Keep momentary input on the widget the cursor is on

Tests across layers, as asked, and the fifth one found a defect older than
this branch: a press fired on a widget the cursor had just left, because the
frame its hover ends is a frame it still gets dispatched on, and `should_run`
only ever looked at the cursor. A button in the corner of a list therefore
clicked when the press landed anywhere else in the row.

A widget that is not under the cursor now sees a cursor with nothing
momentary in it, which settles both halves of the question at once: it is not
its press to receive, and not its press to take from the layers below.

`CursorSense` and `CursorButton` derive `Debug`, so a failure says which
sense fired rather than `left != right`.

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

src/default/sense.rs

@@ -4,14 +4,14 @@ use std::{
     rc::Rc,
 };
 
-#[derive(Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub enum CursorButton {
     Left,
     Right,
     Middle,
 }
 
-#[derive(Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub enum CursorSense {
     PressStart(CursorButton),
     Pressing(CursorButton),
@@ -205,6 +205,19 @@ impl SensorUi for UiRenderState {
                 }
                 hovered_here |= over;
 
+                // Momentary input belongs to whatever the cursor is on. A
+                // widget it has just left still hears its hover ending, but
+                // a press landing elsewhere is neither its press to receive
+                // nor its press to take.
+                let cursor = match over {
+                    true => cursor.clone(),
+                    false => CursorState {
+                        pos: cursor.pos,
+                        exists: cursor.exists,
+                        ..Default::default()
+                    },
+                };
+
                 for senses in rsc.events_mut().get_type::<CursorSense>().registered(*id) {
                     for sense in senses.iter() {
                         if matches(sense, &cursor, sensor.hover) {
@@ -213,8 +226,6 @@ impl SensorUi for UiRenderState {
                     }
                 }
 
-                let cursor = cursor.clone();
-
                 let data = CursorData {
                     pos: cursor.pos - region.top_left,
                     size: region.bot_right - region.top_left,