
/// let (list, atlas) = core.output();
/// assert!(list.quads.iter().any(|q| q.kind == QuadKind::Solid));
/// atlas.dirty = false; // after uploading `atlas.pixels`
///
/// // 5: input, hit-tested against the frame that finished.
/// core.handle_input(InputEvent::CursorMoved(Vec2::new(20.0, 20.0)));
/// core.handle_input(InputEvent::mouse_down(1));
/// let events = core.handle_input(InputEvent::mouse_up());
/// assert_eq!(events.len(), 1);
/// assert_eq!(events[0].payload.as_str(), Some("save"));
///
/// // 6: diagnostics, and whether another frame is owed.
/// assert!(core.take_warnings().is_empty());
/// assert!(!core.animating());
/// ```
///
/// A windowed runner does the same with real time, real input and a
/// renderer consuming [`Core::output`]; `kui-native` is that runner.
pub struct Core {
    /// The caches and registries this window shares with the rest of its
    /// session. Everything a frame needs from it is borrowed inside a
    /// method and dropped before it returns; see `session`'s module doc.
    session: Session,
    /// This window's shaped-text cache and rasterizer. Not the session's:
    /// its cache entries are stamped with `atlas`'s epoch, and `TextId`
    /// indexes its per-frame list.
    pub text: TextSystem,
    /// The frame's cell grids and their glyph tables.
    pub cells: crate::cells::CellStore,
    /// This window's glyph atlas — the CPU side of its renderer's texture,
    /// handed out by `output`.
    pub atlas: GlyphAtlas,
    /// The session's resource registry. A font, image or sound registered
    /// through it is registered for every window in the session.
    pub resources: SharedResources,
    /// The session's audio store: one device for the process, so playback
    /// bookkeeping and the command queue are shared, not per window.
    pub audio: SharedAudio,
    /// The family names of the session's fonts as of this window's last
    /// frame, so `font_family` can lend one out. Refreshed from
    /// `SessionState::fonts_rev`.
    font_names: FxHashMap<FontId, std::rc::Rc<str>>,
    fonts_rev: u64,
    /// The session's `weights_rev` this core's shaped text is of
    /// (`sync_weights`).
    weights_rev: u64,
    /// The session's `images_rev` this core last checked its atlas
    /// against (`sync_dropped`).
    images_rev: u64,
    pub interaction: Interaction,
    pub scroll: ScrollStore,
    pub edit: EditStore,
    /// Transition tweens, keyed by node; see `anim`. Fed by `set_time`.
    pub anim: AnimStore,
    /// Subtrees the view stopped declaring, played out and then dropped;
    /// see `depart`. Empty unless something declares `exit`.
    pub depart: DepartStore,
    /// Frame timing pushed by the frame driver; see `widgets::latency_graph`.
    pub stats: FrameStats,
    /// Host facts pushed by the frame driver (refresh rate, focus).
    pub env: Env,
    /// Where this window's palette comes from; see [`crate::theme`].
    /// `Derived` unless the app said otherwise, so an app that never
    /// mentions themes still follows the OS.
    theme_source: ThemeSource,
    /// `theme_source` resolved against `env.system`, re-resolved at the
    /// start of every frame. Read by the stock widgets, by the core's own
    /// chrome (ring, scrollbar, selection) and by any view that asks.
    theme: Theme,
    /// The sizes the stock widgets are built from: the
    /// palette's other axis, set by the app or
    /// [`Metrics::default`](crate::metrics::Metrics::default).
    metrics: crate::metrics::Metrics,
    /// The titlebar height the driver opened this window's strip at, over
    /// the platform's caption height ([`Core::set_platform_titlebar_h`]);
    /// `None` is the platform's.
    platform_titlebar_h: Option<f32>,
    /// The named colours and lengths each origin declared: the
    /// host's under `OriginId::HOST`, an extension's under its own, so a
    /// guest's declaration never replaces the host's palette. Read through
    /// [`Core::token_lookup`], which puts the running origin's table over
    /// the host's.
    tokens: rustc_hash::FxHashMap<OriginId, crate::tokens::Tokens>,
    /// Window title declared this frame (immediate-mode: cleared each
    /// `begin_frame`; the driver diffs and applies). None = leave as-is.
    window_title: Option<String>,
    /// Whether this frame asked for the window to stay above every other
    /// app's. The title's shape — cleared each `begin_frame`,
    /// the driver diffs and applies on change — but a bool with a default
    /// rather than an option, so a frame that stops asking is what lowers
    /// the window again: the pin button an app draws for this is a
    /// toggle, and the fact follows it.
    always_on_top: bool,
    /// Whether this frame asked for secure keyboard entry while its window
    /// has the keyboard. `always_on_top`'s shape: cleared each
    /// `begin_frame`, so a frame that stops asking is what turns it off.
    secure_input: bool,
    /// Which Option keys this frame asked to act as Alt on macOS.
    /// `always_on_top`'s shape: cleared each `begin_frame`, so a
    /// frame that stops declaring it gives the Option keys back to the
    /// layout's composition.
    option_as_alt: crate::input::OptionAsAlt,
    /// Whether this frame asked for the platform's input method off in
    /// its window. `always_on_top`'s shape: cleared each `begin_frame`,
    /// so a frame that stops asking gives the window its IME back.
    ime_off: bool,
    /// Keyboard focus: the one node key input goes to — an editor (the
    /// edit store mirrors it), an `on_key` sink, a control Tab landed on.
    /// `set_focus` is
    /// the only writer.
    focus: Option<Key>,
    /// Focus got there by keyboard or assistive technology, so it shows:
    /// the default ring, or the node's `focus_bg`. A mouse press clears it.
    focus_visible: bool,
    /// Nodes that declared focus this frame and last (`set_key_focus`):
    /// a declaration takes focus only when it starts, so a repeated one
    /// does not clobber a Tab press.
    declared_focus: Vec<Key>,
    declared_focus_last: Vec<Key>,
    /// Whether the app moved focus through `set_focus` since the last
    /// frame began — the edge a closing modal's restore yields to, like a
    /// `keyFocus` edge. Cleared by `begin_frame`.
    focus_asked: bool,
    /// The `.str`-keyed nodes this frame and last, with their labels
    /// (`open_keyed`): what `key_of` resolves a name through. The same
    /// swap-and-clear pair as the focus declarations, so a frame that
    /// keys nothing costs two clears.
    key_labels: LabelIndex,
    key_labels_last: LabelIndex,
    /// The slots this frame declared (`begin_slot`), by name: what the
    /// `"root"` fill and the `unknown-slot` check read, and what makes a
    /// second declaration of one name a `duplicate-slot`. Cleared each
    /// frame; never compared to the last one, since a slot is a position
    /// and not a declaration the core diffs.
    slot_labels: LabelIndex,
    /// A slot replayed by its host (ADR 0045, `runtime::replay`): the
    /// fill being kept, the ask for the next one to keep, what each slot
    /// kept between frames, and what `slot_replay` answered this frame.
    recording: Option<replay::Recording>,
    keep_next: Option<(String, Key, Value)>,
    kept: rustc_hash::FxHashMap<String, replay::Kept>,
    slot_fills: Vec<(String, replay::SlotFill, Option<String>)>,
    slot_fills_last: Vec<(String, replay::SlotFill, Option<String>)>,
    /// The key namespace of the fill in progress:
    /// while the node stack is exactly `ns_depth` deep, a child's key is
    /// derived from `ns_key` instead of from the node it is opened under,
    /// so an extension's nodes are keyed by the slot and the extension
    /// rather than by whatever the host built around them. `usize::MAX`
    /// when no fill is in progress — one compare on the auto-key path.
    ns_depth: usize,
    ns_key: Key,
    /// Between `begin_frame` and `finish_frame`: `key_labels` is partial
    /// and `key_labels_last` is the last whole frame, and `key_of` reads
    /// both; outside a build `key_labels` is the whole last frame and is
    /// the only one read.
    building: bool,
    /// Windows this frame and last declared (`declare_window`): the same
    /// edge-triggered shape as the focus pair, one level up. The session's
    /// registry diffs the union across every core at `finish_frame`; the
    /// pair here is what makes a frame that changed nothing cost nothing.
    declared_windows: Vec<WindowDecl>,
    declared_windows_last: Vec<WindowDecl>,
    /// The presses delivered to the current focus and not yet released,
    /// in press order. A `KeyUp` is routed only when its press is in here
    /// (so a sink never sees a release it did not see the press of), and
    /// focus leaving synthesizes the missing releases from it.
    keys_held: Vec<KeyPress>,
    /// The modifiers of the `KeyDown` the next input event is the second
    /// channel of (`KeyPress::edit_event`), so the `Key` and `Text` arms
    /// ask the same chord question the raw press did. Set by a `KeyDown`, read
    /// and cleared by whatever
    /// comes next: a `Key` or `Text` with no press before it — a test
    /// driving one channel, a host's editing-key door — has no chord to
    /// agree with and falls back to what its own `Mods` say.
    pressed_mods: Option<KeyMods>,
    pub(crate) tree: Tree,
    /// Whether the host asked for `finish_frame` to copy the frame into
    /// `inspected` (see `runtime/inspect.rs`, `set_inspect`); off unless
    /// it did. The panel's own need is `dt_inspect`, derived each frame
    /// and kept apart, so neither ask can turn the other
    /// off.
    inspect: bool,
    /// The devtools panel's need for the snapshot this frame: its tree
    /// tab is showing, or it is picking.
    dt_inspect: bool,
    inspected: Vec<inspect::NodeInfo>,
    /// The devtools' hold on this window's frame: the
    /// tree index of the app container the host's tree is wrapped in
    /// while the panel is docked, whether this core draws the panel's own
    /// window, and — while the panel's theme override is in force — the
