>>105769327
starting to realize why my colleagues hate me

>>105769319
Found myself an answer in SDL, as always they already have it implemented.
Common Wayland usage error is assuming first enumerated monitor is the primary one (as I already suspected >>105769080) and you need to either use DBus (which understandably nobody wants to deal with) or do some fallback logic like picking the largest monitor or best refresh rate.
Windows have flag for primary monitor in MONITORINFO struct https://learn.microsoft.com/en-us/windows/win32/api/winuser/ns-winuser-monitorinfo
X11 doesn't have anything primary info, so I guess it just consistently announces primary monitor first and SDL picks first from list.