I ran into a problem that was reported as a bug in 27.2. It's not fixed in 27.2.1. Sharing here to help others. It did not occur in Tahoe. macOS did not display the usual hard crash screen on restart; lack of logging was also noted in the bug report.
When switching users with Fast User Switching the Mac froze at login for the new user. No change over 12h. It was easy to replicate 100%. Went away when I disconnected external monitor.
ai suggested it was related to having my MacBook Pro external monitor rotated 90 to portrait mode. It's an old monitor, very standard resolution. I have seen rendering glitches with GG so that seemed plausible. I do have Spaces enabled.
Switching external monitor to landscape mode (standard rotation) fixed the problem.
I do miss my portrait monitor orientation.
Opus suggested other things to try. I might get around to them.
turn off “Displays have separate Spaces” in Desktop & Dock, log out, rotate back to portrait, and retest the switch. Also try 270° instead of 90°. Either result is worth adding to the post, since it narrows the bug for Apple and gives readers a possible workaround. I’m not confident either will work.
Heck, I might even submit a bug report, though historically that has been a waste of time.
