Take input per kind, rather than deciding it once a frame

Reviewing this against the process we agreed: the title claimed per-kind
routing and the code decided it once for the whole frame. A scroll and a
click in the same frame both went to the button, because a widget that
matched any momentary sense consumed everything.

Consumption is now removing an input from the cursor the layers below see.
`CursorSense::take` states what each sense takes -- exhaustively, so a new
sense has to answer the question rather than inherit a default -- and
`is_momentary` is gone with the enumeration it was written on. `should_run`
and consumption share one matcher instead of two copies of the table.

Two tests, each checked to fail without the change: a click and a scroll in
one frame reach different widgets, and leaving a widget still ends its hover.
The second is a regression this review caught in its own first draft, where
the skip condition used `is_off`, which counts `End` -- the one frame a
hover-end handler has to run on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/event/manager.rs

@@ -135,6 +135,11 @@ impl<Rsc: HasEvents + 'static, E: Event> TypeEventManager<Rsc, E> {
         ));
     }
 
+    /// What this widget registered, without running any of it.
+    pub fn registered(&self, id: WidgetId) -> impl Iterator<Item = &E> {
+        self.map.get(&id).into_iter().flatten().map(|(e, _)| e)
+    }
+
     pub fn run_fn<'a>(
         &mut self,
         id: impl IdLike,

src/default/sense.rs

@@ -52,6 +52,19 @@ impl CursorSense {
     pub fn is_dragging(&self) -> bool {
         matches!(self, CursorSense::Pressing(CursorButton::Left))
     }
+
+    /// Takes what this sense answers to out of `cursor`, so a widget below
+    /// does not also get it. Hovering takes nothing: it goes to the topmost
+    /// widget in shape, which is not a question about the input.
+    fn take(&self, cursor: &mut CursorState) {
+        match self {
+            Self::PressStart(button) | Self::Pressing(button) | Self::PressEnd(button) => {
+                *cursor.buttons.select_mut(button) = ActivationState::Off
+            }
+            Self::Scroll => cursor.scroll_delta = Vec2::ZERO,
+            Self::HoverStart | Self::Hovering | Self::HoverEnd => {}
+        }
+    }
 }
 
 #[derive(Default, Clone)]
@@ -78,6 +91,14 @@ impl CursorButtons {
         }
     }
 
+    pub fn select_mut(&mut self, button: &CursorButton) -> &mut ActivationState {
+        match button {
+            CursorButton::Left => &mut self.left,
+            CursorButton::Right => &mut self.right,
+            CursorButton::Middle => &mut self.middle,
+        }
+    }
+
     pub fn end_frame(&mut self) {
         self.left.end_frame();
         self.middle.end_frame();
@@ -164,17 +185,33 @@ impl SensorUi for UiRenderState {
         // state like thing, but local to render state, and is passed to UiRsc events so you can
         // update it there?
         let mut active = std::mem::take(&mut rsc.events_mut().get_type::<CursorSense>().active);
+        // Narrowed as it descends: a widget takes what it answers to, and
+        // what is left is what the layers below see.
+        let mut cursor = cursor;
+        let mut hovered = false;
         for layer in self.layers.indices().rev() {
-            let mut sensed = false;
+            let mut below = cursor.clone();
+            let mut hovered_here = false;
             for (id, sensor) in active.get_mut(&layer).into_flat_iter() {
                 let shape = self.active.get(id).unwrap().region;
                 let region = shape.to_px(window_size);
-                let in_shape = cursor.exists && region.contains(cursor.pos);
-                sensor.hover.update(in_shape);
-                if sensor.hover == ActivationState::Off {
+                let over = cursor.exists && region.contains(cursor.pos);
+                // Hover goes to the topmost widget in shape and no further.
+                sensor.hover.update(over && !hovered);
+                // `is_off` would be wrong here: it counts `End`, which is
+                // the one frame a hover-end handler has to run on.
+                if !over && sensor.hover == ActivationState::Off {
                     continue;
                 }
-                sensed = true;
+                hovered_here |= over;
+
+                for senses in rsc.events_mut().get_type::<CursorSense>().registered(*id) {
+                    for sense in senses.iter() {
+                        if matches(sense, &cursor, sensor.hover) {
+                            sense.take(&mut below);
+                        }
+                    }
+                }
 
                 let cursor = cursor.clone();
 
