Between xcb_grab_server() and xcb_ungrab_server(), XCB's output buffer
might fill up. Thus, the GrabServer request might already have been sent
to the server, but the following UngrabServer request could end up in
XCB's output buffer. There, it might sit around for quite a while and
cause problems.
Since we cannot detect when XCB's output buffer fills up, we just always
flush after generating an UngrabServer request.
Very-likely-Fixes: https://github.com/awesomeWM/awesome/issues/2697
Signed-off-by: Uli Schlachter <psychon@znc.in>
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