Description of problem:
in current implementation of metacity, it keeps _NET_SUPPORTING_WM_CHECK
property in the root window even when it's gone. usually it doesn't work
properly so that the window id that property contains would be invalid. however
it would affects unexpectedly when any applications grabs that window id, and
any applications referring to that value will behaves unexpected thing.
So I'd suggest metacity should removes it before finishing.
See Bug#471927 for the unexpected problem happened without the removal of this property.
Version-Release number of selected component (if applicable):
mclasen's patch applied upstream before 2.28.0, should be OK in F12