fix blank on first frame

Runtime/editor/editor_window.cs

     using UnityEditor;
     public abstract class UIWidgetsEditorWindow : EditorWindow {
         WindowAdapter _windowAdapter;
-        public UIWidgetsEditorWindow()
-        {
+        public UIWidgetsEditorWindow() {
-        
+
         void OnEnable() {
             if (this._windowAdapter == null) {
                 this._windowAdapter = new EditorWindowAdapter(this);
         }
 
         void OnGUI() {
-            this._windowAdapter.OnGUI();
+            this._windowAdapter.OnGUI(Event.current);
         }
 
         void Update() {
         protected abstract Widget rootWidget();
     }
 
-    public class EditorWindowAdapter : WindowAdapter
-    {
+    public class EditorWindowAdapter : WindowAdapter {
-        public EditorWindowAdapter(EditorWindow editorWindow)
-        {
+        public EditorWindowAdapter(EditorWindow editorWindow) {
-        
+
-        public override GUIContent titleContent
-        {
+        public override GUIContent titleContent {
-        
-        protected override double queryDevicePixelRatio()
-        {
+
+        protected override double queryDevicePixelRatio() {
-        protected override Vector2 queryWindowSize()
-        {
+        protected override Vector2 queryWindowSize() {
-    
+
-        
+
-        public WidgetInspectorService widgetInspectorService
-        {
+        public WidgetInspectorService widgetInspectorService {
-        
+
         WidgetsBinding _binding;
         float _lastWindowWidth;
         float _lastWindowHeight;
 
         bool _regenerateLayerTree;
         Surface _surface;
-        
+
-        
-        public bool alive
-        {
+
+        public bool alive {
             get { return this._alive; }
         }
 
             this._lastWindowWidth = size.x;
-            this._lastWindowHeight =size.y;
+            this._lastWindowHeight = size.y;
             this._physicalSize = new Size(
                 this._lastWindowWidth * this._devicePixelRatio,
                 this._lastWindowHeight * this._devicePixelRatio);
         public void OnDisable() {
             _windowAdapters.Remove(this);
             this._alive = false;
-            
+
             this._rasterizer.teardown();
 
             D.assert(this._surface != null);
             public WindowDisposable(WindowAdapter window) {
                 this._window = window;
             }
-            
+
-                
+
-        public void PostPointerEvent(List<PointerData> data)
-        {
-            WithBinding(() =>
-            {
+        public void PostPointerEvent(List<PointerData> data) {
+            WithBinding(() => {
-        
-        public void PostPointerEvent(PointerData data)
-        {
-            PostPointerEvent(new List<PointerData>(){data});
+
+        public void PostPointerEvent(PointerData data) {
+            PostPointerEvent(new List<PointerData>() { data });
-        
+
-        
+
-        
-        public virtual void OnGUI() {
+
+        public virtual void OnGUI(Event evt = null) {
+            evt = evt??Event.current;
             using (this.getScope()) {
                 bool dirty = false;
 
                     }
                 }
 
-                this._doOnGUI();
+                this._doOnGUI(evt);
-        public virtual GUIContent titleContent
-        {
+        public virtual GUIContent titleContent {
-        
+
-        protected virtual Surface createSurface()
-        {
+        protected virtual Surface createSurface() {
-        
-        
+
+
         void _beginFrame() {
             if (this.onBeginFrame != null) {
                 this.onBeginFrame(new TimeSpan(Stopwatch.GetTimestamp()) - this._epoch);
             }
         }
 
-        void _doOnGUI() {
-            var evt = Event.current;
+        void _doOnGUI(Event evt) {
 
             if (evt.type == EventType.Repaint) {
                 if (this._regenerateLayerTree) {
                     this._rasterizer.drawLastLayerTree();
                 }
-
                 return;
             }
 
 
         public void Update() {
             Timer.update();
-            
+
             using (this.getScope()) {
                 this._timerProvider.update(this.flushMicrotasks);
                 this.flushMicrotasks();

Runtime/engine/WidgetCanvas.cs

             base.scheduleFrame(regenerateLayerTree);
             _needsPaint = true;
         }
-        
+
-        public override void OnGUI()
+
+        public override void OnGUI(Event evt)
-            if (Event.current.type == EventType.Repaint)
+            if (evt.type == EventType.Repaint)
-
-            base.OnGUI();
+            base.OnGUI(evt);
         }
 
         protected override Surface createSurface()
             _windowAdapter.OnEnable();
             var root = new WidgetsApp(null, getWidget(), _windowAdapter);
             _windowAdapter.attachRootWidget(root);
-            
             _lastMouseMove = Input.mousePosition;
         }
 
             {
                 this._windowAdapter.Update();
             }
+            this._windowAdapter.OnGUI(new Event() {type=EventType.Repaint});
-        private void OnGUI()
+         private void OnGUI()
-            if (Event.current.type == EventType.Repaint)
-            {
-                this._windowAdapter.OnGUI();
-            }
-
-                if (this._windowAdapter != null)
-                {
-                    this._windowAdapter.OnGUI();
-                }
+                this._windowAdapter.OnGUI(Event.current);
             }
         }