Existing client-state hook isn't sufficient for this, and so easy to
be extended cleanly (*1). Adding a new callback is trivial and cheap,
so preferring this way, instead of trying to tweak the existing hook
for something it's never been designed for.
*1) see discussion here: https://github.com/X11Libre/xserver/pull/1077
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
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