Pass on all unhandeled messages to the owner.

dlls/comctl32/tab.c

@@ -2977,7 +2977,7 @@ TAB_WindowProc (HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
       if (uMsg >= WM_USER)
 	WARN("unknown msg %04x wp=%08x lp=%08lx\n",
 	     uMsg, wParam, lParam);
-      return DefWindowProcA (hwnd, uMsg, wParam, lParam);
+      return SendMessageA(GetParent(hwnd), uMsg, wParam, lParam);
     }
 
     return 0;