Some default values differ between classic and normal sessions. We
used to implement this by overriding the shell's default values in
mini-extensions (or more precisely: reversing the shell's overrides).
Use a mode-specific overridesSchema instead, which has the advantage
that settings defaults will not change unexpectedly when extensions
are disabled/enabled (for instance during screen locks).
https://bugzilla.gnome.org/show_bug.cgi?id=701717