The constraint may be destroyed before the client destroyes the
protocol object, for example if a oneshot constraint was disabled by
alt-tab. Therefore we need to NULL check the constraint in request
handlers and ignore any requests to defunct objects.
The wp_pointer_constraints protocol is a protocol which enables clients
to manipulate the behavior of the pointer cursor associated with a seat.
Currently available constraints are locking the pointer to a static
position, and confining the pointer to a given region.
Currently locking is fully implemented, and confining is implemented for
rectangular confinement regions.
What else is lacking is less troublesome semantics for enabling the lock
or confinement; currently the only requirement implemented is that the
window that appears focused is the one that may aquire the lock.
This means that a pointer could be 'stolen' by creating a new window that
receives active focus, or when using focus-follows-mouse, a pointer
passes a window that has requested a lock. This semantics can be changed
and the protocol itself allows any semantics as seems fit.
https://bugzilla.gnome.org/show_bug.cgi?id=744104
