AppWindow.RequestFullscreen

void RequestFullscreen(bool fullscreen)

Asks for this window to cover its whole display, or to go back to a normal window. This is only ever a request, and it’s never immediate! Window managers can refuse it, browsers wait for a user gesture, and some platforms don’t implement it at all. None of those report back a refusal, so watch the Fullscreen property to see if and when it takes effect. Going fullscreen resizes the window, so expect the render surface to follow along.