
The bind constraint that replaced the Shell.GenericContainer in commit
f4682748fa
is subtly different from the previous code:
It forces the actor to have the same size as the stage, rather than just
requesting that size.
This breaks the magnifier which relies on the UI being able to be bigger
than the display size. Fix by going back to using a custom actor.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/646