nano -w /etc/mpv/mpv.conf
autofit should usually be 100% but on a Plasma 6 wayland session the Plasma taskbar covers a certain aspect of the mpv window so setting it accordingly to e.g. 95% as a workaround prevents the mpv window to stretch over the entire screen.
hwdec=auto
cache=no
gpu-api=vulkan
ao=pipewire
vo=gpu-next
autofit=95%
#autofit=100%
To re-enable seeking with scroll wheel (5 second steps in this example).
nano -w /etc/mpv/input.conf
WHEEL_UP seek 5
WHEEL_DOWN seek -5