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.

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(), []);
+}