--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -1562,16 +1562,21 @@ nsWindow::GetClientOffset()
 }
 
 gboolean
 nsWindow::OnPropertyNotifyEvent(GtkWidget* aWidget, GdkEventProperty* aEvent)
 
 {
   if (aEvent->atom == gdk_atom_intern("_NET_FRAME_EXTENTS", FALSE)) {
     UpdateClientOffset();
+
+    // Send a WindowMoved notification. This ensures that TabParent
+    // picks up the new client offset and sends it to the child process
+    // if appropriate.
+    NotifyWindowMoved(mBounds.x, mBounds.y);
     return FALSE;
   }
 
   if (GetCurrentTimeGetter()->PropertyNotifyHandler(aWidget, aEvent)) {
     return TRUE;
   }
 
   return FALSE;