Bug 1573710 - make window resizing more robust to races. r=sotaro

Differential Revision: https://phabricator.services.mozilla.com/D49634

--HG--
extra : moz-landing-system : lando

widget/windows/WinCompositorWidget.cpp

@@ -334,6 +334,17 @@ void WinCompositorWidget::UpdateCompositorWndSizeIfNecessary() {
     return;
   }
 
+  // This code is racing with the compositor, which needs to reparent
+  // the compositor surface to the actual window (mWnd). To avoid racing
+  // mutations, we refuse to proceed until GetParent returns the
+  // expected result. It's ok to fail to resize here since this function is
+  // called pretty frequently, and the reparenting race only happens when the
+  // window is being created for the first time.
+  HWND realParent = ::GetParent(mCompositorWnds.mCompositorWnd);
+  if (realParent != mWnd) {
+    return;
+  }
+
   LayoutDeviceIntSize size = GetClientSize();
   if (mLastCompositorWndSize == size) {
     return;
@@ -342,7 +353,7 @@ void WinCompositorWidget::UpdateCompositorWndSizeIfNecessary() {
   // Force a resize and redraw (but not a move, activate, etc.).
   if (!::SetWindowPos(mCompositorWnds.mCompositorWnd, nullptr, 0, 0, size.width,
                       size.height,
-                      SWP_NOMOVE | SWP_NOACTIVATE | SWP_NOCOPYBITS |
+                      SWP_NOACTIVATE | SWP_NOCOPYBITS |
                           SWP_NOOWNERZORDER | SWP_NOZORDER)) {
     return;
   }