iris: eliminate retained-frame heap churn

src/rsc/sense.rs

@@ -328,18 +328,17 @@ impl SensorUi for UiRenderState {
         // Platform cancellation reaches every tracker and produces no other sense.
         if cursor.cancelled {
             let captured = pointer.captured.take();
-            let mut told: Vec<WidgetId> = captured.into_iter().collect();
-            for id in pointer.pressed.drain(..) {
-                if Some(id) != captured {
-                    told.push(id);
-                }
-            }
             requests.release();
             pointer.press_origin = None;
             pointer.drag_axis = None;
-            for id in told {
+            if let Some(id) = captured {
                 deliver_cancel(self, rsc, state, id, &cursor, window_size, &requests);
             }
+            for id in pointer.pressed.drain(..) {
+                if Some(id) != captured {
+                    deliver_cancel(self, rsc, state, id, &cursor, window_size, &requests);
+                }
+            }
             rsc.events_mut().get_type::<CursorSense>().global = pointer;
             return;
         }
@@ -442,15 +441,16 @@ impl SensorUi for UiRenderState {
         pointer.captured = requests.holder();
         match pointer.captured {
             Some(winner) => {
-                let losers: Vec<WidgetId> = pointer
-                    .pressed
-                    .iter()
-                    .copied()
-                    .filter(|&id| id != winner)
-                    .collect();
-                pointer.pressed.retain(|&id| id == winner);
-                for loser in losers {
-                    deliver_cancel(self, rsc, state, loser, &cursor, window_size, &requests);
+                let mut winner_was_pressed = false;
+                for id in pointer.pressed.drain(..) {
+                    if id == winner {
+                        winner_was_pressed = true;
+                    } else {
+                        deliver_cancel(self, rsc, state, id, &cursor, window_size, &requests);
+                    }
+                }
+                if winner_was_pressed {
+                    pointer.pressed.push(winner);
                 }
             }
             None if !button_down => pointer.pressed.clear(),