Bug 1178 - No enter / leave events on actors when pointer leaves the
stage window
The patch is mostly thanks to Johan Bilien with small modifications
based on suggestions by Owen Taylor.
The X11 backend now listens for enter and leave notifications. Leave
notifications get translated directly to a CLUTTER_LEAVE
event. Clutter can detect these special events because the source
actor is NULL in which case it sets the source actor to the last known
actor and then sets the last known actor to NULL.
Enter notifications just get translated to CLUTTER_MOTION events which
will cause Clutter to generate an enter event through the usual code
path.
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