In the nested session the startup animation sometimes fails, weirdly
that always happens to me when running a freshly built gnome-shell the
first time. The reason it fails is that mutter fails to aquire a pointer
grab from the xserver, XIGrabDevice() is unsuccessful.
A simple workaround for this race condition in the xserver is to just
grab the devices a bit later, that is after the startups animation
instead of before it.
This was also tested with disabled animations, and seems to work just as
well in that case.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1812>
954131c1c8