frust_widgets/

platform_view.rs

//! `PlatformViewSlot`: the app-facing leaf that reserves layout space for a
//! native platform view and publishes a
//! [`PlatformViewFrame`](frust_core::widget::PlatformViewFrame) every paint.
//!
//! [`platform_view`] takes the `"dev.frust.<Factory>"`-style native factory
//! name and returns a builder ([`PlatformViewView`]) over the params/size
//! contract:
//!
//! ```ignore
//! platform_view("dev.frust.MapFactory")
//!     .params_json(r#"{"style":"dark"}"#)   // optional, default ""
//!     .size(300.0, 400.0)                   // explicit slot size; or:
//!     .expand()                             // fill given constraints
//! ```
//!
//! # Paint contract (Mode A / Mode B)
//!
//! Under **Mode A** (an opaque native sibling view placed on top of the frust
//! surface) this widget **paints nothing**: the native view physically covers
//! this region regardless of what frust paints underneath, so punching a hole
//! would only risk erasing real app content. Under **Mode B** (a translucent
//! frust surface composited over the native view) the slot must **actively
//! clear its rect** ([`PaintScene::clear_rect`]) so the hole survives an opaque
//! app backdrop painted below it (the catalog's `AppBackground`, or any app-root
//! fill). The earlier "paints nothing in Mode B" contract was
//! device-disproven: a translucent app whose page fills the screen sealed every
//! hole opaque, and the hosted native view could never show. The clear is a
//! real destination-clearing composite, not a skipped paint,
//! so it erases whatever the backdrop drew beneath the slot.
//!
//! The widget can't see the surface mode directly (that lives in the shell); it
//! reads the threaded [`PaintCtx::is_translucent`] flag instead (the render root
//! seeds it from the surface's **resolved** translucency — what the GPU backend
//! actually granted, not the app's one-way surface-mode request, so a platform
//! that refuses translucency degrades to Mode A rather than punching holes in
//! an opaque swapchain). Punching is therefore gated on that flag — Mode A
//! keeps the paints-nothing behavior. In every mode
//! [`PlatformViewWidget::paint`] also *publishes* the [`PlatformViewFrame`]
//! describing where/how the native view should be placed; the actual
//! composition happens downstream (the differ, the per-shell channel). See
//! `docs/CODE_STANDARDS.md`'s platform-view paint-contract entry for the full
//! writeup.
//!
//! # Identity
//!
//! `slot_id` is allocated once per **widget instance**
//! ([`frust_core::widget::next_slot_id`], called from
//! [`View::build`](frust_core::View::build)) — never derived from tree
//! position, so a keyed reorder preserves it automatically along
//! with the rest of the retained widget. A `view_type` change across a
//! rebuild is treated like swapping in a widget of a different concrete type
//! (matching the framework's type-swap reconciliation conventions
//! elsewhere): a different native factory is a genuinely different slot, not
//! an update to the existing one, so it gets a fresh `slot_id` and its
//! `params_generation` resets to 0 as if newly built.
//!
//! # No input contract (v1)
//!
//! `PlatformViewWidget` has no [`Widget::event`](frust_core::Widget::event)
//! override — it relies on the trait's `Ignored` default. Mode A routes
//! input to the native sibling view at the OS level (frust never sees it);
//! Mode B has frust own the whole surface, but a platform-view slot's own
//! input contract is out of scope for v1.
//!
//! # Z-shields ([`shield`])
//!
//! An [`interactive`](PlatformViewView::interactive) slot hands a touch-DOWN
//! inside its rect to the native sibling — including a DOWN that landed on
//! frust chrome painted *over* the slot, since the OS-side hit test knows
//! nothing about the frust scene. [`shield`] is the fix: wrap that chrome, and
//! it reports its own painted rect through
//! [`PaintCtx::report_input_shield`](frust_core::PaintCtx::report_input_shield)
//! every paint. The shell-side differ intersects the pass's shield rects
//! against each interactive slot and ships the overlapping subset with that
//! slot's `Update`, so the wire/host contract is unchanged.
//!
//! Auto-collection is the ordinary path;
//! [`shield_local`](PlatformViewView::shield_local) stays as a manual escape
//! hatch for a region no widget paints (the differ unions the two).
//!
//! # Teardown ([`View::teardown`](frust_core::View::teardown))
//!
//! A removed slot reports its `slot_id` to `frust-core`'s pending-retire list
//! ([`frust_core::widget::report_retired_slot`]) so the shell can dispose the
//! native view on its very next frame, instead of waiting out the differ's
//! ~30-frame missing-streak heuristic. A merely
//! *culled* slot never runs `teardown`, so it keeps the missing-streak
//! backstop — which is exactly what keeps a scrolled-offscreen camera preview
//! alive.

use frust_core::accesskit::Role;
use frust_core::widget::{PlatformViewFrame, next_slot_id, report_retired_slot};
use frust_core::{
    AnyView, BoxConstraints, BuildCtx, ChangeFlags, ChildPod, EventCtx, EventResult, InputEvent,
    LayoutCtx, PaintCtx, PaintScene, SemanticsCtx, View, Widget, any,
};
use kurbo::{Point, Rect, Size};
#[cfg(debug_assertions)]
use peniko::Color;

/// How a [`PlatformViewSlot`] resolves its layout size.
///
/// `Expand` is the default (see [`platform_view`]): a native view most often
/// wants to fill whatever space its container gives it (a full-bleed map, a
/// video player), so an app that forgets to call either builder still gets a
/// sensible slot rather than a degenerate zero-size one. [`PlatformViewView::size`]
/// switches to `Explicit` for the common case of a fixed-size embed.
#[derive(Clone, Copy, Debug, PartialEq)]
enum SlotSize {
    /// Fill the incoming constraints' maximum.
    Expand,
    /// A fixed `(width, height)`, clamped into the incoming constraints.
    Explicit(f64, f64),
}

