Expanding notifications automatically just because they popped up where
the pointer is positioned distracts the user. It can also lead to a behavior
that is surprising to the user by, for example, making the user's input
switch to the notification's text entry box.
It is possible to expand a notification that popped up over the pointer
by mousing away from it and then mousing back in immediately.
https://bugzilla.gnome.org/show_bug.cgi?id=617209