Gdm crashes if HDMI monitor connected

When I plug a cable into the HDMI connector on my Pocket (I’ve tried a white Raspberry Pi micro-HDMI cable, as well as a generic black micro-HDMI cable I bought somewhere or other) and connect it to a monitor, gdm crashes on launch after drawing the login screen on the internal monitor.

If I have the HDMI connected at power-up, I get a brief flash of something or other just after power-on, so I believe the cable and monitor are good, and I’m just having some software issue.

I’m not quite current on software, but I was current before the most recent issue and subsequent fix. Just haven’t had the time yet.

Any ideas about what I need to do to get this working? A big monitor is a lot easier on the eyes in the evening.