/// The translucent magenta a [`PlatformViewView::debug_fill`] slot paints —
/// a named constant (deliberately not theme-resolved: nothing here is
/// themed, see the module's Notes), invaluable on desktop where no native
/// host exists to show through the slot.
#[cfg(debug_assertions)]
const DEBUG_FILL_COLOR: Color = Color::from_rgba8(0xE0, 0x00, 0xE0, 0x40);

/// A declarative platform-view slot. See the [module docs](self).
pub struct PlatformViewView {
    view_type: String,
    params_json: String,
    slot_size: SlotSize,
    #[cfg(debug_assertions)]
    debug_fill: bool,
    semantics_label: Option<String>,
    interactive: bool,
    shields_local: Vec<Rect>,
}

/// Reserve layout space for a native platform view created by `view_type`
/// (the `"dev.frust.<Factory>"` convention), filling its container by
/// default (see [`SlotSize::Expand`]). See the [module docs](self) for the
/// full builder contract.
pub fn platform_view(view_type: impl Into<String>) -> PlatformViewView {
    PlatformViewView {
        view_type: view_type.into(),
        params_json: String::new(),
        slot_size: SlotSize::Expand,
        #[cfg(debug_assertions)]
        debug_fill: false,
        semantics_label: None,
        interactive: false,
        shields_local: Vec::new(),
    }
}

impl PlatformViewView {
    /// Set the opaque creation params handed to the native factory (default
    /// empty). Changing this across a rebuild bumps the widget's retained
    /// `params_generation` counter exactly once (see the module's Identity
    /// notes) — the differ uses the bump to decide whether to
    /// re-create or merely update the native view.
    pub fn params_json(mut self, params_json: impl Into<String>) -> Self {
        self.params_json = params_json.into();
        self
    }

    /// Force an explicit `(width, height)` slot size, clamped into whatever
    /// constraints this widget's parent hands it. Overrides
    /// [`PlatformViewView::expand`] when called after it (last builder call
    /// wins, matching the crate's other size-mode builders).
    pub fn size(mut self, width: f64, height: f64) -> Self {
        self.slot_size = SlotSize::Explicit(width, height);
        self
    }

    /// Fill the incoming constraints' maximum — the default (see
    /// [`SlotSize::Expand`]), provided as an explicit builder call for
    /// readability at call sites that want to make the choice visible.
    pub fn expand(mut self) -> Self {
        self.slot_size = SlotSize::Expand;
        self
    }

    /// Debug aid: paint a translucent magenta slab over the slot's bounds
    /// (see [`DEBUG_FILL_COLOR`]) so the reserved region is visible even
    /// with no native host attached — invaluable on desktop, where a
    /// platform view never actually composites. Compiled only into debug
    /// builds, so it can never ship live in a release binary.
    #[cfg(debug_assertions)]
    pub fn debug_fill(mut self) -> Self {
        self.debug_fill = true;
        self
    }

    /// Attach a plain accessible label over the slot's bounds (a
    /// [`Role::GenericContainer`] semantics node) — the v1 accessibility-gap
    /// mitigation: the native view itself carries no frust-visible
    /// semantics, so this is the only signal an assistive technology gets
    /// for the region.
    pub fn semantics_label(mut self, label: impl Into<String>) -> Self {
        self.semantics_label = Some(label.into());
        self
    }

    /// Mode B input forwarding: mark this slot's
    /// native view as pointer-interactive. A touch-DOWN inside the slot's
    /// rect — and outside every shield rect ([`Self::shield_local`]) — hands
    /// the whole gesture to the native sibling instead of the frust surface.
    /// Default off (the v1 no-input contract).
    pub fn interactive(mut self) -> Self {
        self.interactive = true;
        self
    }

    /// Declare a slot-relative region where frust content drawn OVER this
    /// slot must keep receiving input (the z-shield).
    ///
    /// **The manual escape hatch, not the ordinary path.** Wrap the frust
    /// chrome in [`shield`] instead: a `shield(child)` reports the rect it
    /// actually painted every frame, so it can't drift out of sync with a
    /// moving/resizing widget the way a hand-written rect does. Reach for this
    /// builder only for a region no widget paints (a reserved gutter, a
    /// gesture-only zone). Both sources are honored — the shell-side differ
    /// unions a slot's manual rects with the intersecting auto-collected ones.
    ///
    /// The rect is slot-relative; the widget translates it into absolute
    /// window coordinates when it publishes its frame.
    pub fn shield_local(mut self, rect: Rect) -> Self {
        self.shields_local.push(rect);
        self
    }
}

/// The retained widget for a [`PlatformViewView`]. See the [module docs](self).
pub struct PlatformViewWidget {
    /// Stable per-widget-instance id (see the module's Identity notes) —
    /// allocated once at construction, never derived from tree position.
    slot_id: u64,
    view_type: String,
    params_json: String,
    /// Bumped whenever `params_json` changes across a rebuild (never on a
    /// `view_type` swap, which instead resets it to 0 — see the module's
    /// Identity notes).
    params_generation: u64,
    slot_size: SlotSize,
    #[cfg(debug_assertions)]
    debug_fill: bool,
    semantics_label: Option<String>,
    interactive: bool,
    shields_local: Vec<Rect>,
}

impl<State: 'static> View<State> for PlatformViewView {
    type Element = PlatformViewWidget;