    /// source the host had before it, beside the override applied, so a
    /// source the host sets *under* the override is told from it.
    dt_app: Option<usize>,
    /// The dock the frame was wrapped for at `begin_frame`, `None` when
    /// it was not: a panel turned on or re-docked mid-frame (an app's
    /// `set_devtools` from its `view`) is built next frame, which is
    /// asked for, rather than into a root laid out for something else.
    dt_dock: Option<devtools::Dock>,
    dt_window: bool,
    dt_theme: Option<(ThemeSource, ThemeSource)>,
    /// The menu mode the host had before the panel's override went on —
    /// drawn or native, the bar with it — for the override's `platform`
    /// choice to put back.
    dt_menus: Option<(bool, bool)>,
    /// The panel was built at `begin_frame` (a left dock precedes the
    /// app's container in tree order), on this tab, so `finish` must not
    /// build again — and a door that moved the panel, turned it off or
    /// changed its tab since is the next frame's, which `finish` asks
    /// for.
    dt_built: Option<devtools::Shown>,
    /// The devtools tabs declared this frame, in order: what
    /// the panel's strip lists, moved into the session's state at the
    /// end of the main window's frame. Empty on a frame nobody declares
    /// one, which is what every other frame pays.
    dt_tabs: Vec<devtools::TabDecl>,
    /// The host's viewport in window coordinates: the whole window, or
    /// what the dock leaves of it while the panel is docked.
    /// What `Core::viewport` reports, what a `resize` is measured on,
    /// what the host's viewport floats resolve against, and the origin
    /// every coordinate the host is handed or hands in is relative to.
    dt_area: Rect,
    /// The previous frame's tree, kept only while a frame declares `exit`
    /// — `begin_frame` swaps the two buffers instead of clearing one, so
    /// the frame that notices a node gone still has the node. Empty (and
    /// untouched) for every frame that declares no exit.
    prev_tree: Tree,
    /// The frame's strokes, indexed by the `line` nodes' `LineId`s; the
    /// previous frame's kept alongside on the same terms as `prev_tree`.
    pub(crate) lines: crate::line::LineStore,
    /// The frame's paths, on the same terms as the strokes.
    pub(crate) paths: crate::path::PathStore,
    /// What each `path` key last declared and when its ops last changed,
    /// and whether the key is animating: one whose ops changed twice
    /// within `path::ANIMATING_WINDOW` frames, whose masks leave the atlas
    /// until it has held still for `path::SETTLED_AFTER`.
    pub(crate) path_motion: rustc_hash::FxHashMap<Key, crate::path::Motion>,
    /// The ops of each `d` string a `path` key last declared, so a string
    /// handed over every frame is parsed once.
    pub(crate) path_parsed: rustc_hash::FxHashMap<Key, crate::path::Parsed>,
    /// The masks drawn from a texture of their own rather than the atlas:
    /// too big for a page, or animating.
    pub(crate) path_textures: crate::path::PathTextures,
    /// Whether [`Core::output`] has been asked for since the frame was
    /// built: what says its `dropped_textures` and `dropped_fragments`
    /// reached a backend. A frame nobody read hands them to the next.
    output_read: bool,
    pub(crate) fragments: crate::fragment::FragmentList,
    /// The stock polygon fragment's handle, once a `polygon` node has
    /// asked for it this session. Forgotten by
    /// `remove_fragment` if a host removes it, so the next node registers
    /// it again rather than drawing nothing — and not re-checked per node,
    /// which was a session lock per polygon and cost more than the six
    /// segment quads a closed stroke of the same outline emits.
    pub(crate) stock_polygon: Option<crate::resources::FragmentId>,
    /// The hit shapes the frame being emitted builds beside its regions,
    /// handed to `interaction` with them at the end of
    /// emission; the previous frame's buffers, cleared, in between.
    pub(crate) hit_shapes: crate::input::HitShapes,
    pub(crate) display: DisplayList,
    pub(crate) viewport: Size,
    pub(crate) scale: f32,
    // Frame-builder state.
    stack: Vec<u32>,
    counters: Vec<u64>,
    origin: OriginId,
    /// Per-node inherited clip (logical), rebuilt each finish_frame.
    /// The access tree cuts each node's rect to it, so what a
    /// reader finds is what a pointer can hit.
    clips: Vec<Clip>,
    /// The `DisplayList::clips` index each of those became, so a node
    /// whose clip is its parent's names the entry the parent already
    /// interned instead of asking again. Parallel to `clips`.
    clip_ids: Vec<ClipId>,
    /// Per-node inherited group opacity (the product down the ancestors),
    /// rebuilt each finish_frame and only materialized when something
    /// actually fades.
    opacity: Vec<f32>,
    /// Per node, the layer it paints in: the index of its nearest floating
    /// ancestor-or-self, `NIL` in flow. Only filled on a frame
    /// that floats something.
    float_root: Vec<u32>,
    /// The float layers as the last frame painted them, bottom to top:
    /// each root's key and its rank among that frame's float roots in
    /// tree order. A root the next frame keeps stays where it is, one it
    /// opens goes on top, one it closes leaves — so the stack is the
    /// order the layers opened in. Bounded by the
    /// frame's own float count; nothing to evict.
    float_stack: Vec<(Key, u32)>,
    /// The hover hints of the nodes open right now that declared a
    /// `tooltip` (`Core::hint`), each with the stack depth it was opened
    /// at, so `close` knows whose turn it is. Only nodes that declared one
    /// are here: a frame without a tooltip pays one length check per close.
    hints: Vec<(usize, Key, String)>,
    /// The context menu this window has open, the keys the stock renderer
    /// gave its rows (so their clicks can be told from the app's), and
    /// what choosing one left for the host to do.
    menu: Option<crate::menu::Menu>,
    menu_actions: Vec<crate::menu::MenuAction>,
    /// The editor that held focus when the menu opened, since the menu's
    /// own rows take focus from it — what Cut, Copy and Select All act on.
    menu_editor: Option<Key>,
    /// The node that held focus when the menu opened, whatever it was:
    /// where a `replay` row's chord goes once the menu is gone (backlog
    /// F151), since a key goes where the keyboard was.
    menu_focus: Option<Key>,
    /// Whether the host draws menus itself (`set_native_menus`).
    native_menus: bool,
    /// The submenus open in the drawn context menu and in the drawn bar's
    /// open menu (backlog F128): retained, like `menu_bar_open`, because
    /// the frame cannot derive which row the pointer last rested on.
    menu_sub: menu_api::Submenus,
    menu_bar_sub: menu_api::Submenus,
    /// The application menu this frame has in force, and a count bumped
    /// whenever it changes, so a driver diffs against one integer rather
    /// than against a tree.
    /// `None` is a declaration nobody has made; an empty `MenuBar` is one
    /// that took the bar away.
    menu_bar: Option<crate::menu::MenuBar>,
    menu_bar_rev: u64,
    /// Who declared it, and so who hears the events its items post — the
    /// host, or the extension whose view declared the bar.
    menu_bar_origin: OriginId,
    /// Which of the drawn bar's menus is open, and the bar's root this
    /// frame — where a menu-bar event lands. Its titles and rows are known
    /// by their origin (`OriginId::MENU_BAR`), not by key.
    menu_bar_open: Option<usize>,
    menu_bar_root: Option<Key>,
    /// The select fields this frame built, each with its menu's rows
    /// (`widgets::select`); a click on one opens that menu.
    selects: Vec<(Key, Vec<crate::menu::MenuItem>)>,
    /// Whether the platform owns the menu bar (`set_native_menu_bar`), in
    /// which case the drawn one draws nothing and the driver hands the
    /// declaration over instead.
    native_menu_bar: bool,
    /// Whether the host can show a definition panel
    /// (`set_lookup_available`).
    lookup_available: bool,
    /// The window's text selection outside an editor, and what the
    /// frame resolved it to: `sel_ords` numbers the text nodes of the
    /// selection's scope in emission order (`u32::MAX` for a node
    /// outside it), and `sel_ends` is the pair of ends in reading order,
    /// `None` when this frame builds neither end.
    selection: Option<crate::select::Selection>,
    /// The window's selection when it is in a `cells` grid instead of in
    /// text. One selection per window: starting either clears the other,
    /// which `Core::set_selection` / `set_cell_selection` enforce in one
    /// place each.
    cell_selection: Option<crate::select::CellSelection>,
    /// Whether a `selectionrange` ask is outstanding.
    awaiting_selection: bool,
    /// Whether a paste ask is outstanding — queued, or taken by the
    /// driver and not yet answered with a `Commit`. A
    /// second ask while one is out is dropped, so a view that asks every
    /// frame until the answer lands asks once.
    awaiting_paste: bool,
    /// The one file-dialog ask: queued, taken by the host,
    /// or none.
    file_ask: crate::dialog::FileAsk,
    /// The drag a press is running through a selection scope, if any: set
    /// on the press inside a scope, cleared on release. The counterpart of
    /// `EditStore::dragging` for text nobody is editing.
    select_dragging: Option<crate::select::SelectDrag>,
    /// The held drag — a caret drag or a drag-select — following its
    /// scroller: the pointer's last position, the scroller found for it,
    /// and what that scroller was at when the live end was last placed.
    /// Set with either drag, cleared with both.
    drag_follow: Option<follow::DragFollow>,
    /// The frame clock's reading at the last frame, for the edge drag's
    /// rate (`follow::follow_drag`); `None` before a clock is set.
    last_frame_time: Option<f64>,
    /// The fraction of a line the last delta over an `on_scroll` grid —
