We are applying the :insensitive pseudo class to unreactive widgets,
or at least we are supposed to. As we currently only update the style
on notify, we don't apply it to initially unreactive widgets.
This was covered up partially until recently when Clutter started to
use G_PARAM_EXPLICIT_NOTIFY. Before that, the notify handler would run
when explicitly setting :reactive to FALSE at construction time.
Make sure we always apply the pseudo class correctly by updating it
after construct properties have been set.
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3685
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1648>