    fn build(&self, _ctx: &mut BuildCtx<'_>) -> PlatformViewWidget {
        PlatformViewWidget {
            slot_id: next_slot_id(),
            view_type: self.view_type.clone(),
            params_json: self.params_json.clone(),
            params_generation: 0,
            slot_size: self.slot_size,
            #[cfg(debug_assertions)]
            debug_fill: self.debug_fill,
            semantics_label: self.semantics_label.clone(),
            interactive: self.interactive,
            shields_local: self.shields_local.clone(),
        }
    }

    fn rebuild(
        &self,
        prev: &Self,
        element: &mut PlatformViewWidget,
        _ctx: &mut BuildCtx<'_>,
    ) -> ChangeFlags {
        let mut flags = ChangeFlags::NONE;

        if prev.view_type != self.view_type {
            // A different native factory is a different slot outright — treat
            // like a type swap (module docs' Identity section): fresh
            // slot_id, generation resets to 0 as if freshly built.
            element.slot_id = next_slot_id();
            element.view_type = self.view_type.clone();
            element.params_json = self.params_json.clone();
            element.params_generation = 0;
            flags |= ChangeFlags::LAYOUT | ChangeFlags::PAINT;
        } else if prev.params_json != self.params_json {
            element.params_json = self.params_json.clone();
            element.params_generation += 1;
            flags |= ChangeFlags::PAINT;
        }

        if prev.slot_size != self.slot_size {
            element.slot_size = self.slot_size;
            flags |= ChangeFlags::LAYOUT | ChangeFlags::PAINT;
        }

        #[cfg(debug_assertions)]
        if prev.debug_fill != self.debug_fill {
            element.debug_fill = self.debug_fill;
            flags |= ChangeFlags::PAINT;
        }

        if prev.semantics_label != self.semantics_label {
            // Semantics are recomputed each pass from the widget (mirroring
            // `IconView`'s label), so adopting the new value needs no
            // layout/paint dirtiness of its own.
            element.semantics_label = self.semantics_label.clone();
        }

        if prev.interactive != self.interactive || prev.shields_local != self.shields_local {
            element.interactive = self.interactive;
            element.shields_local = self.shields_local.clone();
            // The next paint must republish the frame so the differ sees the
            // new input contract.
            flags |= ChangeFlags::PAINT;
        }

        flags
    }

    fn teardown(&self, element: &mut PlatformViewWidget, _ctx: &mut BuildCtx<'_>) {
        // Prompt retire (see the module's Teardown section): tell core this slot
        // is gone for good, so the shell disposes the native view on its next
        // frame rather than waiting out the differ's missing-streak heuristic —
        // which cannot distinguish "torn down" from "culled" on its own.
        report_retired_slot(element.slot_id);
    }
}

impl Widget for PlatformViewWidget {
    fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
        // Nothing platform-y here — plain box-constraint arithmetic, exactly
        // like any other leaf.
        match self.slot_size {
            SlotSize::Explicit(width, height) => bc.constrain(Size::new(width, height)),
            SlotSize::Expand => bc.max(),
        }
    }

    fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
        let rect = Rect::from_origin_size(ctx.origin(), ctx.size());

        // `clip`/`visible`: intersect against a threaded scroll-ancestor
        // viewport when one exists; no constraint means fully visible (see
        // `PlatformViewFrame`'s doc comment). `overlaps` (not a zero-area
        // check on the intersection) is the same edge-inclusive test `Flex`'s
        // paint-time cull uses, so a slot that merely shares an edge with the
        // viewport still counts as visible.
        let (clip, visible) = match ctx.visible_rect() {
            Some(visible_rect) => (
                Some(visible_rect.intersect(rect)),
                visible_rect.overlaps(rect),
            ),
            None => (None, true),
        };

        // Publish exactly once per paint — the whole point of this widget.
        ctx.publish_platform_view(PlatformViewFrame {
            slot_id: self.slot_id,
            view_type: self.view_type.clone(),
            params_json: self.params_json.clone(),
            params_generation: self.params_generation,
            rect,
            clip,
            visible,
            interactive: self.interactive,
            shields: self
                .shields_local
                .iter()
                .map(|r| *r + ctx.origin().to_vec2())
                .collect(),
        });

        // Mode B hole-punch: on a translucent surface,
        // actively clear the slot's rect so an opaque app backdrop painted below
        // it (the catalog's `AppBackground`) doesn't seal the hole the hosted
        // native view shows through. Gated on the threaded translucent flag —
        // Mode A (opaque) keeps the paints-nothing contract, since a clear there
        // would erase real app content behind the slot (and be disregarded by an
        // opaque surface anyway; see `PaintScene::clear_rect`). The clear covers
        // the full slot rect, not the scroll-intersected `clip`: the shell-side
        // differ clips the native view to `clip`, so over-clearing beyond the
        // visible viewport is harmless (nothing composites there) and keeps the
        // punch geometry identical to the published `rect`.
        if ctx.is_translucent() {
            scene.clear_rect(ctx.origin(), ctx.size());
        }

        // Nothing else is painted here (see the module's Paint contract section)
        // except the debug-only fill aid, which paints OVER any punch so the
        // reserved region stays visible on desktop (where no native host and no
        // translucent surface exist).
        #[cfg(debug_assertions)]
        if self.debug_fill {
            scene.fill_rect(ctx.origin(), ctx.size(), DEBUG_FILL_COLOR);
        }
    }

    fn semantics(&self, ctx: &mut SemanticsCtx) {
        if let Some(label) = &self.semantics_label {
            ctx.push_node(Role::GenericContainer, |node| {
                node.set_label(label.as_str());
            });
        }
    }
}

