mpv on kde fullscreen transition bug

Important Information

Provide following Information:

• mpv version: mpv 1:0.33.0-4
• Linux Distribution and Version: Arch KDE
• Source of the mpv binary: Arch repo
• Window Manager and version: kwin - 5.21.2-1
• GPU driver and version: mesa - 20.3.4-3

Reproduction steps

Use KDE, play a video, and go full-screen. It will be instant and without the full-screen animation.

Important info: this bug only happens when you launch mpv. So it seems that on initial creation of the window, something goes wrong. If I move the window on the screen first, before going full-screen, there will be a full-screen transition animation. But if I leave the window as is, going full-screen is instant, but after that, the transition animation will always play, until I start a new instance of mpv and then it is as described earlier.

I reported this first on KDE, but they checked and forwarded me to here since the bug is in mpv (and that is true, I checked with smplayer and mplayer and they do work correctly).

Expected behavior

I expected a full-screen transition animation.

Actual behavior

mpv went fullscreen instantly, skipping the animation.

Log file

https://gist.githubusercontent.com/Sojiro84/517efe43c4821ea902cc73de692684e2/raw/685bf193c3df844d29cf6a7f3a259de389c22477/gistfile1.txt

Start by reproducing the fullscreen transition on Arch KDE with the reported mpv, KWin, and Mesa versions, then inspect the linked log file for window-state details. Done means identifying and fixing the initial-window condition so fullscreen consistently triggers KDE's transition animation without affecting later instances.