@@ -191,7 +228,9 @@ impl SensorUi for UiRenderState {
                 };
                 rsc.run_event::<CursorSense>(*id, data, state);
             }
-            if sensed {
+            hovered |= hovered_here;
+            cursor = below;
+            if hovered && !is_momentary(&cursor) {
                 break;
             }
         }
@@ -204,20 +243,27 @@ pub fn should_run(
     cursor: &CursorState,
     hover: ActivationState,
 ) -> Option<CursorSense> {
-    for sense in senses.iter() {
-        if match sense {
-            CursorSense::PressStart(button) => cursor.buttons.select(button).is_start(),
-            CursorSense::Pressing(button) => cursor.buttons.select(button).is_on(),
-            CursorSense::PressEnd(button) => cursor.buttons.select(button).is_end(),
-            CursorSense::HoverStart => hover.is_start(),
-            CursorSense::Hovering => hover.is_on(),
-            CursorSense::HoverEnd => hover.is_end(),
-            CursorSense::Scroll => cursor.scroll_delta != Vec2::ZERO,
-        } {
-            return Some(*sense);
-        }
+    senses
+        .iter()
+        .find(|sense| matches(sense, cursor, hover))
+        .copied()
+}
+
+fn matches(sense: &CursorSense, cursor: &CursorState, hover: ActivationState) -> bool {
+    match sense {
+        CursorSense::PressStart(button) => cursor.buttons.select(button).is_start(),
+        CursorSense::Pressing(button) => cursor.buttons.select(button).is_on(),
+        CursorSense::PressEnd(button) => cursor.buttons.select(button).is_end(),
+        CursorSense::HoverStart => hover.is_start(),
+        CursorSense::Hovering => hover.is_on(),
+        CursorSense::HoverEnd => hover.is_end(),
+        CursorSense::Scroll => cursor.scroll_delta != Vec2::ZERO,
     }
-    None
+}
+
+/// Whether anything is happening to the cursor beyond where it rests.
+fn is_momentary(cursor: &CursorState) -> bool {
+    cursor.scroll_delta != Vec2::ZERO || cursor.buttons.iter().any(|(_, state)| !state.is_off())
 }
 