/// A declarative z-shield wrapper. See [`shield`] and the [module docs](self).
pub struct ShieldView<State: 'static> {
    child: AnyView<State>,
}

/// Mark `child` as frust content that must keep receiving pointer input even
/// where it paints over an [`interactive`](PlatformViewView::interactive)
/// platform-view slot (the z-shield — see the [module docs](self)).
///
/// Layout/paint/event/semantics transparent: the child is laid out under the
/// wrapper's own constraints at its own size, painted unchanged, and routed
/// events unchanged. The only added behavior is one
/// [`PaintCtx::report_input_shield`](frust_core::PaintCtx::report_input_shield)
/// call per paint, carrying the rect the wrapper just painted into.
///
/// Wrap the *narrowest* chrome that actually needs input — a shield is a hole
/// in the native view's input region, so an over-wide one silently takes touches
/// away from it.
///
/// ```ignore
/// any(Stack(vec![
///     any(platform_view("dev.frust.MapFactory").interactive()),
///     any(shield(button("recenter", |s: &mut State| s.recenter()))),
/// ]))
/// ```
pub fn shield<State: 'static, V: View<State>>(child: V) -> ShieldView<State> {
    ShieldView { child: any(child) }
}

/// The retained widget for a [`ShieldView`]. See [`shield`].
pub struct ShieldWidget {
    child: ChildPod,
}

impl<State: 'static> View<State> for ShieldView<State> {
    type Element = ShieldWidget;

    fn build(&self, ctx: &mut BuildCtx<'_>) -> ShieldWidget {
        ShieldWidget {
            child: crate::authoring::build_child(&self.child, ctx),
        }
    }

    fn rebuild(
        &self,
        prev: &Self,
        element: &mut ShieldWidget,
        ctx: &mut BuildCtx<'_>,
    ) -> ChangeFlags {
        crate::authoring::rebuild_child(&prev.child, &self.child, &mut element.child, ctx)
    }

    fn teardown(&self, element: &mut ShieldWidget, ctx: &mut BuildCtx<'_>) {
        crate::authoring::teardown_child(&self.child, &mut element.child, ctx);
    }
}

impl Widget for ShieldWidget {
    fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
        // Fully transparent: the child sees the incoming constraints unchanged
        // and the wrapper takes exactly the child's size.
        self.child.set_origin(Point::ZERO);
        self.child.layout_child(ctx, bc)
    }

    fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
        // Paint the child FIRST, then report — the reported rect describes what
        // was just drawn, and a child that culls itself entirely still occupies
        // the wrapper's layout box (so the shield stands regardless).
        self.child.paint_child(ctx, scene);
        ctx.report_input_shield(Rect::from_origin_size(ctx.origin(), ctx.size()));
    }

    fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
        crate::authoring::route_event_single(&mut self.child, ctx, event)
    }

    fn semantics(&self, ctx: &mut SemanticsCtx) {
        // Transparent wrapper: forward to the single child (Semantics
        // Conventions — a container that skips this drops its whole subtree).
        self.child.semantics_child(ctx);
    }

    crate::authoring::visit_children!(child);
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{Axis, EdgeInsets, FlexView, Padding, PaddingView, Row, SizedBox, keyed};
    use frust_core::{BuildCtx, ChildPod, any};
    use kurbo::Point;

    fn build(view: &PlatformViewView) -> PlatformViewWidget {
        let mut counter = 0u64;
        <PlatformViewView as View<()>>::build(view, &mut BuildCtx::new(&mut counter))
    }

    fn rebuild(
        prev: &PlatformViewView,
        next: &PlatformViewView,
        element: &mut PlatformViewWidget,
    ) -> ChangeFlags {
        let mut counter = 0u64;
        <PlatformViewView as View<()>>::rebuild(
            next,
            prev,
            element,
            &mut BuildCtx::new(&mut counter),
        )
    }

    /// A recording scene capturing the draw ops (in order) the punch tests
    /// assert on; the frame-shape tests ignore `ops` and inspect the published
    /// `PlatformViewFrame`s instead.
    #[derive(Default)]
    struct NullScene {
        ops: Vec<SceneOp>,
    }

    #[derive(Debug, PartialEq)]
    enum SceneOp {
        Clear { origin: Point, size: Size },
        Fill { origin: Point, size: Size },
    }

    impl PaintScene for NullScene {
        fn fill_rect(&mut self, origin: Point, size: Size, _color: peniko::Color) {
            self.ops.push(SceneOp::Fill { origin, size });
        }
        fn draw_text(&mut self, _origin: Point, _text: &str) {}
        fn clear_rect(&mut self, origin: Point, size: Size) {
            self.ops.push(SceneOp::Clear { origin, size });
        }
    }

    // -- layout --------------------------------------------------------

    #[test]
    fn expand_fills_the_incoming_constraints_max() {
        let view = platform_view("dev.frust.MapFactory");
        let mut w = build(&view);
        let mut lctx = LayoutCtx::new();
        let size = w.layout(&mut lctx, &BoxConstraints::loose(Size::new(300.0, 200.0)));
        assert_eq!(size, Size::new(300.0, 200.0));
    }

    #[test]
    fn explicit_size_is_clamped_into_constraints() {
        let view = platform_view("dev.frust.MapFactory").size(1000.0, 1000.0);
        let mut w = build(&view);
        let mut lctx = LayoutCtx::new();
        let size = w.layout(&mut lctx, &BoxConstraints::loose(Size::new(300.0, 200.0)));
        assert_eq!(size, Size::new(300.0, 200.0));
    }

