Bug 1486958 - Sync FlushRendering() only during resizing on windows r=mattwoodrow

gfx/layers/wr/WebRenderLayerManager.cpp

@@ -637,8 +637,12 @@ WebRenderLayerManager::FlushRendering()
   }
   MOZ_ASSERT(mWidget);
 
-  // When DirectComposition and compositor window are used, we do not need to do sync FlushRendering.
-  if (WrBridge()->GetCompositorUseDComp()) {
+  // If value of IsResizingNativeWidget() is nothing, we assume that resizing might happen.
+  bool resizing = mWidget && mWidget->IsResizingNativeWidget().valueOr(true);
+
+  // Limit async FlushRendering to !resizing and Win DComp.
+  // XXX relax the limitation
+  if (WrBridge()->GetCompositorUseDComp() && !resizing) {
     cBridge->SendFlushRenderingAsync();
   } else if (mWidget->SynchronouslyRepaintOnResize() || gfxPrefs::LayersForceSynchronousResize()) {
     cBridge->SendFlushRendering();

widget/nsIWidget.h

@@ -778,6 +778,11 @@ class nsIWidget : public nsISupports
                         double aHeight,
                         bool   aRepaint) = 0;
 
+    virtual mozilla::Maybe<bool> IsResizingNativeWidget()
+    {
+        return mozilla::Nothing();
+    }
+
     /**
      * Resize the widget so that the inner client area has the given size.
      *

widget/windows/nsWindow.cpp

@@ -1988,6 +1988,15 @@ nsWindow::Resize(double aX, double aY, double aWidth,
   NotifyRollupGeometryChange();
 }
 
+mozilla::Maybe<bool>
+nsWindow::IsResizingNativeWidget()
+{
+  if (mResizeState == RESIZING) {
+    return Some(true);
+  }
+  return Some(false);
+}
+
 nsresult
 nsWindow::BeginResizeDrag(WidgetGUIEvent* aEvent,
                           int32_t aHorizontal,

widget/windows/nsWindow.h

@@ -131,6 +131,7 @@ public:
   virtual void            Move(double aX, double aY) override;
   virtual void            Resize(double aWidth, double aHeight, bool aRepaint) override;
   virtual void            Resize(double aX, double aY, double aWidth, double aHeight, bool aRepaint) override;
+  virtual mozilla::Maybe<bool> IsResizingNativeWidget() override;
   virtual MOZ_MUST_USE nsresult
                           BeginResizeDrag(mozilla::WidgetGUIEvent* aEvent,
                                           int32_t aHorizontal,