SUMMARY So I can find a discussion link mentioning something similar to this bug already in KDE discussion. It's weird that it hasn't been fixed yet or that I don't seem to find any bug reports of it here. https://discuss.kde.org/t/black-screen-on-fullscreen-with-wayland/29368/11 Now the specific bug I've encountered is not completely the same as this discussion link mentions. But basically whenever the PC goes to sleep (or I think just the screen automatically turning off might be enough too, I know it by when the message "new audio device" detected shows up), and if adaptive sync is either on "always" or "automatic" (so basically enabled), when going to fullscreen, the monitor just turns black, upon exiting fullscreen, picture returns. If adaptive sync is disabled (which to be honest, while the discussion does suggest that, adaptive sync is useful), this issue doesn't happen, however, this time (now I've only tested this with the mpv mediaplayer on fullscreen), occasionally when on fullscreen the screen turns black for a few seconds and then returns, so it's not really a complete workaround. The best workaround I've found around this time is to just logout and log back in, which fixes this issue. It's a bit annoying though having to basically logout and login each time. For info, this is on Wayland, with an AMD GPU (haven't really tested with anything else). Here is my GPU, CPU and monitor: AMD 7900xtx AMD Ryzen 9 7900 MSI G274QPX STEPS TO REPRODUCE 1. Have adaptive sync on in your settings 2. Let the PC go to sleep 3. Wake the PC up and go fullscreen OBSERVED RESULT Screen goes black EXPECTED RESULT Screen should show stuff in fullscreen SOFTWARE/OS VERSIONS Windows: macOS: (available in the Info Center app, or by running `kinfo` in a terminal window) Linux/KDE Plasma: KDE Plasma Version: KDE Frameworks Version: Qt Version: ADDITIONAL INFORMATION Like I said, I know the bug has happened when "New audio device detected" shows up, which is basically always when the PC wakes up. So maybe there is something to do with that? Though I think the message is just something that always happens as a result of sleep/waking up. Me having an AMD GPU might be important in reproducing this bug, as I said I haven't tested with anything else. I am thinking this is a kwin bug, but feel free to move this somewhere else if it's something else, and I am not sure which component to select, so I selected "general".
I am not sure how to edit my original comment, so here's some additional info that I want to add: I think this might mainly happen when the PC automatically goes to sleep after waiting and the screen turning off, not when you manually select sleep. (It honestly might just be triggered by the screen first turning off, than the sleep part) I haven't tested enough, but it might be triggered by just unplugging the displayport cable and plugging it back in too.
Some further additional info: The "New audio device detected" is a discord popup not a KDE popup, since I often end up in discord, I confused it and thought it was a KDE popup. Turning adaptive sync on the monitor off and on again, also seems to fix this, which is arguably quicker than logging out and in.