    // -- absolute rect under nesting + scroll offset --------------------

    #[test]
    fn publishes_absolute_rect_under_padding_flex_nesting_and_scroll_offset() {
        let mut counter = 0u64;
        let insets = EdgeInsets::all(10.0);
        let row: FlexView<()> = Row(vec![
            any(SizedBox::<()>(Some(50.0), Some(30.0))),
            any(platform_view("dev.frust.MapFactory").size(80.0, 60.0)),
        ]);
        let padded: PaddingView<()> = Padding(insets, row);

        let widget = padded.build(&mut BuildCtx::new(&mut counter));
        let mut lctx = LayoutCtx::new();

        // Wrap in a `ChildPod` and translate it by (-15, -25) — the same
        // shape `ScrollView` uses to offset scrolled content
        // (`self.child.set_origin(Point::new(0.0, -self.offset))`) —
        // standing in for "a simulated scroll offset".
        let mut pod = ChildPod::new(Box::new(widget));
        pod.set_origin(Point::new(-15.0, -25.0));
        pod.layout_child(&mut lctx, &BoxConstraints::loose(Size::new(1000.0, 1000.0)));

        let mut scene = NullScene::default();
        let mut pctx = PaintCtx::new(Point::ZERO, Size::new(2000.0, 2000.0));
        pod.paint_child(&mut pctx, &mut scene);

        let frames = pctx.take_platform_views();
        assert_eq!(frames.len(), 1);
        // Row places the SizedBox(50x30) first, so the platform_view sits at
        // main-axis offset 50; then the Padding's (10, 10) top-left inset;
        // then the (-15, -25) scroll translation.
        let expected_origin = Point::new(-15.0 + 10.0 + 50.0, -25.0 + 10.0);
        assert_eq!(
            frames[0].rect,
            Rect::from_origin_size(expected_origin, Size::new(80.0, 60.0))
        );
    }

    // -- clip / visibility ------------------------------------------------

    #[test]
    fn clip_is_intersected_with_visible_rect_when_straddling_edge() {
        let view = platform_view("dev.frust.MapFactory").size(100.0, 100.0);
        let mut w = build(&view);
        let mut lctx = LayoutCtx::new();
        w.layout(&mut lctx, &BoxConstraints::tight(Size::new(100.0, 100.0)));

        let mut scene = NullScene::default();
        let mut pctx = PaintCtx::new(Point::new(50.0, 0.0), Size::new(100.0, 100.0));
        pctx.constrain_visible_rect(Rect::from_origin_size(Point::ZERO, Size::new(100.0, 100.0)));
        w.paint(&mut pctx, &mut scene);

        let frames = pctx.take_platform_views();
        assert_eq!(frames.len(), 1);
        let frame: &PlatformViewFrame = &frames[0];
        assert_eq!(
            frame.rect,
            Rect::from_origin_size(Point::new(50.0, 0.0), Size::new(100.0, 100.0))
        );
        assert_eq!(frame.clip, Some(Rect::new(50.0, 0.0, 100.0, 100.0)));
        assert!(frame.visible, "an edge-straddling slot is still visible");
    }

    #[test]
    fn fully_outside_visible_rect_publishes_not_visible() {
        let view = platform_view("dev.frust.MapFactory").size(100.0, 100.0);
        let mut w = build(&view);
        let mut lctx = LayoutCtx::new();
        w.layout(&mut lctx, &BoxConstraints::tight(Size::new(100.0, 100.0)));

        let mut scene = NullScene::default();
        // Slot paints at (200, 0)-(300, 100); visible rect covers only
        // (0,0)-(100,100) — no overlap at all.
        let mut pctx = PaintCtx::new(Point::new(200.0, 0.0), Size::new(100.0, 100.0));
        pctx.constrain_visible_rect(Rect::from_origin_size(Point::ZERO, Size::new(100.0, 100.0)));
        w.paint(&mut pctx, &mut scene);

        let frames = pctx.take_platform_views();
        assert_eq!(frames.len(), 1);
        assert!(!frames[0].visible);
    }

    #[test]
    fn no_visible_rect_constraint_means_fully_visible_with_no_clip() {
        let view = platform_view("dev.frust.MapFactory").size(100.0, 100.0);
        let mut w = build(&view);
        let mut lctx = LayoutCtx::new();
        w.layout(&mut lctx, &BoxConstraints::tight(Size::new(100.0, 100.0)));

        let mut scene = NullScene::default();
        let mut pctx = PaintCtx::new(Point::ZERO, Size::new(100.0, 100.0));
        w.paint(&mut pctx, &mut scene);

        let frames = pctx.take_platform_views();
        assert_eq!(frames[0].clip, None);
        assert!(frames[0].visible);
    }

    #[test]
    fn parent_culling_of_the_whole_subtree_yields_no_frame_at_all() {
        let mut counter = 0u64;
        let row: FlexView<()> = Row(vec![any(
            platform_view("dev.frust.MapFactory").size(50.0, 50.0)
        )]);
        let mut widget = row.build(&mut BuildCtx::new(&mut counter));
        let mut lctx = LayoutCtx::new();
        widget.layout(&mut lctx, &BoxConstraints::loose(Size::new(1000.0, 1000.0)));

        let mut scene = NullScene::default();
        let mut pctx = PaintCtx::new(Point::ZERO, Size::new(1000.0, 1000.0));
        // Far outside even the one-viewport warm margin `Flex` extends the
        // cull test by, so the child never paints at all.
        pctx.constrain_visible_rect(Rect::from_origin_size(
            Point::new(5000.0, 5000.0),
            Size::new(10.0, 10.0),
        ));
        widget.paint(&mut pctx, &mut scene);

        let frames = pctx.take_platform_views();
        assert!(
            frames.is_empty(),
            "a culled subtree must publish no frame at all, not a hidden one"
        );
    }

