diff --git a/widget/src/windows/nsWindow.cpp b/widget/src/windows/nsWindow.cpp
index d7c63976b689..f9f51713621b 100644
--- a/widget/src/windows/nsWindow.cpp
+++ b/widget/src/windows/nsWindow.cpp
@@ -2020,6 +2020,20 @@ void nsWindow::SetTransparencyMode(nsTransparencyMode aMode)
   GetTopLevelWindow(PR_TRUE)->SetWindowTranslucencyInner(aMode);
 }
 
+namespace {
+  BOOL CALLBACK AddClientAreaToRegion(HWND hWnd, LPARAM lParam) {
+    nsIntRegion *region = reinterpret_cast<nsIntRegion*>(lParam);
+
+    RECT clientRect;
+    ::GetWindowRect(hWnd, &clientRect);
+    nsIntRect clientArea(clientRect.left, clientRect.top,
+                         clientRect.right - clientRect.left,
+                         clientRect.bottom - clientRect.top);
+    region->Or(*region, clientArea);
+    return TRUE;
+  }
+}
+
 void nsWindow::UpdatePossiblyTransparentRegion(const nsIntRegion &aDirtyRegion,
                                                const nsIntRegion &aPossiblyTransparentRegion) {
 #if MOZ_WINSDK_TARGETVER >= MOZ_NTDDI_LONGHORN
@@ -2029,13 +2043,25 @@ void nsWindow::UpdatePossiblyTransparentRegion(const nsIntRegion &aDirtyRegion,
   HWND hWnd = GetTopLevelHWND(mWnd, PR_TRUE);
   nsWindow* topWindow = GetNSWindowPtr(hWnd);
 
+  if (!mIsTopWidgetWindow)
+    return;
+
   mPossiblyTransparentRegion.Sub(mPossiblyTransparentRegion, aDirtyRegion);
   mPossiblyTransparentRegion.Or(mPossiblyTransparentRegion, aPossiblyTransparentRegion);
 
+  nsIntRegion childWindowRegion;
+
+  ::EnumChildWindows(mWnd, AddClientAreaToRegion, reinterpret_cast<LPARAM>(&childWindowRegion));
+  RECT r;
+  ::GetWindowRect(mWnd, &r);
+
+  childWindowRegion.MoveBy(-r.left, -r.top);
+
   nsIntRect clientBounds;
   topWindow->GetClientBounds(clientBounds);
   nsIntRegion opaqueRegion;
   opaqueRegion.Sub(clientBounds, mPossiblyTransparentRegion);
+  opaqueRegion.Or(opaqueRegion, childWindowRegion);
   MARGINS margins = { 0, 0, 0, 0 };
   DWORD_PTR dwStyle = ::GetWindowLongPtrW(hWnd, GWL_STYLE);
   // If there is no opaque region or hidechrome=true then full glass