This also affects me, with a dual-monitor - laptop screen + one external
monitor - setup (it's harder to tell if it also happens without the
second monitor but I can check that if useful.) The second monitor is
connected by either HDMI direct or HDMI via a thunderbolt dongle, and it
happens in both cases. The only GPU in this laptop is i915, and it's
running Ubuntu 22.04.

It's maybe notable that in some but not all cases, I'll return and the
mouse pointer is visible on the otherwise black screen.

Also, it's not smooth. If I lock with super-L:
* I get the lock screen
* after a second, the lock screen fades to black, the laptop screen goes blank 
(maybe DPMS-off, it's hard to tell) and the external monitor reports "no 
signal" and turns itself off after a short moment
* they remains off for a few seconds
* they then wake up again and show the lock screen
* after some time (20 seconds maybe) the external says "no signal" again and 
then powers off, laptop screen goes blank
* it wakes up again, back at the lock screen
* it settles on on with the lock screen showing, no change for a few minutes

That's where I stopped, but I suspect that after some time the screen
blanker turns the display blank but doesn't power things off. I can get
a more precise sequence if it'd be useful. I ran these with both colour
profiles and nightlight disabled, though they were previously enabled.

My hypothesis is that the monitor turning off causes some event that
causes Gnome to wake it up (in the past I've noticed windows moving from
one screen to another, as if it were unplugged, but I haven't seen that
recently), and then some other "let's not repeatedly power cycle things"
check stops it happening too much. But that's 100% speculation.

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https://bugs.launchpad.net/bugs/1973467

Title:
  [Wayland] Backlight stays on with black screen when monitor should be
  sleeping

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