    // -- Mode B hole-punch ---------------------------------------------

    #[test]
    fn translucent_surface_punches_the_slot_rect() {
        // Mode B: the slot actively clears its rect so an opaque backdrop below
        // it doesn't seal the hole.
        let view = platform_view("dev.frust.MapFactory").size(100.0, 80.0);
        let mut w = build(&view);
        let mut lctx = LayoutCtx::new();
        w.layout(&mut lctx, &BoxConstraints::tight(Size::new(100.0, 80.0)));

        let mut scene = NullScene::default();
        let mut pctx =
            PaintCtx::new(Point::new(20.0, 30.0), Size::new(100.0, 80.0)).with_translucent(true);
        w.paint(&mut pctx, &mut scene);

        assert_eq!(
            scene.ops,
            vec![SceneOp::Clear {
                origin: Point::new(20.0, 30.0),
                size: Size::new(100.0, 80.0),
            }],
            "a translucent slot must clear its own absolute rect"
        );
    }

    #[test]
    fn opaque_surface_does_not_punch_mode_a_unchanged() {
        // Mode A (the default): paints-nothing must be preserved — a punch there
        // would erase real app content behind the slot.
        let view = platform_view("dev.frust.MapFactory").size(100.0, 80.0);
        let mut w = build(&view);
        let mut lctx = LayoutCtx::new();
        w.layout(&mut lctx, &BoxConstraints::tight(Size::new(100.0, 80.0)));

        let mut scene = NullScene::default();
        // No `.with_translucent(true)` — opaque Mode A.
        let mut pctx = PaintCtx::new(Point::new(20.0, 30.0), Size::new(100.0, 80.0));
        w.paint(&mut pctx, &mut scene);

        assert!(
            scene.ops.is_empty(),
            "an opaque-surface slot must paint nothing (no punch), Mode A unchanged"
        );
        // The frame is still published in both modes.
        assert_eq!(pctx.take_platform_views().len(), 1);
    }

    // -- Resolved-translucency flip -----------------------------

    /// Count the `ClearRect` commands in a real display list — the punch as the
    /// GPU backend will actually see it, one level below the recording
    /// `PaintScene` the tests above use (a recording-level assertion once
    /// missed a real defect in this exact path).
    fn clear_rects(scene: &frust_scene::Scene) -> usize {
        scene
            .commands()
            .iter()
            .filter(|c| matches!(c, frust_scene::Command::ClearRect { .. }))
            .count()
    }

    #[test]
    fn a_resolved_translucency_downgrade_stops_the_punch_in_the_real_display_list() {
        // The shells push the surface's RESOLVED
        // translucency (`SurfaceRenderer::surface_resolved_translucent`), not
        // the request latch, so a surface that asked for translucency and
        // fell back to an opaque swapchain flips this to `false` — and the
        // punch must stop, or the `DestOut` composite zeroes real pixels and
        // presents black rectangles.
        fn logic(_: &mut ()) -> PlatformViewView {
            platform_view("dev.frust.MapFactory").size(100.0, 80.0)
        }

        let mut root: frust_core::RenderRoot<(), PlatformViewView> = frust_core::RenderRoot::new();
        let mut state = ();
        let mut scene = frust_scene::Scene::new();

        // Frame 1: the surface resolved translucent (Mode B) — the slot punches.
        root.set_surface_translucent(true);
        root.rebuild(&mut logic, &mut state);
        root.layout(Size::new(200.0, 200.0));
        {
            let mut builder = frust_scene::SceneBuilder::new(&mut scene);
            root.paint(&mut builder, frust_core::FrameTime::ZERO);
        }
        assert_eq!(
            clear_rects(&scene),
            1,
            "a translucent-resolved surface must punch exactly one slot rect"
        );

        // Frame 2: a (re)install resolved OPAQUE — the shell pushes `false`.
        root.set_surface_translucent(false);
        assert!(
            root.has_pending_change_flags(),
            "a translucency downgrade must mark the tree dirty so the next \
             frame actually repaints without the punch"
        );
        root.rebuild(&mut logic, &mut state);
        root.layout(Size::new(200.0, 200.0));
        scene.reset();
        {
            let mut builder = frust_scene::SceneBuilder::new(&mut scene);
            root.paint(&mut builder, frust_core::FrameTime::ZERO);
        }
        assert_eq!(
            clear_rects(&scene),
            0,
            "an opaque-resolved surface must encode no punch at all (Mode A)"
        );
    }

    #[cfg(debug_assertions)]
    #[test]
    fn debug_fill_paints_over_the_punch_on_a_translucent_surface() {
        // Desktop debug affordance: with the surface translucent AND debug_fill
        // on, the clear runs first, then the magenta fill over it.
        let view = platform_view("dev.frust.MapFactory")
            .size(100.0, 80.0)
            .debug_fill();
        let mut w = build(&view);
        let mut lctx = LayoutCtx::new();
        w.layout(&mut lctx, &BoxConstraints::tight(Size::new(100.0, 80.0)));

        let mut scene = NullScene::default();
        let mut pctx = PaintCtx::new(Point::ZERO, Size::new(100.0, 80.0)).with_translucent(true);
        w.paint(&mut pctx, &mut scene);

        assert_eq!(
            scene.ops,
            vec![
                SceneOp::Clear {
                    origin: Point::ZERO,
                    size: Size::new(100.0, 80.0),
                },
                SceneOp::Fill {
                    origin: Point::ZERO,
                    size: Size::new(100.0, 80.0),
                },
            ],
            "debug_fill must paint OVER the punch, not under it"
        );
    }

