I implemented setState on macOS.

So one of the problems this highlights on Mac is that SFML doesn't properly handle the two different "fullscreen" options on Mac:

• Classic fullscreen, as SFML currently uses, where it's a fullscreen window on the main desktop akin to borderless fullscreen
• Fullscreen space, the more modern approach to fullscreen on Mac where the app gets it's own space/desktop to itself

This is really an existing bug, as you can make the window fullscreen via the Mac menu bar, keyboard shortcut or window button but internally SFML still thinks it's not fullscreen (although you do get a resized event)

This API highlights the problem more, as you can get the window state which won't say it's fullscreen if you've made it a fullscreen space. Options are:

• Maintain the existing behaviour but disable the default NSWindow fullscreen menu option so users can't use the fullscreen space
• Change to use fullscreen space only and remove the legacy behaviour
• Expose the option to the users via some flag (build or runtime)

Personally I would rule out the first option, as it's the worst user experience on Mac. For reference, SDL gives the user the choice via a hint: https://wiki.libsdl.org/SDL2/SDL_HINT_VIDEO_MAC_FULLSCREEN_SPACES