 impl ActivationState {

tests/pointer_routing.rs

@@ -0,0 +1,180 @@
+//! A widget takes only what it answers to: a button over a list takes the
+//! click and leaves the scroll. These drive `run_sensors` directly, which
+//! needs no GPU and no window.
+
+use iris::prelude::*;
+use std::{cell::Cell, rc::Rc};
+
+struct SenseRsc {
+    ui: UiData,
+    events: EventManager<SenseRsc>,
+}
+
+impl UiRsc for SenseRsc {
+    fn ui(&self) -> &UiData {
+        &self.ui
+    }
+    fn ui_mut(&mut self) -> &mut UiData {
+        &mut self.ui
+    }
+    fn on_draw(&mut self, active: &ActiveData) {
+        self.events.draw(active);
+    }
+    fn on_undraw(&mut self, active: &ActiveData) {
+        self.events.undraw(active);
+    }
+    fn on_remove(&mut self, id: WidgetId) {
+        self.events.remove(id);
+    }
+}
+
+impl HasState for SenseRsc {
+    type State = ();
+}
+
+impl HasEvents for SenseRsc {
+    fn events(&self) -> &EventManager<Self> {
+        &self.events
+    }
+    fn events_mut(&mut self) -> &mut EventManager<Self> {
+        &mut self.events
+    }
+}
+
+fn cursor_at(pos: Vec2) -> CursorState {
+    CursorState {
+        pos,
+        exists: true,
+        buttons: Default::default(),
+        scroll_delta: Vec2::ZERO,
+    }
+}
+
+/// A button covering a list, on the layer above it: the list scrolls, the
+/// button clicks, and the returned flags say which fired.
+fn button_over_list() -> (UiRenderState, SenseRsc, Rc<Cell<bool>>, Rc<Cell<bool>>) {
+    let mut rsc = SenseRsc {
+        ui: UiData::default(),
+        events: EventManager::default(),
+    };
+
+    // Both cover the whole window: the button "sitting over" the list.
+    let list = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
+    let list_weak = list.weak();
+    let button = rsc.ui.widgets.add_strong(Rect::new(UiColor::RED));
+    let button_weak = button.weak();
+
+    let scrolled = Rc::new(Cell::new(false));
+    let clicked = Rc::new(Cell::new(false));
+    {
+        let scrolled = scrolled.clone();
+        rsc.register_event(list_weak, CursorSense::Scroll, move |_ctx, _rsc| {
+            scrolled.set(true);
+        });
+    }
+    {
+        let clicked = clicked.clone();
+        rsc.register_event(button_weak, CursorSense::click(), move |_ctx, _rsc| {
+            clicked.set(true);
+        });
+    }
+
+    // A Stack draws each child on its own layer, in order.
+    let root = rsc
+        .ui
+        .widgets
+        .add_strong(Stack {
+            children: vec![list.any(), button.any()],
+            size: StackSize::default(),
+        })
+        .any();
+
+    let mut render = UiRenderState::new();
+    render.resize((100.0, 100.0));
+    render.update(&root, &mut rsc);
+
+    (render, rsc, scrolled, clicked)
+}
+
+#[test]
+fn a_button_over_a_list_scrolls_the_list_and_still_clicks() {
+    let (render, mut rsc, scrolled, clicked) = button_over_list();
+    let mut state = ();
+
+    let mut scroll = cursor_at((50.0, 50.0).into());
+    scroll.scroll_delta = (0.0, 10.0).into();
+    render.run_sensors(&mut rsc, &mut state, scroll, (100.0, 100.0).into());
+
+    assert!(
+        scrolled.get(),
+        "a scroll over the button must still reach the list underneath it"
+    );
+    assert!(
+        !clicked.get(),
+        "a scroll is not a click; the button must not have fired"
+    );
+
+    let mut click = cursor_at((50.0, 50.0).into());
+    click.buttons.left = ActivationState::Start;
+    render.run_sensors(&mut rsc, &mut state, click, (100.0, 100.0).into());
+
+    assert!(
+        clicked.get(),
+        "the button on top must still receive an actual click"
+    );
+}
+
+#[test]
+fn a_click_and_a_scroll_in_one_frame_go_to_different_widgets() {
+    let (render, mut rsc, scrolled, clicked) = button_over_list();
+    let mut state = ();
+
+    let mut both = cursor_at((50.0, 50.0).into());
+    both.scroll_delta = (0.0, 10.0).into();
+    both.buttons.left = ActivationState::Start;
+    render.run_sensors(&mut rsc, &mut state, both, (100.0, 100.0).into());
+
+    assert!(
+        clicked.get(),
+        "the button takes the click it registered for"
+    );
+    assert!(
+        scrolled.get(),
+        "taking the click must not take the scroll with it"
+    );
+}
+
+#[test]
+fn leaving_a_widget_still_ends_its_hover() {
+    let mut rsc = SenseRsc {
+        ui: UiData::default(),
+        events: EventManager::default(),
+    };
+    let widget = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
+    let ended = Rc::new(Cell::new(false));
+    {
+        let ended = ended.clone();
+        rsc.register_event(widget.weak(), CursorSense::HoverEnd, move |_ctx, _rsc| {
+            ended.set(true);
+        });
+    }
+
+    let root = widget.any();
+    let mut render = UiRenderState::new();
+    render.resize((100.0, 100.0));
+    render.update(&root, &mut rsc);
+
+    let mut state = ();
+    render.run_sensors(
+        &mut rsc,
+        &mut state,
+        cursor_at((50.0, 50.0).into()),
+        (100.0, 100.0).into(),
+    );
+    assert!(!ended.get(), "the cursor is still on it");
+
+    let mut gone = cursor_at((50.0, 50.0).into());
+    gone.exists = false;
+    render.run_sensors(&mut rsc, &mut state, gone, (100.0, 100.0).into());
+    assert!(ended.get(), "leaving a widget ends its hover");
+}