Logging into systemd-homed user briefly shows "no space left on device" notification
After logging out, systemd-homed tries to shrink the homed partition as small as possible. Upon logging back in, the drive is mounted and then grown again to make space for new files.
This leads to the situation where the homed partition has very little or zero space left on it for a short amount of time, which triggers a "No space left on device" notification shown during login.
I am not sure if this is an issue in GDM or Shell as the popup is shown after logging in but before the Shell properly appears.