To avoid messing up St.Buttons' internal state with a pointer grab,
we wait for the pointer to leave the actor before starting the
drag operation manually. This works generally fine, but makes starting
a drag operation harder than necessary. To fix, enforce a reasonable
button state when starting the drag, rather than special-casing buttons
before the drag.
https://bugzilla.gnome.org/show_bug.cgi?id=637103