So far it was missing because in Qt4 we did this via QX11Info which is no longer available. I have just found in qtest.h that qt5 has a special setting for this, so we can easily fix it.
Created attachment 128478 [details] Patch
Comment on attachment 128478 [details] Patch Very nice solution!
Comment on attachment 128478 [details] Patch Clearing flags on attachment: 128478 Committed r108790: <http://trac.webkit.org/changeset/108790>
All reviewed patches have been landed. Closing bug.