    // -- params generation --------------------------------------------------

    #[test]
    fn params_json_change_bumps_generation_exactly_once_unchanged_does_not() {
        let prev = platform_view("dev.frust.MapFactory").params_json("a");
        let mut w = build(&prev);
        assert_eq!(w.params_generation, 0);

        let same = platform_view("dev.frust.MapFactory").params_json("a");
        rebuild(&prev, &same, &mut w);
        assert_eq!(w.params_generation, 0, "unchanged params must not bump");

        let changed = platform_view("dev.frust.MapFactory").params_json("b");
        rebuild(&same, &changed, &mut w);
        assert_eq!(w.params_generation, 1);

        let changed_again = platform_view("dev.frust.MapFactory").params_json("c");
        rebuild(&changed, &changed_again, &mut w);
        assert_eq!(w.params_generation, 2);
    }

    #[test]
    fn view_type_change_allocates_a_fresh_slot_id_and_resets_generation() {
        let prev = platform_view("dev.frust.MapFactory").params_json("a");
        let mut w = build(&prev);
        w.params_generation = 3; // pretend a couple of param bumps already happened
        let old_slot_id = w.slot_id;

        let swapped = platform_view("dev.frust.OtherFactory").params_json("a");
        rebuild(&prev, &swapped, &mut w);

        assert_ne!(
            w.slot_id, old_slot_id,
            "a view_type swap must allocate a new slot id"
        );
        assert_eq!(
            w.params_generation, 0,
            "a swap resets generation like a fresh build"
        );
        assert_eq!(w.view_type, "dev.frust.OtherFactory");
    }

    // -- keyed reorder identity ---------------------------------------------

    #[test]
    fn slot_id_survives_a_keyed_reorder() {
        // slot_id lives on the WIDGET (module docs' Identity notes), so the
        // existing keyed-reconciliation machinery preserves it across a
        // reorder with no platform-view-specific code needed — this proves
        // that by keying two slots, swapping their positions across a
        // rebuild, and checking each keeps its original slot_id.
        let mut counter = 0u64;
        let a: FlexView<()> = FlexView::new(
            Axis::Horizontal,
            vec![
                keyed(1u64, platform_view("dev.frust.A")),
                keyed(2u64, platform_view("dev.frust.B")),
            ],
        );
        let mut widget = a.build(&mut BuildCtx::new(&mut counter));
        let mut lctx = LayoutCtx::new();
        widget.layout(&mut lctx, &BoxConstraints::loose(Size::new(1000.0, 1000.0)));

        let mut scene = NullScene::default();
        let mut pctx = PaintCtx::new(Point::ZERO, Size::new(1000.0, 1000.0));
        widget.paint(&mut pctx, &mut scene);
        let before = pctx.take_platform_views();
        assert_eq!(before.len(), 2);
        let (id_a, id_b) = (before[0].slot_id, before[1].slot_id);
        assert_ne!(id_a, id_b);

        // Rebuild with the two keyed children swapped in position.
        let b: FlexView<()> = FlexView::new(
            Axis::Horizontal,
            vec![
                keyed(2u64, platform_view("dev.frust.B")),
                keyed(1u64, platform_view("dev.frust.A")),
            ],
        );
        b.rebuild(&a, &mut widget, &mut BuildCtx::new(&mut counter));
        widget.layout(&mut lctx, &BoxConstraints::loose(Size::new(1000.0, 1000.0)));

        let mut pctx2 = PaintCtx::new(Point::ZERO, Size::new(1000.0, 1000.0));
        widget.paint(&mut pctx2, &mut scene);
        let after = pctx2.take_platform_views();
        assert_eq!(after.len(), 2);
        // Position 0 is now key 2 ("B"), position 1 is key 1 ("A") — each
        // keeps its original slot_id despite the position swap.
        assert_eq!(after[0].slot_id, id_b, "key 2's slot survives the reorder");
        assert_eq!(after[1].slot_id, id_a, "key 1's slot survives the reorder");
    }

    // -- Z-shield auto-collection ------------------------------------

    #[test]
    fn shield_reports_its_absolute_painted_rect_and_paints_the_child_unchanged() {
        // A shield nested under Padding/Row must report the rect it actually
        // painted into (absolute window coordinates, the same space
        // `PlatformViewFrame::rect` uses), and must not alter the child's paint.
        let mut counter = 0u64;
        let row: FlexView<()> = Row(vec![
            any(SizedBox::<()>(Some(50.0), Some(30.0))),
            any(shield(crate::test_support::leaf(80.0, 60.0))),
        ]);
        let padded: PaddingView<()> = Padding(EdgeInsets::all(10.0), row);

        let mut widget = padded.build(&mut BuildCtx::new(&mut counter));
        let mut lctx = LayoutCtx::new();
        widget.layout(&mut lctx, &BoxConstraints::loose(Size::new(1000.0, 1000.0)));

        let mut scene = NullScene::default();
        let mut pctx = PaintCtx::new(Point::ZERO, Size::new(1000.0, 1000.0));
        widget.paint(&mut pctx, &mut scene);

        let expected = Rect::from_origin_size(Point::new(60.0, 10.0), Size::new(80.0, 60.0));
        assert_eq!(pctx.take_input_shields(), vec![expected]);
        // Paint-transparent: the child's own fill is the ONLY recorded op, at
        // the same rect (a `Leaf` fills its bounds).
        assert_eq!(
            scene.ops,
            vec![SceneOp::Fill {
                origin: Point::new(60.0, 10.0),
                size: Size::new(80.0, 60.0),
            }]
        );
    }

