`--window-maximized` opens mpv's window on the wrong screen on kwin/wayland

Important Information

Provide following Information:

• mpv version
  mpv 1:0.33.0-4
• Linux Distribution and Version
  Arch (rolling - up2date)
• Source of the mpv binary
  Arch community repo
• If known which version of mpv introduced the problem
  n/a
• Window Manager and version
  KWin 5.21.0, using wayland
• GPU driver and version
  linux-git 5.11.r0.gf40ddce88593-2 / mesa 20.3.4-1
• Possible screenshot or video of visual glitches
  n/a

Reproduction steps

Just gave kwin on wayland another shot today and noticed an issue in mpv regarding its window position.

I'm using a dual-screen setup, with the right hand one being the primary one. On X11, this screen could be set as "primary" one, but on wayland, there doesn't seem to be an equivalent of primary/secondary screens.

The issue I'm seeing with this is that mpv will always open on the left screen when --window-maximized is set, even if the window position is supposed to be on a different screen, eg. when --screen or --geometry are set, or when kwin would usually manage it on its own, depending on the mouse cursor position (or when a custom window rule is set). This can be reliably reproduced:

# this opens the window on the screen where the mouse cursor is currently at
mpv --no-config video.mp4

# this always opens the window on the left hand screen (my secondary screen)
mpv --no-config --window-maximized video.mp4
mpv --no-config --window-maximized --screen=0 video.mp4
mpv --no-config --window-maximized --screen=1 video.mp4

Log file

1. https://gist.github.com/bastimeyer/075a91d2db75edad66d8ebf351a70272#file-log-default-txt
2. https://gist.github.com/bastimeyer/075a91d2db75edad66d8ebf351a70272#file-log-window-maximized-txt
3. https://gist.github.com/bastimeyer/075a91d2db75edad66d8ebf351a70272#file-log-window-maximized-0-txt
4. https://gist.github.com/bastimeyer/075a91d2db75edad66d8ebf351a70272#file-log-window-maximized-1-txt

Start by reproducing the behavior with the listed mpv commands on a dual-screen KWin/Wayland setup, comparing the default, --screen, and --geometry cases with and without --window-maximized. Read the linked logs and trace the existing window placement and maximization handling. Done means maximized mpv opens on the intended screen instead of always choosing the left screen.