add recreateWidget.

Runtime/engine/UIWidgetsPanel.cs

         protected virtual Widget createWidget() {
             return null;
         }
+        
+        public void recreateWidget() {
+            Widget root;
+            using (this._windowAdapter.getScope()) {
+                root = this.createWidget();
+            }
+
+            this._windowAdapter.attachRootWidget(root);
+        }
 
         internal void applyRenderTexture(Rect screenRect, Texture texture, Material mat) {
             this.texture = texture;

Runtime/ui/painting/canvas_impl.cs

             D.assert(this._layers[0].states.Count == 1);
 
             var layer = this._currentLayer;
-            if (layer.draws.Count == 0) {
-                D.assert(layer.layers.Count == 0);
-                return;
-            }
-
             using (var cmdBuf = new CommandBuffer()) {
                 cmdBuf.name = "CommandBufferCanvas";
 
                         break;
                 }
             }
+            
+            if (toClear) {
+                this._setRenderTarget(cmdBuf, layer.rtID, ref toClear);
+            }
+            
+            D.assert(!toClear);
 
             foreach (var subLayer in layer.layers) {
                 cmdBuf.ReleaseTemporaryRT(subLayer.rtID);