    /// a wheel notch or an edge step — did not cover, with the grid it
    /// was over: the next delta on the same grid adds to it.
    line_carry: Option<(Key, f32)>,
    /// The targets the scroll gesture under way latched, per axis.
    scroll_latch: gesture::ScrollLatch,
    sel_ords: Vec<u32>,
    sel_ends: Option<crate::select::Ends>,
    /// Per-node innermost enclosing selection scope — the key of the
    /// nearest ancestor (or the node itself) declaring `selectable`, and
    /// `None` outside every scope. Filled only on a frame that declares
    /// one at all (`Tree::any_selectable`), so an app that never selects
    /// anything pays nothing for it.
    scopes: Vec<Option<Key>>,
    /// Per-node enclosing virtualised row index, filled beside `scopes`:
    /// what places an endpoint whose own node is no longer built.
    rows: Vec<Option<u64>>,
    /// The frame's modal scope: the tree range `[i, subtree_end(i))` of the
    /// last node declaring `modal`, and its key. Everything outside it is
    /// inert and out of the Tab ring. Recomputed by `finish_frame`.
    modal: Option<(usize, usize, Key)>,
    /// The modals declared by the last finished frame, in tree order, each
    /// with the focus it displaced: a modal that stops being declared gives
    /// that focus back.
    modal_focus: Vec<(Key, Option<Key>)>,
    /// Per-node inherited opacity while a departing subtree is replayed
    /// (`depart`), reused across ghosts and frames.
    ghost_opacity: Vec<f32>,
    /// Per-node inherited clip and painted rect while a departing subtree
    /// is replayed: the clips its own clippers establish, since a ghost
    /// draws outside every clip its ancestors held (`depart`).
    ghost_clip: Vec<Clip>,
    /// The interned index of each of those, as `clip_ids` is for `clips`.
    ghost_clip_ids: Vec<ClipId>,
    ghost_rect: Vec<Rect>,
    /// A view asked for one more frame (`request_frame`); cleared by
    /// `begin_frame`, reported through `animating`.
    frame_requested: bool,
    /// The earliest frame-clock time a view or a handler asked for a frame
    /// at (`request_frame_at`, backlog F135): kept until a frame is begun
    /// at or after it, and read by a driver through `next_frame_at` as a
    /// deadline to wake for rather than a frame to draw now.
    frame_due: Option<f64>,
    /// Exits named for a removal (`set_exit`, backlog F136), by key: read
    /// by the departures of the frame that finishes next, over the exit
    /// the kept frame declared, and cleared when it finishes.
    exits_named: rustc_hash::FxHashMap<Key, crate::enter::Enter>,
    /// Why frames run: the reasons, and — traced — who held an owed one
    /// and whether a frame changed anything (`runtime/cause.rs`).
    trace: cause::Trace,
    /// The focused editor's caret rect (logical, viewport coords) as of the
    /// last finish_frame — where drivers should anchor the OS IME window.
    ime_rect: Option<Rect>,
    /// A custom editor's caret as of the last frame: the `line` under the
    /// focused sink that declares `caret`, and the offset it declares.
    /// What the blink clock is armed on when no stock
    /// editor is focused; `None` with nothing to blink.
    sink_caret: Option<(Key, u32)>,
    /// Whether that line declared its caret `caret_solid` — a block caret
    /// in a modal editor's normal mode: still the IME's anchor and the
    /// access tree's caret, but not a caret to blink, so `has_caret`
    /// leaves it out and an idle app draws no frame for it.
    sink_caret_solid: bool,
    /// Bumped whenever `sink_caret` changes between frames — the caret
    /// moved, or focus came to or left a custom editor — so the driver
    /// re-arms the blink solid, the way `EditStore::caret_stamp` does for
    /// the stock editor. The two are summed in `caret_stamp`.
    sink_caret_stamp: u64,
    /// Events raised by the frame driver's own reports rather than by input
    /// — a changed viewport becoming a `resize`. Drained alongside the
    /// interaction's pending queue.
    pending: Vec<UiEvent>,
    /// Whether any frame has begun yet: the first one establishes the
    /// viewport instead of resizing it.
    framed: bool,
    /// The OS settings the last frame was begun with. A driver pushes
    /// them into `env` whenever it learns of a change, and the difference
    /// between two frames is what becomes a `system` event — the same
    /// bookkeeping `viewport` does for `resize`.
    system_seen: SystemEnv,
    /// The session's `system_fonts_rev` the last frame was begun with; the
    /// difference is what becomes a `fonts` event.
    system_fonts_seen: u64,
    /// Frames begun so far; stamps the per-key stores below.
    frame_no: u64,
    /// The rect last reported for each `on_layout` node and the frame it
    /// was seen: a different rect, or a node not seen last frame, posts a
    /// `layout` event (see `emit_layout_events`).
    layouts: FxHashMap<Key, (Rect, u64)>,
    /// One-off announcements queued since the last drain (see
    /// [`Self::announce`]). The fourth of
    /// the four drained channels, and the same shape as the other three:
    /// the core appends, a driver drains, a headless test asserts on what
    /// it drained.
    announcements: Vec<crate::access::Announcement>,
    /// The last announcement's text, the frame it was queued on and the
    /// [`Self::events_answered`] reading then: the same text on two
    /// consecutive frames with no event handed to the app between them is
    /// what an unguarded `ui.announce(...)` in a view looks like, and
    /// `announcement-repeated` says so. The event count is what tells a
    /// window that redraws only on input apart from one shouting every
    /// frame — two Copy presses in a row are two consecutive frames there.
    last_announcement: Option<(String, u64, u64)>,
    /// How many times `handle_input` handed the app at least one event.
    events_answered: u64,
    /// The `reveal(key)`s waiting for a layout to resolve against, in the
    /// order asked: the next `finish_frame` scrolls each node's scrolling
    /// ancestor to show it, then clears them. Within one container the
    /// last ask wins; asks aimed at different containers all land.
    pending_reveal: Vec<Key>,
    /// `reveal_label` and `set_scroll_label` asks, with the origin that
    /// asked: resolved when the frame finishes, where a label named before
    /// its node is declared — later in the same build, or by the next
    /// frame — has a node to find.
    pending_reveal_labels: Vec<(String, crate::tree::OriginId)>,
    /// The `on_focus` nodes the focus was last reported inside, outermost
    /// first, with each one's origin and tag — what `report_focus` diffs
    /// the focus against.
    focus_reported: Vec<(Key, crate::tree::OriginId, Value)>,
    pending_scroll_labels: Vec<(String, crate::tree::OriginId, Vec2)>,
    /// Type-ahead inside a composite: the
    /// characters typed so far, and the frame clock reading of the last
    /// keystroke. The buffer is cleared at the start of the first frame,
    /// or the first keystroke, more than [`TYPE_AHEAD_SECS`] after it —
    /// the keystroke's against the clock its driver stamped before the
    /// input (backlog F139), since a parked window draws no frame in
    /// between. With no
    /// clock set `type_ahead_at` is None and every keystroke starts a
    /// fresh search, which is the useful half of type-ahead.
    type_ahead: String,
    type_ahead_at: Option<f64>,
    /// Reused by the composite walks (the ring, arrow motion), so a frame
    /// with composites in it pays one allocation rather than one each.
    items_scratch: Vec<usize>,
    /// A `focus_next` / `focus_prev` waiting for a frame to walk: `true`
    /// forward. The Tab ring is the finished tree's, and a request made
    /// while a frame is being built has no tree to walk yet (`begin_frame`
    /// cleared it), so the step is held until `finish_frame` — after the
    /// modal scope is resolved, which is what scopes the ring. Last writer
    /// wins, and an applied step beats a `set_focus` from the same frame.
    pending_focus_step: Option<bool>,
    /// The focus region in effect — the key of the node whose subtree Tab
    /// walks — or `None` for the main ring (the tree minus every region).
    /// Follows focus: `set_focus` moves it to the region enclosing the
    /// focused node, a press settles it on the region under the pointer,
    /// and it is kept across a blur so Tab re-enters where the user was.
    region: Option<Key>,
    /// Whether a press settled `region` somewhere other than the focused
    /// node's own region — dead space in the dock, focus on the root sink
    /// the press bubbled to — so the region stops following that focus
    /// until it moves (decision 3's second sentence). Cleared by any
    /// `set_focus` that changes the focus.
    region_held: bool,
    /// The focus each region last held, main (`None`) included: what
    /// `focus_region` lands on when entering it again, and what main gets
    /// back when the region in effect stops being declared.
    region_focus: Vec<(Option<Key>, Option<Key>)>,
    /// A `focus_region` waiting for the frame to finish, for the reason
    /// `pending_focus_step` waits: the ring it enters is the finished
    /// tree's, and the caller may name a node the last frame did not have.
    /// Last writer wins.
    pending_region: Option<RegionTarget>,
    /// Silent-misconfiguration detection; see `diag`.
    diag: Diagnostics,
    /// The access tree of the last finished frame, built on demand (see
    /// `access_tree`) and stamped with the frame it was built from.
    access: crate::access::AccessTree,
    access_built: u64,
    /// The hash of the inputs `self.access` was derived from, so a frame
    /// whose access-relevant state is unchanged keeps it. `None` when the last
    /// frame could not be hashed, which
    /// forces the next derivation.
    access_inputs: Option<u64>,
    /// How many times the access tree has actually been derived, as against
    /// asked for. Tests read it to tell a cache hit from a miss; nothing in
    /// the library acts on it.
    access_rebuilds: u64,
}

