Pin the hover-then-scroll case, and say what the line means

A wheel makes `position_only` false, so a button already hovered in a
layer above does not consume it -- but the line read as though it might.
`hovering_a_button_above_does_not_stop_a_later_scroll` is that case in the
two frames a window actually delivers it in, and the comment now leads
with it. `resting` is renamed to `position_only`, so the same word is used
throughout.

src/default/sense.rs

@@ -182,7 +182,7 @@ 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);
-        let resting = cursor.position_only();
+        let position_only = cursor.position_only();
         for layer in self.layers.indices().rev() {
             let mut consumed = false;
             for (id, sensor) in active.get_mut(&layer).into_flat_iter() {
@@ -206,13 +206,13 @@ impl SensorUi for UiRenderState {
                     sense: CursorSense::Hovering,
                     render: self,
                 };
-                // A cursor that is only resting stops at the layer it is
-                // over, answered or not -- but not at a widget it has just
-                // left, which is here only to end its hover. An action stops
-                // where something answered it, so a button over a list does
-                // not swallow the list's scrolling.
+                // A press or a scroll stops where something answered it, so a
+                // button over a list does not swallow the list's scrolling. A
+                // cursor doing neither stops at the widget it is over, so
+                // hovering does not reach through one -- but not at a widget
+                // it has just left, which is here only to end its hover.
                 let answered = rsc.run_event::<CursorSense>(*id, data, state);
-                consumed |= answered || (resting && in_shape);
+                consumed |= answered || (position_only && in_shape);
             }
             if consumed {
                 break;

tests/pointer_routing.rs

@@ -263,3 +263,28 @@ fn leaving_a_widget_does_not_block_the_layer_below() {
         "ending a hover above must not stop the hover below"
     );
 }
+
+#[test]
+fn hovering_a_button_above_does_not_stop_a_later_scroll() {
+    let ui = &mut Ui::new();
+    let (list, button) = (full(ui), full(ui));
+    let scrolled = ui.listen(&list, CursorSense::Scroll);
+    let clicked = ui.listen(&button, CursorSense::click());
+    ui.stack(vec![list.any(), button.any()]);
+
+    // The hover arrives in its own frame, as a window delivers it.
+    let hover = ui.cursor((50.0, 50.0));
+    ui.run(hover);
+    assert_eq!(scrolled.take(), []);
+
+    let mut wheel = ui.cursor((50.0, 50.0));
+    wheel.scroll_delta = (0.0, 10.0).into();
+    ui.run(wheel);
+
+    assert_eq!(
+        scrolled.take(),
+        [CursorSense::Scroll],
+        "a hover already resting on the button must not consume the wheel"
+    );
+    assert_eq!(clicked.take(), []);
+}