Fix handling of notification disappearing under cursor

Previously, notifications disappearing under cursor (e.g., because closed
manually or open from another device) did not notify the manager
properly, as the leaveEventHook was not triggered. This could lead to
notifications staying around when not supposed to (see telegramdesktop#28813).

This commit fixes that by explicitly notifying manager when the
notification widget disappears under the cursor.

Fixes telegramdesktop#28813.

Telegram/SourceFiles/window/notifications_manager_default.cpp

@@ -500,7 +500,13 @@ void Widget::opacityAnimationCallback() {
 	updateOpacity();
 	update();
 	if (!_a_opacity.animating() && _hiding) {
-		manager()->removeWidget(this);
+		if (underMouse()) {
+			// The notification is leaving from under the cursor, but in such case leave hook is not
+			// triggered automatically. But we still want the manager to start hiding notifications
+			// (see #28813).
+			manager()->startAllHiding();
+		}
+		manager()->removeWidget(this);  // Deletes `this`
 	}
 }