Add a config file option to enable it, but leave it off by default. Exposay
still triggers too many lock-ups or stuck grabs and triggers under X even
when the Wayland window doesn't have keyboard focus.
Set the internal pointer for the client to NULL. This fixes a
segmentation fault at shutdown, where the shell would hang up before
and cause libwayland to call wl_client_destroy(). When the shell was
destroyed later, another call to wl_client_destroy() would cause the
crash.
https://bugs.freedesktop.org/show_bug.cgi?id=72550
