I just tested this a bit further. I booted the laptop connected to a monitor. Then I disconnected it while still in mirror mode. After that, I let it fall into lock mode with a black screen. As expected, I could not see the password I was typing in until I connected an external monitor. I typed the password, but did not yet hit Enter because I wanted to see what would happen if I unplugged and plugged the HDMI several times in succession, with plenty of resync time in between. I ended up hanging the machine with the password dialog showing after maybe 5 cycles. Maybe this wouldn't have occurred, had I allowed more resync time between plug events. Then again, robust software would not need extra time in order to prevent hanging the machine. Unfortunately, I have no idea where in the stack between me and the hardware the problem actually resides.

I have a similar issue (not sure if it should be a separate ticket). My external display is set to primary. When my screens have been blanked and I unplug the HDMI, I get the login screen background on my laptop but no top panel or login prompt. Typing does not log me in (might have been a number lock issue), nor can I get my pointer to show up on the laptop screen. Plugging the HDMI back in for even a fraction of a second brings the login prompt to the laptop screen.

This was the error when I disconnected:
[drm:drm_edid_block_valid [drm]] *ERROR* EDID checksum is invalid, remainder is 23

This is the error when plugging the HDMI back in for a moment:
[drm:drm_edid_block_valid [drm]] *ERROR* EDID checksum is invalid, remainder is 191
Raw EDID:

00 ff ff ff ff ff ff 00 04 69 a4 32 01 01 01 01
0f 1a 01 03 80 47 28 78 ea 87 d5 a8 5f ff ff ff
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff

Considering that you got the login background but could not see the prompt, which is centered, that sounds like a resolution mismatch between your monitor and your laptop screen, which is not being handled properly when you're logged out. This is a delicate issue because it begs the question of what settings the driver should be able to see when you're logged out, vs. what it shouldn't. (I suppose this is one reason why Solus quite sensibly separates logged in from logged out background images.)

Maybe the checksum errors are just benign side effects of the plug event in the middle of a scanline trace by the video chip. Or maybe not.

The issue described in this ticket also affects me (intel card as well). Besides I need to plug out-plugin the external monitor after a screen lock to 'wake it up'.

Does this issue still persist under a fully upgraded Solus 3?

I’ve been completely mobile the last couple of months. However I tried to use HDMI two days ago for audio output and had no HDMI source listed. I’ve not had time to investigate further.