/// How long a type-ahead search buffer survives without a keystroke.
/// Aged at
/// the start of a frame, so a core with no clock never ages one.
const TYPE_AHEAD_SECS: f64 = 1.0;

/// One step along a list of `len` items from `at`, wrapping or clamping.
/// None = the step ran off the end of a list that clamps.
fn step(at: usize, delta: isize, len: usize, wrap: bool) -> Option<usize> {
    let n = len as isize;
    let p = at as isize + delta;
    if (0..n).contains(&p) {
        return Some(p as usize);
    }
    wrap.then(|| (((p % n) + n) % n) as usize)
}

/// What a frame left owed, by kind: [`Core::owed`]. `any()` is what
/// [`Core::animating`] answers; `beyond_cycles()` is the same with a
/// keyframe cycle — which never ends — left out.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub struct Owed {
    /// A finite transition — a leg or a spring — still mid-flight.
    pub transition: bool,
    /// A keyframe cycle running; it always is, while its node is drawn.
    pub cycle: bool,
    /// An exit animation (a ghost) still departing.
    pub depart: bool,
    /// A frame a view asked for: `request_frame`, or an `animate` row —
    /// or one the session's fonts ask for, moved under the text this
    /// window shaped through another window (a new fallback list, a
    /// rescan).
    pub requested: bool,
    /// A held drag scrolling its container.
    pub autoscroll: bool,
    /// A scroll container easing a programmatic offset change — a
    /// `reveal` or a `set_scroll` on a container with a `transition`.
    pub scroll: bool,
}

impl Owed {
    /// Anything at all: the driver's reading.
    pub fn any(self) -> bool {
        self.transition
            || self.cycle
            || self.depart
            || self.requested
            || self.autoscroll
            || self.scroll
    }

    /// Anything but a cycle: what a test waits on when the view has a
    /// cycle that will never let `any()` clear. An eased scroll is a
    /// finite leg like a transition, so it is in.
    pub fn beyond_cycles(self) -> bool {
        self.transition || self.depart || self.requested || self.autoscroll || self.scroll
    }
}

impl Core {
    /// This window's palette, as of this frame.
    /// Resolved from [`Core::theme_source`] and `env.system` at the start
    /// of every frame, so it is already right by the time a view runs.
    ///
    /// Frame-stable on purpose: every widget in one frame paints from the
    /// same palette, whatever the driver does to `env` while the view
    /// runs. A host that writes `env.system` *directly* and wants the new
    /// answer before its next frame calls [`Core::refresh_theme`]; every
    /// env setter a binding exposes already does.
    ///
    /// Read inside a fill being kept (ADR 0045), the palette is one of
    /// the facts the replay is checked against: a fill that drew from it
    /// runs again when it changes (backlog F155).
    pub fn theme(&self) -> &Theme {
        self.note_read_once(replay::ONCE_THEME, || {
            replay::Read::Theme(Box::new(self.theme))
        });
        &self.theme
    }

    /// Re-resolve the palette from `env.system` now, rather than at the
    /// start of the next frame. What an env setter calls after writing.
    pub fn refresh_theme(&mut self) {
        self.theme = self.theme_source.resolve(&self.env.system);
    }

    /// Writes what the user set in the OS and re-resolves the palette from
    /// it, so a host that pushes the appearance and reads the theme back
    /// before its next frame sees the answer. The one door for
    /// a binding's env setter: writing `env.system` by hand and forgetting
    /// the refresh was a decision each of them had to remember.
    pub fn set_system(&mut self, system: SystemEnv) {
        self.env.system = system;
        self.refresh_theme();
    }

    /// The env reading's inputs (`schema::ENV_FIELDS`): the stored env and
    /// the frame's own facts — viewport, scale, focus — that ride in the
    /// same reading.
    pub fn env_facts(&self) -> crate::schema::EnvFacts {
        let facts = self.env_facts_raw();
        // A fill being kept read the frame's facts (ADR 0045).
        self.note_read(|| replay::Read::Env(facts));
        facts
    }

    pub(crate) fn env_facts_raw(&self) -> crate::schema::EnvFacts {
        crate::schema::EnvFacts {
            env: self.env,
            // The frame's viewport, as its `ENV_FIELDS` row says: what the
            // dock leaves, not the window it was begun with (backlog F43
            // — this read `self.viewport`, and an app under `KUI_DEVTOOLS`
            // sized its tiers to a window it did not have).
            viewport: self.viewport(),
            scale: self.scale,
            focus: self.focus(),
            focus_visible: self.focus_visible(),
            region: self.region(),
            // Read raw, not through the noting doors: the reading carries
            // both for a binding to hand out, and a kept fill compares the
            // reading less these two (`replay::env_same`); a view that
            // wants them counted reads `Ui::caret_visible` / `Ui::now`.
            caret_visible: self.edit.blink_visible(),
            now: self.anim.time().unwrap_or(0.0),
        }
    }

    /// Whether anyone actually *chose* the accent — the OS reported one,
    /// or the app set or pinned one — as opposed to the palette falling
    /// back to kui's own blue.
    ///
    /// The question the `accent` row asks before it repaints anything:
    /// that row has always meant "the accent colour where there is one,
    /// the `bg` I declared where there is not", so a view can name its
    /// own fallback and a host that knows nothing changes nothing. The
    /// theme widened *where* the accent comes from without widening
    /// *whether* there is one.
    pub fn has_accent(&self) -> bool {
        let chosen = self.has_accent_raw();
        self.note_read_once(replay::ONCE_ACCENT, || replay::Read::Accent(chosen));
        chosen
    }

    /// [`Self::has_accent`], not noted for a kept fill.
    pub(crate) fn has_accent_raw(&self) -> bool {
        match self.theme_source {
            ThemeSource::Derived => self.env.system.accent.is_some(),
            ThemeSource::DerivedWithAccent(_) | ThemeSource::Pinned(_) => true,
        }
    }

    /// Where the palette comes from. [`ThemeSource::Derived`] by default.
    pub fn theme_source(&self) -> ThemeSource {
        self.theme_source
    }

    /// Change where the palette comes from. Takes effect on the next
    /// frame, and immediately for anything reading [`Core::theme`] after
    /// this call, so a host may set it before its first frame or in a
    /// handler and get the same answer either way.
    pub fn set_theme_source(&mut self, source: ThemeSource) {
        self.theme_source = source;
        self.refresh_theme();
    }

    /// Pin a palette: this exact [`Theme`], following neither the OS's
    /// appearance nor its accent. Shorthand for
    /// [`ThemeSource::Pinned`].
    pub fn set_theme(&mut self, theme: Theme) {
        self.set_theme_source(ThemeSource::Pinned(theme));
    }

    /// Keep following the OS's light/dark, but paint this accent instead
    /// of the OS's. Shorthand for [`ThemeSource::DerivedWithAccent`], and
    /// what an app with a brand colour wants.
    pub fn set_accent(&mut self, accent: Color) {
        self.set_theme_source(ThemeSource::DerivedWithAccent(accent));
    }

    /// Go back to following the OS for both — the default.
    pub fn derive_theme(&mut self) {
        self.set_theme_source(ThemeSource::Derived);
    }

    /// The sizes the stock widgets are built from — the palette's other
    /// axis ([`crate::metrics`]). [`Metrics::default`](crate::metrics::Metrics::default) until
    /// the app sets one; nothing in the OS is followed. Noted for a fill
    /// being kept, as [`Self::theme`] is.
    pub fn metrics(&self) -> &crate::metrics::Metrics {
        self.note_read_once(replay::ONCE_METRICS, || {
            replay::Read::Metrics(Box::new(self.metrics))
        });
        &self.metrics
    }

    /// Declare the tokens the running origin references by name:
    /// the host's outside a
    /// fill, the filling extension's inside one. Replaces that origin's
    /// table whole, so an app whose lengths change with a viewport tier
    /// declares again on `resize`. A name a role owns is dropped with a
    /// `reserved-token` warning, once per name. A derived token whose
    /// source did not resolve is dropped with `unknown-token`, naming both.
    pub fn set_tokens(&mut self, tokens: crate::tokens::Tokens) {
        for name in tokens.reserved() {
            let w = Warning {
                code: crate::diag::RESERVED_TOKEN,
                key: Key::ROOT.str(crate::diag::RESERVED_TOKEN).str(name),
                message: format!(
                    "`{name}` is a theme or metrics role, so the token is dropped: `${name}` \
                     always means the role's value, and an app does not shadow one"
                ),
            };
            self.diag.raise(w);
        }
        // A derived token whose source did not resolve (ADR 0028) is
        // dropped the same way, and the warning names both ends: the
        // token that is gone and the name that was not there for it.
        for u in tokens.unresolved() {
            let w = Warning {
                code: crate::diag::UNKNOWN_TOKEN,
                key: Key::ROOT.str(crate::diag::UNKNOWN_TOKEN).str(&u.token),
                message: format!(
                    "`${}` is dropped: it derives from `${}`, which is no colour token \n                     declared before it and no theme role",
                    u.token, u.source
                ),
            };
            self.diag.raise(w);
        }
        self.tokens.insert(self.origin, tokens);
    }

    /// Whether `origin` has declared a table — what an extension asks
    /// before declaring the one it was loaded with, so a per-frame `view`
    /// declares once.
    pub fn tokens_declared(&self, origin: OriginId) -> bool {
        self.tokens.contains_key(&origin)
    }

    /// The running origin's own table, if it declared one.
    pub fn tokens(&self) -> Option<&crate::tokens::Tokens> {
        self.tokens.get(&self.origin)
    }

    /// What a `$name` in a prop resolves to this frame: the running
    /// origin's table over the host's, the theme's and metrics' roles in
    /// front of both. What every binding lowers a reference through.
    ///
    /// A lookup can answer a theme or metrics role, and a binding bakes
    /// what it answers into the spec, so a fill being kept that looks a
    /// name up has read both (ADR 0045, backlog F155).
    pub fn token_lookup(&self) -> crate::tokens::TokenLookup<'_> {
        self.note_read_once(replay::ONCE_THEME, || {
            replay::Read::Theme(Box::new(self.theme))
        });
        self.note_read_once(replay::ONCE_METRICS, || {
            replay::Read::Metrics(Box::new(self.metrics))
        });
        let own = self.tokens.get(&self.origin);
        let host = if self.origin == OriginId::HOST {
            None
        } else {
            self.tokens.get(&OriginId::HOST)
        };
        crate::tokens::TokenLookup {
            own,
            host,
            theme: &self.theme,
            metrics: &self.metrics,
            session: Some(&self.session),
        }
    }

