Bug 1656266 - Make the window client size atomic to avoid a read/write race. r=mstange

Differential Revision: https://phabricator.services.mozilla.com/D90582

widget/gtk/GtkCompositorWidget.h

@@ -76,7 +76,15 @@ class GtkCompositorWidget : public CompositorWidget,
   nsWindow* mWidget;
 
  private:
-  LayoutDeviceIntSize mClientSize;
+  // Webrender can try to poll this while we're handling a window resize event.
+  // This read/write race is largely benign because it's fine if webrender and
+  // the window desync for a frame (leading to the page displaying
+  // larger/smaller than the window for a split second) -- nobody expects
+  // perfect rendering while resizing a window. This atomic doesn't change the
+  // fact that the window and content can display at different sizes, but it
+  // does make it Not Undefined Behaviour, and also ensures webrender only
+  // ever uses the old or new size, and not some weird synthesis of the two.
+  Atomic<LayoutDeviceIntSize> mClientSize;
 
   WindowSurfaceProvider mProvider;