So I have been having a problem where the dock just would not recognize HDMI in both desktop and handheld modes. I tried every fix I could find to no avail.

Then I started thinking it could be about KDE and not the deck or dock. So I found that you can reset monitor configs by deleting ~/.config/kwinoutputconfig.json. You have to do this while your logged out of KDE. The easiest way I found was to add dolphin file manager as a non steam game and remove the config in handheld mode.

After that I did a hard reset holding the power button till the light flashes a few times and powering on with the dock connected to HDMI.

This allowed me to get things working again.

If I remember right, I once had to delete that file (or maybe it was ~/.local/share/kscreen/?) to get desktop mode working at all. Basically I could use game mode fine, but switching to desktop mode didn't work and just gave a black screen. Ended up connecting a keyboard, using ctrl+alt+F3 to swap to a command line, and deleting it that way.

Actually I did delete ~/.local/share/kscreen as well but I had done that a day before I deleted the other file. Maybe it needed both. Wont be able to know unless it's tested and I don't know how to get back to a broken state.

Good catch though, i didn't even think that might be relevant till you said something!

I don't think deleting kwinoutputconfig.json will work being logged in, according to this KDE discussion where I got my info for the fix. I guess you could probably login to the terminal as root if you have root setup. [ comming to think about it probably wont work as the file is generated on user logout and loaded on login ]

Hope this thread helps the next person that has the issue!