    /// A binding lowered a `family` that names nothing installed or
    /// loaded: raise `unknown-family`, once per name. The text
    /// shapes as sans, which is what the message says.
    pub fn warn_unknown_family(&mut self, name: &str) {
        self.diag.raise(Warning {
            code: crate::diag::UNKNOWN_FAMILY,
            key: Key::ROOT.str(crate::diag::UNKNOWN_FAMILY).str(name),
            message: format!(
                "`family` named {name:?}, and no installed or loaded font family has that name, \
                 so the text shapes as sans; `systemFonts()` lists the names there are, and \
                 `sans`, `serif` and `mono` are kui's own"
            ),
        });
        // A family a sibling prop registered in the same parse is listed
        // by the next frame's sync; nothing to do here.
    }

    /// A binding lowered a reference that did not resolve: raise
    /// `unknown-token`, once per name, saying which slot asked. The slot
    /// keeps its default, which is what the message says.
    pub fn warn_unknown_token(&mut self, err: &crate::tokens::TokenError) {
        let name = match err {
            crate::tokens::TokenError::Unknown(n)
            | crate::tokens::TokenError::Kind { name: n, .. } => n,
        };
        let w = Warning {
            code: crate::diag::UNKNOWN_TOKEN,
            key: Key::ROOT.str(crate::diag::UNKNOWN_TOKEN).str(name),
            message: err.to_string(),
        };
        self.diag.raise(w);
    }

    /// Makes `metrics` the frame's: every stock widget from the next node
    /// on is built from it, and `ui.metrics()` reads it back. Logical px,
    /// before `env.scale`; a density is the app's to choose
    /// (`Metrics::compact`, `Metrics::scaled`).
    ///
    /// `titlebar_h` is the platform's row: left at the stock number (the
    /// platform's caption height, which every stock set carries), it is
    /// the height the driver opened this window's strip at
    /// ([`Core::set_platform_titlebar_h`]), so an app that sets a density
    /// keeps the titlebar its launcher asked for. Any other number is the
    /// app's and stands.
    pub fn set_metrics(&mut self, metrics: crate::metrics::Metrics) {
        let mut metrics = metrics;
        if metrics.titlebar_h == crate::metrics::Metrics::comfortable().titlebar_h {
            metrics.titlebar_h = self.platform_titlebar_h();
        }
        self.metrics = metrics;
    }

    /// What the platform's caption height is for this window: `h` for a
    /// strip the driver knows is another height (backlog W22) — the
    /// runner's `Launcher::titlebar` under custom chrome off macOS, the
    /// titlebar it measured under macOS custom chrome — `None` for the
    /// platform's own. The `titlebar_h` metric follows it
    /// while it is at the stock number, now and through every later
    /// [`Core::set_metrics`]; an app that set a number of its own keeps
    /// it. A driver's call, made before the first frame.
    pub fn set_platform_titlebar_h(&mut self, h: Option<f32>) {
        let stock = self.metrics.titlebar_h == self.platform_titlebar_h();
        self.platform_titlebar_h = h.filter(|h| h.is_finite() && *h > 0.0);
        if stock {
            self.metrics.titlebar_h = self.platform_titlebar_h();
        }
    }

    /// The titlebar height that stands for "the platform's" in this
    /// window: the driver's, or the stock caption height.
    fn platform_titlebar_h(&self) -> f32 {
        self.platform_titlebar_h
            .unwrap_or(crate::metrics::Metrics::comfortable().titlebar_h)
    }

    /// A core with a session of its own — one window, nothing shared.
    pub fn new() -> Self {
        Self::new_in(&Session::new())
    }

    /// A core joining an existing session: it draws with the same fonts,
    /// images, sounds, shaping caches and glyph atlas as every other core
    /// constructed against `session`, and plays through the same audio
    /// device. Everything else — tree, focus, scroll, viewport — is this
    /// window's alone.
    pub fn new_in(session: &Session) -> Self {
        let mut core = Self {
            session: session.clone(),
            text: TextSystem::new(),
            cells: crate::cells::CellStore::new(),
            atlas: GlyphAtlas::new(),
            resources: SharedResources::new(session),
            audio: SharedAudio::new(session),
            font_names: FxHashMap::default(),
            fonts_rev: u64::MAX,
            weights_rev: 0,
            images_rev: 0,
            interaction: Interaction::default(),
            scroll: ScrollStore::default(),
            edit: EditStore::default(),
            anim: AnimStore::default(),
            depart: DepartStore::default(),
            stats: FrameStats::default(),
            env: Env::default(),
            theme_source: ThemeSource::Derived,
            theme: Theme::default(),
            tokens: Default::default(),
            metrics: crate::metrics::Metrics::default(),
            platform_titlebar_h: None,
            window_title: None,
            always_on_top: false,
            secure_input: false,
            option_as_alt: crate::input::OptionAsAlt::None,
            ime_off: false,
            focus: None,
            focus_visible: false,
            declared_focus: Vec::new(),
            declared_focus_last: Vec::new(),
            focus_asked: false,
            key_labels: LabelIndex::default(),
            key_labels_last: LabelIndex::default(),
            slot_labels: LabelIndex::default(),
            recording: None,
            keep_next: None,
            kept: Default::default(),
            slot_fills: Vec::new(),
            slot_fills_last: Vec::new(),
            ns_depth: usize::MAX,
            ns_key: Key::ROOT,
            dt_app: None,
            dt_dock: None,
            dt_window: false,
            dt_theme: None,
            dt_menus: None,
            dt_built: None,
            dt_tabs: Vec::new(),
            dt_area: Rect::new(0.0, 0.0, 0.0, 0.0),
            building: false,
            declared_windows: Vec::new(),
            declared_windows_last: Vec::new(),
            keys_held: Vec::new(),
            pressed_mods: None,
            access: Default::default(),
            access_built: 0,
            access_inputs: None,
            access_rebuilds: 0,
            tree: Tree::new(),
            inspect: false,
            dt_inspect: false,
            inspected: Vec::new(),
            prev_tree: Tree::new(),
            lines: Default::default(),
            paths: Default::default(),
            path_motion: Default::default(),
            path_parsed: Default::default(),
            path_textures: crate::path::PathTextures::new(session.clone()),
            output_read: true,
            fragments: Default::default(),
            stock_polygon: None,
            hit_shapes: Default::default(),
            display: DisplayList::default(),
            viewport: Size::ZERO,
            scale: 1.0,
            stack: Vec::new(),
            counters: Vec::new(),
            origin: OriginId::HOST,
            clips: Vec::new(),
            clip_ids: Vec::new(),
            opacity: Vec::new(),
            float_root: Vec::new(),
            float_stack: Vec::new(),
            hints: Vec::new(),
            menu: None,
            menu_actions: Vec::new(),
            menu_editor: None,
            menu_focus: None,
            native_menus: false,
            menu_sub: Default::default(),
            menu_bar_sub: Default::default(),
            menu_bar: None,
            menu_bar_rev: 0,
            menu_bar_origin: OriginId::HOST,
            menu_bar_open: None,
            menu_bar_root: None,
            selects: Vec::new(),
            native_menu_bar: false,
            lookup_available: false,
            selection: None,
            cell_selection: None,
            awaiting_selection: false,
            awaiting_paste: false,
            file_ask: Default::default(),
            select_dragging: None,
            drag_follow: None,
            last_frame_time: None,
            line_carry: None,
            scroll_latch: Default::default(),
            sel_ords: Vec::new(),
            sel_ends: None,
            scopes: Vec::new(),
            rows: Vec::new(),
            modal: None,
            modal_focus: Vec::new(),
            type_ahead: String::new(),
            type_ahead_at: None,
            items_scratch: Vec::new(),
            ghost_opacity: Vec::new(),
            ghost_clip: Vec::new(),
            ghost_clip_ids: Vec::new(),
            ghost_rect: Vec::new(),
            frame_requested: false,
            frame_due: None,
            exits_named: Default::default(),
            trace: cause::Trace::default(),
            pending_reveal: Vec::new(),
            pending_reveal_labels: Vec::new(),
            focus_reported: Vec::new(),
            pending_scroll_labels: Vec::new(),
            pending_focus_step: None,
            region: None,
            region_held: false,
            region_focus: Vec::new(),
            pending_region: None,
            ime_rect: None,
            sink_caret: None,
            sink_caret_solid: false,
            sink_caret_stamp: 0,
            pending: Vec::new(),
            framed: false,
            system_seen: SystemEnv::default(),
            system_fonts_seen: 0,
            frame_no: 0,
            layouts: FxHashMap::default(),
            announcements: Vec::new(),
            last_announcement: None,
            events_answered: 0,
            diag: Diagnostics::default(),
        };
        core.sync_font_names();
        core
    }

    // -- Measurement ----------------------------------------------------
    // The layout engine measures text every frame; these hand the same
    // numbers to the view, so it never re-derives them by hand.

