fix flutter bug (refer to flutter/flutter@b3a5be8)

Author: zhuxingwei

Runtime/gestures/mouse_tracking.cs

@@ -75,7 +75,6 @@ public void attachAnnotation(MouseTrackerAnnotation annotation) {
 
         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]));
             }
@@ -125,6 +124,10 @@ _TrackedAnnotation _findAnnotation(MouseTrackerAnnotation annotation) {
 
             return trackedAnnotation;
         }
+        
+        bool isAnnotationAttached(MouseTrackerAnnotation annotation) {
+            return this._trackedAnnotations.ContainsKey(annotation);
+        }
 
         public void collectMousePositions() {
             void exitAnnotation(_TrackedAnnotation trackedAnnotation, int deviceId) {

Runtime/rendering/proxy_box.cs

@@ -1942,6 +1942,11 @@ public PointerExitEventListener onPointerExit {
         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);
             }
@@ -1960,6 +1965,8 @@ void _updateAnnotations() {
             else {
                 this._hoverAnnotation = null;
             }
+            
+            this.markNeedsPaint();
         }
 
         public override void attach(Object owner) {