Fix SplitterProxy not clearing when above another QSplitterHandle

When two SplitterHandles are next to each other, like at the intersection of a
horizontal and vertical splitter (|-), then it's possible that hiding the proxy
of one of those handles causes the other handle to gain focus immediately,
which activates the SplitterProxy again. Before this patch, it would then
continue clearing after reenabling itself, leading to an inconsistent state.

BUG: 431921
(cherry picked from commit d201a1f1)