Bug 543831 - [OOPP] Hang with embedded youtube video. r=bent.

dom/plugins/PluginInstanceParent.cpp

@@ -897,7 +897,11 @@ PluginInstanceParent::PluginWindowHookProc(HWND hWnd,
 
     switch (message) {
         case WM_SETFOCUS:
-        self->CallSetPluginFocus();
+        // Widget may be calling us back from AnswerPluginGotFocus(), make
+        // sure we don't end up sending this back over. If we're not in
+        // SendMessage, this is coming from the dom / focus manager.
+        if ((::InSendMessageEx(NULL) & ISMEX_SEND|ISMEX_REPLIED) != ISMEX_SEND)
+            self->CallSetPluginFocus();
         break;
 
         case WM_CLOSE: