fix flutter bug (refer to b3a5be86)

Runtime/gestures/mouse_tracking.cs

 
         public void detachAnnotation(MouseTrackerAnnotation annotation) {
             _TrackedAnnotation trackedAnnotation = this._findAnnotation(annotation);
-            D.assert(trackedAnnotation != null, $"Tried to detach an annotation that wasn't attached: {annotation}");
             foreach (int deviceId in trackedAnnotation.activeDevices) {
                 annotation.onExit(PointerExitEvent.fromHoverEvent((PointerHoverEvent) this._lastMouseEvent[deviceId]));
             }
             }
             
             return trackedAnnotation;
+        }
+        
+        bool isAnnotationAttached(MouseTrackerAnnotation annotation) {
+            return this._trackedAnnotations.ContainsKey(annotation);
         }
 
         public void collectMousePositions() {

Runtime/rendering/proxy_box.cs

         MouseTrackerAnnotation _hoverAnnotation;
 
         void _updateAnnotations() {
+            D.assert(this._onPointerEnter != this._hoverAnnotation.onEnter || 
+                     this._onPointerHover != this._hoverAnnotation.onHover ||
+                     this._onPointerExit != this._hoverAnnotation.onExit,
+                "Shouldn't call _updateAnnotations if nothing has changed.");
+            
             if (this._hoverAnnotation != null && this.attached) {
                 RendererBinding.instance.mouseTracker.detachAnnotation(this._hoverAnnotation);
             }
             else {
                 this._hoverAnnotation = null;
             }
+            
+            this.markNeedsPaint();
         }
 
         public override void attach(Object owner) {