https://bugs.winehq.org/show_bug.cgi?id=57456
--- Comment #5 from alexandros.frantzis@collabora.com ---
Not sure if winewayland improved it or KWin did?
I did a (Wine) bisection and it turns out that enabling EGL_EXT_present_opaque is what triggers the behavioral change in KWin. I also tried explicitly setting the wl_surface opaque region (while EGL_EXT_present_opaque was disabled) and got the same behavior (surface centered + black background). So KWin seems to be treating fully-opaque surfaces differently from non-opaque for fullscreen purposes (I don't know if such behavior was introduced recently or has been there for some time). I guess all this is in line with protocol changes the KDE folks are proposing (e.g., https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/37...).