Idle delay fade out is broken
The transition to black for the idle delay is broken. We are just turning the screen off immediately. It's annoying because you have no chance to cancel the lock, and by default the screen is off but not locked for a few seconds.
AFAICT the problem is that the fade out "long" light box is not mapped, and so clutter is optimising away the transition. With
CLUTTER_DEBUG=animation, you can see
gnome-shell: [ 699783661]:[ANIMATION]:../clutter/clutter/clutter-actor.c:19753: Skipping implicit transition for '<unnamed>[<StBin>:0x562eac4c9cc0]::opacity'
To reproduce easily, set
gsettings set org.gnome.desktop.session idle-delay 5 and wait 5 seconds.