Index: vncviewer/X11PixelBuffer.cxx
===================================================================
--- vncviewer/X11PixelBuffer.cxx	(revision 5186)
+++ vncviewer/X11PixelBuffer.cxx	(working copy)
@@ -158,6 +158,8 @@
   Bool pixmaps;
   XErrorHandler old_handler;
   Status status;
+  Window test_window;
+  GC test_gc;
 
   if (!XShmQueryVersion(fl_display, &major, &minor, &pixmaps))
     return 0;
@@ -187,6 +189,12 @@
   old_handler = XSetErrorHandler(XShmAttachErrorHandler);
 
   XShmAttach(fl_display, shminfo);
+  test_window = XCreateSimpleWindow(fl_display, XDefaultRootWindow(fl_display), 0, 0, width(), height(), 0, 0, 0);
+  test_gc = XCreateGC(fl_display, test_window, 0, 0);
+  XShmPutImage(fl_display, test_window, test_gc, xim, 0, 0, 0, 0, width(), height(), False);
+  XFreeGC(fl_display, test_gc);
+  XDestroyWindow(fl_display, test_window);
+
   XSync(fl_display, False);
 
   XSetErrorHandler(old_handler);