The assertion in PointerConstraint.confine() can currently still be
triggered if the input region of a surface is changed and the pointer is
moved outside of the new intersection of input region and constraint
region before PointerConstraint.updateState() is called.
This can happen, for example, when a client is made non-fullscreen at
the same time as the pointer is moved across the boundary of the new,
post-fullscreen, input region. If the pointer crosses the boundary
before the transaction completes and updateState() is called, the
assertion in PointerConstraint.confine() will fail.
To fix this, listen for the surface commit event rather than the
set_region event to handle possible deactivation on region changes.
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