    /// Measures `content` in `style` without adding a node: its unwrapped
    /// size, or with `max_w` (logical px) its size once wrapped to that
    /// width — what layout would give a text node with that content and
    /// style, `wrap` / `max_lines` / `ellipsis` included. Logical px at
    /// the scale of the current or last frame (1 before any frame).
    /// Shapes through the text cache, so measuring a string and then
    /// drawing it shapes once.
    pub fn measure_text(
        &mut self,
        content: &str,
        style: &TextStyle,
        max_w: Option<f32>,
    ) -> TextMetrics {
        self.note_read(|| replay::Read::Measure(self.text_rev()));
        let sess = &mut *self.session.state();
        self.text
            .measure(content, style, &sess.resources, &mut sess.fonts, max_w)
    }

    /// `measure_text` for a rich-text paragraph.
    pub fn measure_rich_text(
        &mut self,
        spans: &[Span<'_>],
        base: &TextStyle,
        max_w: Option<f32>,
    ) -> TextMetrics {
        let sess = &mut *self.session.state();
        self.text
            .measure_rich(spans, base, &sess.resources, &mut sess.fonts, max_w)
    }

    /// Where a point lands in the text node `key` drew: a byte offset into
    /// its content and the visual row within that node — counted across
    /// every run the key covers by where the rows sit, so a `line` row of
    /// inline runs is one row and a wrapped run as many as it wrapped to;
    /// not the ordinal `line` node a pointer event names
    /// — or `None` for a key that is not a text node or was not drawn.
    /// A `role="none"` subtree under the key (a gutter) is
    /// not its text, as the access tree reads it. `point` is logical
    /// viewport px — the `x`/`y` a click or drag event carries — so a
    /// custom editor turns the event into a caret position with one call
    /// instead of measuring prefixes or assuming a cell width. Answered
    /// from the frame that finished: between frames that is the layout the
    /// pointer was over, and during a build it is the last one, since the
    /// node being declared has no layout yet. A wrapped node answers in
    /// the width it was drawn at.
    pub fn text_hit(&self, key: Key, point: Vec2) -> Option<TextHit> {
        let hit = self.text_hit_raw(key, point);
        self.note_read(|| replay::Read::TextHit(key, point, hit));
        hit
    }

    pub(crate) fn text_hit_raw(&self, key: Key, point: Vec2) -> Option<TextHit> {
        // The host's point is its own viewport's; the turns, like the
        // text, are the window's, so the shift comes first.
        self.text.hit_at(
            key,
            self.unturned(key, point.plus(self.dt_shift())),
            self.building,
        )
    }

    /// `p`, a window point, pulled back through every turn the node `key`
    /// was drawn under in the frame that finished (ADR 0043): where on the
    /// node's upright layout — the space its text, its caret and its
    /// content origin are in — the point falls. The point itself on a
    /// frame that turns nothing, which pays one branch, and during a
    /// build, which has no finished node to read the turn off.
    pub(crate) fn unturned(&self, key: Key, p: Vec2) -> Vec2 {
        if self.building || !self.tree.any_transform {
            return p;
        }
        self.tree
            .index_of(key)
            .map_or(p, |i| self.unturned_at(i, p))
    }

    /// [`Self::unturned`] for the node at tree index `i` of the frame that
    /// finished, for a caller that has the index.
    pub(crate) fn unturned_at(&self, i: usize, p: Vec2) -> Vec2 {
        if self.building || !self.tree.any_transform {
            return p;
        }
        match self.clips.get(i) {
            Some(c) if c.turned() => c.transform.unapply(p),
            _ => p,
        }
    }

    /// `r`, a rect in the node `key`'s upright layout, as drawn: the box
    /// it covers through the node's turns (ADR 0043), the way the access
    /// rect is a turned node's bounding box. `r` itself when nothing turns.
    pub(crate) fn turned_rect(&self, key: Key, r: Rect) -> Rect {
        if self.building || !self.tree.any_transform {
            return r;
        }
        match self.tree.index_of(key).and_then(|i| self.clips.get(i)) {
            Some(c) if c.turned() => c.transform.bounds(r),
            _ => r,
        }
    }

    /// The caret rect for byte `byte` of the text node `key` drew: logical
    /// viewport px, zero wide, one line tall — where a caret, an IME
    /// candidate window or a selection edge goes. `byte` past the content
    /// is the end. Answered from the same frame `text_hit` is.
    pub fn caret_rect(&self, key: Key, byte: usize) -> Option<Rect> {
        let shift = self.dt_shift();
        self.text
            .caret_at(key, byte, self.building)
            // Turned in the window's space, then moved into the host's.
            .map(|r| self.turned_rect(key, r))
            .map(|r| Rect::new(r.x - shift.x, r.y - shift.y, r.w, r.h))
    }

    // -- Announcements ---------------------------------------------------

    /// Says something once, with no node behind it: "Saved", "3 results".
    /// Queued for [`Self::take_announcements`], the way `play` queues an
    /// audio command — an announcement is a consequence of an event, and
    /// the frame's tree, which is a function of state, has no place to
    /// keep one.
    /// A region whose text changes on screen is the other half, and is
    /// the `live` prop instead.
    ///
    /// [`Live::Off`](crate::access::Live::Off) and an empty string are both no-ops — the first so a
    /// caller can gate politeness without an `if`, the second because
    /// every platform needs a name to say.
    pub fn announce(&mut self, text: &str, live: crate::access::Live) {
        if live == crate::access::Live::Off || text.is_empty() {
            return;
        }
        if let Some((last, frame, answered)) = &self.last_announcement
            && last == text
            && *frame + 1 >= self.frame_no
            && *answered == self.events_answered
        {
            self.diag.raise(crate::diag::announcement_repeated(text));
        }
        self.last_announcement = Some((text.to_string(), self.frame_no, self.events_answered));
        self.announcements.push(crate::access::Announcement {
            text: text.to_string(),
            live,
        });
    }

    /// Drains the announcements queued since the last drain. Windowed
    /// runners drain every frame whether or not assistive technology is
    /// attached, and discard what they cannot deliver, so a real app never
    /// accumulates and nothing is spoken minutes late; headless drivers
    /// assert on what comes back.
    pub fn take_announcements(&mut self) -> Vec<crate::access::Announcement> {
        std::mem::take(&mut self.announcements)
    }

    /// Announcements queued and not yet drained (what `pending` is for
    /// audio commands).
    pub fn pending_announcements(&self) -> &[crate::access::Announcement] {
        &self.announcements
    }

    // -- Diagnostics ----------------------------------------------------

    /// Drains the warnings raised since the last drain (see [`crate::diag`]):
    /// silent misconfigurations the core noticed while finishing frames,
    /// each distinct (code, node) pair once. Windowed runners print them;
    /// headless tests assert on them.
    pub fn take_warnings(&mut self) -> Vec<Warning> {
        // Handles of other sessions resolve where the registry can see
        // them and this core cannot (shaping, the audio backend), so the
        // registry keeps them and the core draining warnings reports them.
        for f in self.session.state().resources.take_foreign() {
            self.diag.raise(crate::diag::foreign_resource(&f));
        }
        if let Some(w) = crate::diag::size_expressions_full() {
            self.diag.raise(w);
        }
        self.diag.take()
    }

    /// Every warning this core has raised, drained or not, oldest first —
    /// for a reader that is not the driver. The runner drains
    /// [`Self::take_warnings`] after every frame and prints them, so a
    /// view that wants to *show* them (a development overlay) would
    /// otherwise never see one; this is the log the drain leaves behind.
    pub fn warnings_raised(&self) -> &[Warning] {
        self.diag.raised()
    }

    /// Raises a warning a binding built (see [`crate::diag::unknown_prop`]):
    /// a frontend sees declarations the tree walk cannot, because a prop
    /// name nothing claims never becomes part of a node. Behind the same
    /// [`Self::set_diagnostics`] gate and the same once-per-(code, key)
    /// dedup as the checks, so a binding may raise one per node per frame.
    pub fn warn(&mut self, warning: Warning) {
        self.diag.raise(warning);
    }

    /// Turns the diagnostic checks on or off. A bare `Core` has them on;
    /// drivers set them for the build they are in (the runner: debug on,
    /// release off; Node loops: off under `NODE_ENV=production`; a
    /// standalone C context: off until asked).
    pub fn set_diagnostics(&mut self, on: bool) {
        self.diag.enabled = on;
    }

    pub fn diagnostics(&self) -> bool {
        self.diag.enabled
    }

    /// Wheel line-deltas (e.g. winit's LineDelta) to logical px.
    pub fn lines_to_px(lines: f32) -> f32 {
        lines * SCROLL_LINE_PX
    }

    /// Rasterizes outline glyphs as LCD subpixel coverage
    /// (`QuadKind::GlyphSubpixel`) instead of alpha masks. Drivers set it
    /// from what their renderer can blend per channel; flipping it drops
    /// the glyph atlas so every glyph re-rasterizes in the new mode.
    pub fn set_subpixel_text(&mut self, on: bool) {
        if self.text.set_subpixel(on) {
            self.atlas.clear();
        }
    }

    pub fn subpixel_text(&self) -> bool {
        self.text.subpixel()
    }

    /// The byte budget for the shaped-text cache: every
    /// text a frame draws is shaped once and kept, and past this many
    /// estimated bytes the least recently drawn entries go, down to three
    /// quarters of it, at the start of the next frame. What the last
    /// frame drew is never evicted, so a budget too small for one
    /// screenful costs re-shaping nothing — it only stops keeping what
    /// scrolled away. Default `DEFAULT_TEXT_CACHE_BYTES` (64 MB): a
    /// terminal streaming new lines lowers it, a document viewer that
    /// wants every page it showed to stay warm raises it. The clock that
    /// empties an idle cache after 300 frames is unchanged.
    pub fn set_text_cache_budget(&mut self, bytes: usize) {
        self.text.set_budget(bytes);
    }

    pub fn text_cache_budget(&self) -> usize {
        self.text.budget()
    }

    /// What the shaped-text cache holds, as the estimate the budget is
    /// charged against (a fixed floor per entry plus a per-glyph rate,
    /// calibrated against a counting allocator; see `text.rs`).
    pub fn text_cache_bytes(&self) -> usize {
        self.text.bytes()
    }

