SUMMARY If I set Variable Refresh Rate to Automatic then disable VRR via my Monitor's OSD, any fullscreen window blanks the monitor indefinitely until said fullscreen window is closed or changed to Windowed mode. Changing Plasma's VRR setting to "Never" prevents the bug happening. STEPS TO REPRODUCE 1. Plug in a variable refresh rate monitor, make sure VRR is turned on via the Monitor OSD. 2. in System Settings, set Variable Refresh to automatic or always. 3. Open any web browser, load a YouTube video and fullscreen the YouTube video. 4. (Optional) Open any other fullscreen application. OBSERVED RESULT Monitor goes blank but never actually loses signal. EXPECTED RESULT Application fullscreens correctly. SOFTWARE/OS VERSIONS Linux: 6.12.13-273-tkg-eevdf, Arch Linux KDE Plasma Version: 6.3.2 KDE Frameworks Version: 6.11 Qt Version: 6.8.2 GPU: NVIDIA GeForce RTX 3060 Ti Driver version: NVIDIA-Open 570.124.04 Monitor: Acer RG241Y 165hz connected via DisplayPort. ADDITIONAL INFORMATION If I change Variable Refresh to "Never" after disabling VRR via the Monitor OSD, the bug no longer reproduces. I remember on a previous version of Plasma the settings used to vanish entirely if I disabled VRR via the monitor OSD. This points to either a regression or an NVIDIA Driver bug as I previously attempted the repro steps in normal usage while I still had an AMD graphics card.
More info: another user on Discord failed to reproduce this bug on an AMD graphics card; I now suspect a potential NVIDIA driver bug with how monitor settings are reloaded.
Yeah, that's definitely a driver bug - if adaptive sync isn't supported anymore, the driver needs to handle that (and tell KWin about it too). Please report this at https://github.com/NVIDIA/open-gpu-kernel-modules/issues