Comment 6 for bug 1110138

OK, I've uploaded a branch with the fix in tact. Unfortunately, that part of the code is not straightforward to get under test, and we can't do an integration test either because decor requires opengl.

I note here that AWT's behaviour is probably not 100% reliable - unless it waited around for an updated _NET_FRAME_EXTENTS, in which case, their behaviour was fine and the bug was ours.