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>

src/default/sense.rs

@@ -53,16 +53,17 @@ impl CursorSense {
         matches!(self, CursorSense::Pressing(CursorButton::Left))
     }
 
-    /// True for a sense that names something happening this frame (a
-    /// button transitioning, a scroll), as opposed to the ambient
-    /// `Hover*` family that is on for as long as the cursor rests there.
-    /// Only a momentary sense can consume an input -- see
-    /// `SensorUi::run_sensors`.
-    pub fn is_momentary(&self) -> bool {
-        !matches!(
-            self,
-            CursorSense::HoverStart | CursorSense::Hovering | CursorSense::HoverEnd
-        )
+    /// 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 => {}
+        }
     }
 }
 
@@ -90,6 +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();
@@ -175,45 +184,34 @@ impl SensorUi for UiRenderState {
         // this would probably be done through a generic parameter that adds yet another rsc /
         // state like thing, but local to render state, and is passed to UiRsc events so you can
         // update it there?
-        // Whether anything momentary is happening this frame at all; see
-        // `consumed` below, which is only judged per kind when it is.
-        let momentary_active =
-            cursor.scroll_delta != Vec2::ZERO || cursor.buttons.iter().any(|(_, a)| !a.is_off());
-
         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;
                 }
+                hovered_here |= over;
 
-                // A widget in shape always still runs: a hover-only
-                // highlight must fire on the topmost thing under the
-                // cursor even while a scroll passes through it. What is
-                // judged per input kind is whether it *consumes* that
-                // input, stopping a lower layer from seeing it. With
-                // nothing momentary happening, being in shape is
-                // consumption and the topmost widget wins the hover; with
-                // a scroll or a press happening, only a widget that
-                // registered a matching momentary sense consumes it, so a
-                // button registered for `click()` alone cannot block a
-                // scroll meant for the list behind it.
-                let consumed = if momentary_active {
-                    rsc.events_mut()
-                        .get_type::<CursorSense>()
-                        .registered(*id)
-                        .any(|senses| {
-                            matches!(should_run(senses, &cursor, sensor.hover), Some(s) if s.is_momentary())
-                        })
-                } else {
-                    true
-                };
-                sensed |= consumed;
+                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();
 
@@ -230,7 +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;
             }
         }
@@ -243,20 +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 {