    /// How many shaped texts the cache holds (a long line's chunks each
    /// count).
    pub fn text_cache_len(&self) -> usize {
        self.text.len()
    }

    /// How many long lines — no-wrap texts past `LONG_LINE_BYTES`, shaped
    /// in chunks — are held.
    pub fn long_lines(&self) -> usize {
        self.text.long_lines()
    }

    /// The frame clock for transitions: monotonic seconds, any origin.
    /// Drivers set it before every frame, and before handing the core
    /// input or the app its events (backlog F139) — the windowed runner
    /// does both — so a handler that reads [`Self::now`] after an idle
    /// stretch reads the time it runs at, not the last frame's. A driver
    /// that never sets it gets snapping instead of animation.
    pub fn set_time(&mut self, now_secs: f64) {
        self.anim.set_time(now_secs);
        self.scroll.set_time(now_secs);
    }

    /// The frame clock (backlog F134): the driver's monotonic seconds the
    /// transitions and cycles of this frame read, from whatever origin the
    /// driver chose; 0 before a driver sets one. What a view reads for
    /// "is this toast due", so that the app's deadlines and the core's
    /// easing agree, and a test that moves the clock (`Drive::advance`,
    /// Node's `advance`, `kui_set_time`) moves both. Read from an event
    /// handler it is the time the input was handled at: the windowed
    /// runner stamps the clock before input as well as before a frame
    /// (backlog F139), so a deadline set in a click handler after the
    /// window sat idle counts from the click.
    pub fn now(&self) -> f64 {
        self.note_read(|| replay::Read::Clock);
        self.anim.time().unwrap_or(0.0)
    }

    /// True when the last frame left a transition mid-flight, or a view
    /// asked for another frame — drivers schedule one without waiting for
    /// input. One bool over every source; [`owed`](Self::owed) is the
    /// same reading by kind.
    pub fn animating(&self) -> bool {
        self.owed().any()
    }

    /// What the last frame left owed, by kind. To a driver
    /// the kinds are one — it schedules the frame either way — but a
    /// test that wants to know whether the *transitions* have run out
    /// under a keyframe cycle that never will reads `cycle` apart from
    /// the rest: [`Owed::beyond_cycles`] is that wait's predicate.
    pub fn owed(&self) -> Owed {
        let (transition, cycle) = self.anim.owes();
        Owed {
            transition,
            cycle,
            depart: self.depart.animating(),
            requested: self.frame_requested
                || self.tree.any_animate
                || self.fonts_moved()
                || self.submenu_waiting(),
            autoscroll: self.autoscrolling(),
            scroll: self.scroll.animating(),
        }
    }

    /// Whether the session's fonts moved under the text this window shaped
    /// since its last frame began — a new fallback list, a rescan, a face of
    /// a registered family come or gone, through whichever window of the
    /// session — so the window owes a frame to shape it again (backlog
    /// RG118). The window that made the change asks for its own frame;
    /// this is how every other window hears, through the `animating` a
    /// driver polls for each of its windows, with no list of the session's
    /// windows to wake. A window that has not drawn has nothing to shape
    /// again. `try_state`, since a driver may ask while the session is
    /// borrowed, where it reads as nothing owed.
    fn fonts_moved(&self) -> bool {
        self.framed
            && self
                .session
                .try_state()
                .is_some_and(|sess| sess.weights_rev != self.weights_rev)
    }

    /// Asks the driver for one more frame right after this one. A view
    /// that sets up a transition by drawing a starting state (a new split
    /// drawn collapsed so it can slide open) needs the next frame to come
    /// without waiting for input — the starting state itself snaps, so
    /// nothing is mid-flight yet to request it.
    ///
    /// Traced ([`Self::set_frame_trace`]), the calling line is kept as
    /// the frame's [`cause::FrameRequest`], which is why this tracks its
    /// caller.
    #[track_caller]
    pub fn request_frame(&mut self) {
        self.taint_kept("it asked for a frame");
        self.frame_requested = true;
        self.trace_request();
    }

    /// Asks for a frame at `at` on the frame clock (`Core::now`'s seconds,
    /// backlog F135): a toast's expiry, a sequence's next beat. Not a
    /// frame now and not one every vsync — the driver wakes at that time
    /// (`next_frame_at`) and the view runs; until then nothing is owed, so
    /// `animating()` stays false. The earliest of the times asked for wins
    /// and is kept until a frame is begun at or past it, so a view need not
    /// ask again every frame (asking again is harmless). A time already
    /// past, or with no clock, is a frame now, as `request_frame` is. A
    /// time that is not a number is ignored.
    pub fn request_frame_at(&mut self, at: f64) {
        self.taint_kept("it asked for a frame at a time");
        // NaN asks for nothing, and so does a time that never comes.
        if at.is_nan() || at == f64::INFINITY {
            return;
        }
        if self.anim.time().is_none_or(|now| at <= now) {
            self.request_frame();
            return;
        }
        self.frame_due = Some(self.frame_due.map_or(at, |d| d.min(at)));
    }

    /// The exit `key` leaves by if it leaves in the frame that finishes
    /// next (backlog F136), over the `exit` the node declared: a card
    /// thrown left by a button that was resting with its exit aimed the
    /// other way. Called from a handler between frames, or from the view
    /// of the frame that stops declaring the node; cleared when that frame
    /// finishes, so a node that stays is not left with it. It aims an
    /// `exit` the node declares — a node that declares none leaves no
    /// picture to replay — with the node's own `transition`.
    pub fn set_exit(&mut self, key: Key, exit: crate::enter::Enter) {
        self.exits_named.insert(key, exit);
    }

    /// The frame-clock time a driver should next draw at for a
    /// `request_frame_at`, `None` when nothing was asked for: what a
    /// runner folds into the deadline it sleeps to.
    pub fn next_frame_at(&self) -> Option<f64> {
        self.frame_due
    }

    /// Starts a frame. Build the tree through the returned `Ui` (or the
    /// `Core` builder methods directly), then `Ui::finish` — the one door
    /// out of a frame, which runs the extension fills, the devtools panel
    /// and the open menu before layout. A driver holding a bare `Core`
    /// mid-frame finishes through `Ui::wrap(core).finish()`.
    pub fn frame(&mut self, viewport: Size, scale: f32) -> Ui<'_> {
        self.begin_frame(viewport, scale);
        Ui::new(self)
    }

    /// `frame` with something to fill the slots the view declares — the
    /// runner's extension list (`[Box<dyn Extension>]` is a `Fill`), or a
    /// test's stand-in. `Ui::slot` calls it in place, and `Ui::finish`
    /// lets it fill `"root"` and report unknown slots before layout.
    pub fn frame_with<'a>(
        &'a mut self,
        viewport: Size,
        scale: f32,
        filler: &'a mut dyn crate::slot::Fill,
    ) -> Ui<'a> {
        self.begin_frame(viewport, scale);
        Ui::with_filler(self, filler)
    }

    /// The session this core draws from. Hand it to `Core::new_in` to open
    /// another window sharing its fonts, images, sounds and glyph atlas.
    pub fn session(&self) -> &Session {
        &self.session
    }

    /// Runs an edit-store operation against the session's font system —
    /// the one the text cache shapes with, so an editor and a text node
    /// measure the same. The session borrow lasts exactly the call.
    fn edit_with_fonts<T>(
        &mut self,
        f: impl FnOnce(&mut EditStore, &mut cosmic_text::FontSystem) -> T,
    ) -> T {
        let sess = &mut *self.session.state();
        f(&mut self.edit, &mut sess.fonts)
    }

    /// The viewport (logical px) the current frame was begun with — the
    /// window, less the devtools' dock while the panel is docked:
    /// what the host lays out into. Changes to it
    /// arrive as `resize` events (see `take_pending_events`), a dock
    /// coming, going or resizing among them.
    pub fn viewport(&self) -> Size {
        Size::new(self.dt_area.w, self.dt_area.h)
    }

    /// The device pixel ratio the current frame was begun with.
    pub fn scale(&self) -> f32 {
        self.scale
    }

    /// The origin nodes opened right now are tagged with: `OriginId::HOST`
    /// in the host's own view, the filling extension's inside a fill (see
    /// `Core::fill`).
    pub fn origin(&self) -> OriginId {
        self.origin
    }

    /// The finished frame's draw data: display list plus the glyph atlas the
    /// renderer mirrors (mutable so it can clear the dirty flag).
    pub fn output(&mut self) -> (&DisplayList, &mut GlyphAtlas) {
        self.output_read = true;
        (&self.display, &mut self.atlas)
    }