    #[test]
    fn two_shields_in_one_pass_both_survive() {
        // The `Vec`-extend discipline end to end (an overwrite-shaped channel
        // would leave only the last one).
        let mut counter = 0u64;
        let row: FlexView<()> = Row(vec![
            any(shield(crate::test_support::leaf(20.0, 20.0))),
            any(shield(crate::test_support::leaf(30.0, 30.0))),
        ]);
        let mut widget = row.build(&mut BuildCtx::new(&mut counter));
        let mut lctx = LayoutCtx::new();
        widget.layout(&mut lctx, &BoxConstraints::loose(Size::new(1000.0, 1000.0)));

        let mut scene = NullScene::default();
        let mut pctx = PaintCtx::new(Point::ZERO, Size::new(1000.0, 1000.0));
        widget.paint(&mut pctx, &mut scene);

        let shields = pctx.take_input_shields();
        assert_eq!(shields.len(), 2, "both shields must reach the render root");
        assert_eq!(shields[0].size(), Size::new(20.0, 20.0));
        assert_eq!(shields[1].size(), Size::new(30.0, 30.0));
    }

    #[test]
    fn shield_local_still_publishes_its_absolute_rect_escape_hatch() {
        // The manual escape hatch survives auto-collection: a slot-relative
        // rect is translated into the slot's absolute space on the published
        // frame, and rides the differ's union with whatever `shield` collected.
        let view = platform_view("dev.frust.MapFactory")
            .size(100.0, 80.0)
            .interactive()
            .shield_local(Rect::new(0.0, 0.0, 20.0, 10.0));
        let mut w = build(&view);
        let mut lctx = LayoutCtx::new();
        w.layout(&mut lctx, &BoxConstraints::tight(Size::new(100.0, 80.0)));

        let mut scene = NullScene::default();
        let mut pctx = PaintCtx::new(Point::new(20.0, 30.0), Size::new(100.0, 80.0));
        w.paint(&mut pctx, &mut scene);

        let frames = pctx.take_platform_views();
        assert!(frames[0].interactive);
        assert_eq!(frames[0].shields, vec![Rect::new(20.0, 30.0, 40.0, 40.0)]);
        assert!(
            pctx.take_input_shields().is_empty(),
            "a manual rect is NOT reported through the auto-collection channel"
        );
    }

    // -- Teardown retire ---------------------------------------------

    /// Serializes the tests draining `frust-core`'s process-wide retire list,
    /// which `cargo test`'s parallel threads would otherwise interleave.
    static RETIRE_TEST_LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());

    /// Drain and discard whatever a previous test left in the process-wide
    /// retire list, so an assertion below sees only its own reports.
    fn drain_retired() -> Vec<u64> {
        frust_core::widget::take_retired_slots()
    }

    #[test]
    fn removing_a_slot_from_a_container_reports_its_id_for_prompt_retire() {
        let _guard = RETIRE_TEST_LOCK.lock().unwrap_or_else(|e| e.into_inner());
        let _ = drain_retired();

        // The real removal path: an `AnyView` concrete-type swap inside a Flex
        // child, which runs `teardown_child` -> `PlatformViewView::teardown`.
        let mut counter = 0u64;
        let prev: FlexView<()> = Row(vec![any(platform_view("dev.frust.MapFactory"))]);
        let mut widget = prev.build(&mut BuildCtx::new(&mut counter));

        let mut scene = NullScene::default();
        let mut lctx = LayoutCtx::new();
        widget.layout(&mut lctx, &BoxConstraints::loose(Size::new(100.0, 100.0)));
        let mut pctx = PaintCtx::new(Point::ZERO, Size::new(100.0, 100.0));
        widget.paint(&mut pctx, &mut scene);
        let slot_id = pctx.take_platform_views()[0].slot_id;
        assert!(
            drain_retired().is_empty(),
            "a live slot reports no retire while it keeps painting"
        );

        let next: FlexView<()> = Row(vec![any(SizedBox::<()>(Some(10.0), Some(10.0)))]);
        next.rebuild(&prev, &mut widget, &mut BuildCtx::new(&mut counter));

        assert_eq!(
            drain_retired(),
            vec![slot_id],
            "a torn-down slot reports its id exactly once"
        );
    }

    #[test]
    fn a_culled_slot_reports_no_retire() {
        // The keep-alive contract: culling is not teardown, so a slot scrolled
        // out of the viewport must NOT be retired (it keeps the differ's
        // missing-streak backstop instead — camera's A6 keep-alive depends on
        // this distinction).
        let _guard = RETIRE_TEST_LOCK.lock().unwrap_or_else(|e| e.into_inner());
        let _ = drain_retired();

        let mut counter = 0u64;
        let row: FlexView<()> = Row(vec![any(
            platform_view("dev.frust.MapFactory").size(50.0, 50.0)
        )]);
        let mut widget = row.build(&mut BuildCtx::new(&mut counter));
        let mut lctx = LayoutCtx::new();
        widget.layout(&mut lctx, &BoxConstraints::loose(Size::new(1000.0, 1000.0)));

        let mut scene = NullScene::default();
        let mut pctx = PaintCtx::new(Point::ZERO, Size::new(1000.0, 1000.0));
        pctx.constrain_visible_rect(Rect::from_origin_size(
            Point::new(5000.0, 5000.0),
            Size::new(10.0, 10.0),
        ));
        widget.paint(&mut pctx, &mut scene);
        assert!(pctx.take_platform_views().is_empty(), "the slot was culled");

        // A rebuild that keeps the slot in the tree (only its paint was culled).
        row.rebuild(&row, &mut widget, &mut BuildCtx::new(&mut counter));
        assert!(
            drain_retired().is_empty(),
            "a culled-but-live slot must never be retired"
        );
    }
}