When the last touch has been released the stage on the
corresponding master device (eg. the virtual core pointer) is set
to NULL and no mouse events can be delivered until an ENTER event
has occurred and the stage pointer restored.
This is due to the fact that the master devices can send both
touch events and mouse events, forwarding events coming from the
attached slave devices.
To restore delivery of mouse events we need to ensure that the
stage is set on each ButtonPress, ButtonRelease and Motion event
coming from master devices.
https://bugzilla.gnome.org/show_bug.cgi?id=684509
35faaf604c