    /// Begins a frame without handing out a [`Ui`]: what [`Core::frame`]
    /// calls first. A binding that drives the builder methods on the core
    /// directly starts here and ends with `Ui::wrap(core).finish()`.
    pub fn begin_frame(&mut self, viewport: Size, scale: f32) {
        // A frame at or past the time asked for is that frame.
        if let (Some(due), Some(now)) = (self.frame_due, self.anim.time())
            && due <= now
        {
            self.frame_due = None;
        }
        // First, while the last frame's tree and every store's reading of
        // it are still whole: why this frame runs, and who held it
        // (backlog F111).
        self.trace_begin_frame();
        // Against the finished frame, before anything below clears it: a
        // held drag re-places its live end where the last layout moved
        // the text under the pointer, and steps its scroller when the
        // pointer is past the edge (ADR 0029).
        self.follow_drag();
        self.age_type_ahead();
        // A window that changed size is a fact the driver reports, so the
        // core turns it into data like any other: `{kind="resize", width,
        // height, scale}` on the root, pending for the driver to route
        // after the frame. The first frame establishes the viewport rather
        // than resizing it.
        // The host's viewport is what the dock leaves of the window
        // (ADR 0024), so a dock that comes, goes or is dragged is a
        // resize too.
        let area = self.devtools_area(viewport);
        if self.framed
            && (area.w != self.dt_area.w || area.h != self.dt_area.h || scale != self.scale)
        {
            self.pending.push(UiEvent {
                origin: OriginId::HOST,
                window: WindowId::MAIN,
                key: Key::ROOT,
                payload: Value::map([
                    ("kind", Value::str("resize")),
                    ("width", Value::Float(area.w as f64)),
                    ("height", Value::Float(area.h as f64)),
                    ("scale", Value::Float(scale as f64)),
                ]),
                slot: None,
            });
        }
        self.dt_area = area;
        // And what the user set in the OS — and whether assistive
        // technology is listening, which rides in the same reading. A
        // driver that learns of a change writes it into `env` and asks
        // for a redraw — which is
        // enough for a host whose view is a function the runner calls
        // every frame, and nothing at all for one that retains the tree
        // it was handed (Node, C, Lua): its `view` runs when a message
        // changes the model, so the change has to *be* a message. The
        // first frame establishes the reading rather than reporting it,
        // the way the viewport does.
        if self.framed && self.env.system != self.system_seen {
            let sys = self.env.system;
            self.pending.push(UiEvent {
                origin: OriginId::HOST,
                window: WindowId::MAIN,
                key: Key::ROOT,
                payload: Value::map([
                    ("kind", Value::str("system")),
                    ("appearance", Value::str(sys.appearance.name())),
                    (
                        "accent",
                        sys.accent
                            .map_or(Value::Null, |c| Value::Int(c.to_hex() as i64)),
                    ),
                    ("motion", Value::str(sys.motion.name())),
                    (
                        "locale",
                        sys.locale.map_or(Value::Null, |l| Value::str(l.as_str())),
                    ),
                    ("assistive", Value::str(sys.assistive.name())),
                ]),
                slot: None,
            });
        }
        self.system_seen = self.env.system;
        // And the installed fonts, rescanned since the last frame and found
        // changed (`reload_system_fonts`, which the winit runner calls when
        // the OS says so): a message for the same reason as `system`, an
        // app holding `systemFonts()` in its model having nothing else to
        // re-read it on. The first frame establishes it, as above.
        let fonts_rev = self.session.state().system_fonts_rev;
        if self.framed && fonts_rev != self.system_fonts_seen {
            self.pending.push(UiEvent {
                origin: OriginId::HOST,
                window: WindowId::MAIN,
                key: Key::ROOT,
                payload: Value::map([("kind", Value::str("fonts"))]),
                slot: None,
            });
        }
        self.system_fonts_seen = fonts_rev;
        // The palette is a function of what the OS said and what the app
        // asked for, so it is recomputed rather than invalidated: a
        // couple of dozen float ops once a frame, against a cache that
        // would have to be poked from every writer of `env.system`.
        self.refresh_theme();
        self.framed = true;
        self.frame_no += 1;
        self.replay_begin_frame();
        // The layout rects a node reported, swept on the one cadence
        // every by-last-use store sweeps on (`retain::sweep_cutoff`, AR45).
        if let Some(cutoff) = crate::retain::sweep_cutoff(self.frame_no) {
            self.layouts.retain(|_, (_, seen)| *seen >= cutoff);
        }
        self.viewport = viewport;
        self.scale = scale;
        self.window_title = None;
        self.always_on_top = false;
        self.secure_input = false;
        self.option_as_alt = crate::input::OptionAsAlt::None;
        self.ime_off = false;
        // The drawn menu bar's root is this frame's: a view that stops
        // calling `widgets::menu_bar` leaves nothing behind for the next
        // event to land on. Re-recorded while the widget builds.
        self.menu_bar_root = None;
        // And the select fields, re-declared by the ones the view builds.
        self.selects.clear();
        // Last frame's focus declarations are what this frame's are
        // compared against (see `set_key_focus`).
        std::mem::swap(&mut self.declared_focus, &mut self.declared_focus_last);
        self.declared_focus.clear();
        // And the labels `key_of` resolves through, the same way.
        std::mem::swap(&mut self.key_labels, &mut self.key_labels_last);
        self.key_labels.clear();
        // Slots are positions, not declarations to diff: one clear.
        self.slot_labels.clear();
        self.ns_depth = usize::MAX;
        self.building = true;
        // And the window declarations, which `finish_frame` diffs the same
        // way (see `declare_window`).
        std::mem::swap(&mut self.declared_windows, &mut self.declared_windows_last);
        self.declared_windows.clear();
        // A frame that declared an `exit` may be the last one some node is
        // ever seen in, so it is kept whole: the two tree buffers swap
        // roles instead of one being cleared, which costs an allocation
        // that already existed and no copying. Nothing else keeps it — a
        // frame with no exits empties the spare, so a stale tree can never
        // be diffed against.
        let keep_prev = self.tree.any_exit;
        if keep_prev {
            std::mem::swap(&mut self.tree, &mut self.prev_tree);
        } else {
            self.prev_tree.clear();
        }
        self.tree.clear();
        // The drops a frame carried and no backend read - a frame built
        // twice before one is drawn, a window that framed and did not
        // render - ride on, or the texture a path animated out of would
        // be the backend's for good (RG112). Bounded, for a core that is
        // never read at all and so has no backend to tell.
        let unread = (!self.output_read).then(|| {
            const MOST: usize = 4096;
            let mut t = std::mem::take(&mut self.display.dropped_textures);
            let mut f = std::mem::take(&mut self.display.dropped_fragments);
            t.drain(..t.len().saturating_sub(MOST));
            f.drain(..f.len().saturating_sub(MOST));
            (t, f)
        });
        self.display.clear();
        if let Some((t, f)) = unread {
            self.display.dropped_textures = t;
            self.display.dropped_fragments = f;
        }
        self.output_read = false;
        // Removed handles the backend has not heard of, and this window's
        // atlas slots for removed images (AR8); kept on the session
        // because a removal can land between frames, after the list was
        // cleared, and through a window that never draws again.
        self.sync_dropped();
        // Before anything is emitted: the one point where the atlas may
        // empty its page — one the last frame extended, or one about to
        // fill — without a quad sampling what it dropped (F99).
        self.atlas.begin_frame();
        // An image's spare buffer outlives its stream by a few frames (W20).
        self.session.state().resources.release_spares();
        // Before the text store begins its frame, as a scale change is.
        self.sync_weights();
        // The text list goes with the tree: a kept frame's text nodes carry
        // that frame's `TextId`s, and nothing else can resolve them.
        self.text.begin_frame(
            &mut self.session.state().fonts,
            scale,
            keep_prev,
            self.frame_no,
        );
        // And the strokes, for the same reason: a kept frame's `line`
        // nodes index that frame's list.
        self.lines.begin_frame(keep_prev);
        self.paths.begin_frame(keep_prev);
        let cutoff = self.frame_no.saturating_sub(crate::path::ANIMATING_WINDOW);
        if self.path_motion.len() > 1024 {
            self.path_motion.retain(|_, m| m.seen >= cutoff);
        }
        if self.path_parsed.len() > 1024 {
            self.path_parsed.retain(|_, p| p.seen >= cutoff);
        }
        self.fragments.begin_frame(keep_prev);
        self.cells.begin_frame(scale);
        self.sync_font_names();
        self.anim.begin_frame(self.frame_no);
        self.depart.begin_frame(self.frame_no);
        // The two stores that keep state by key across a key's absence:
        // they stamp this frame onto what it declares, and cap what it
        // does not (backlog F26).
        self.edit.begin_frame(self.frame_no);
        self.scroll.begin_frame(self.frame_no);
        self.tree.push(
            NIL,
            Key::ROOT,
            OriginId::HOST,
            NodeSpec::column().fill(),
            NodeContent::Container,
        );
        self.stack.clear();
        self.stack.push(0);
        self.counters.clear();
        self.counters.push(0);
        // A frame that ended with nodes unclosed must not leak its hints
        // into the next one.
        self.hints.clear();
        self.origin = OriginId::HOST;
        self.frame_requested = false;
        // And the lines that asked, with it: an ask the reset above
        // forgets is not one the next frame was held by.
        self.trace_forget_requests();
        self.devtools_begin_frame();
    }
}

impl Default for Core {
    fn default() -> Self {
        Self::new()
    }
}

/// A frontend that draws into the shared tree each frame — the trait the
/// runner uses to host Lua (or any other) extensions without knowing what
/// they are. Origins are assigned by the runner.
///
/// Where it draws is a slot the host declared:
/// `slots` names
/// the ones it fills, `view` is called once per frame for each of them
/// with which one it is, and an extension naming none is called once
/// after the host's view for the reserved `"root"` slot — the sequence
/// every extension got before slots existed. `on_event` answers with
/// replies: values the runner hands to the host's own `on_event`, with
/// this extension's origin on them.
pub trait Extension {
    fn name(&self) -> &str;
    /// The slot names this extension fills; empty means `"root"`. The one
    /// entry [`crate::slot::ANY_SLOT`] (`"*"`) means every name declared
    /// under its namespace, for an extension whose slots are not known
    /// when it loads — a Lua host whose `init.lua` registers views at
    /// runtime, one slot per view — and it then gets no `unknown-slot`
    /// warning, since there is no list to check against.
    fn slots(&self) -> &[String] {
        &[]
    }
    fn view(&mut self, slot: &crate::slot::Slot<'_>, ui: &mut Ui<'_>) -> Result<(), String>;
    /// One of this extension's events; the values returned are replies
    /// to the host, delivered in order.
    fn on_event(&mut self, ev: &UiEvent) -> Vec<Value>;
}