SDL: Incorrect picture size on RPi4 in KMSDRM mode

For comparison’s sake, here’s how window sizing with respect to fullscreen options works on other platforms (and therefore how I’d expect it to work when the KMSDRM backend is used) – I believe all of what I’m writing is accurate but please correct me if I’m wrong:

• Creating a window without any fullscreen flags will attempt to make the window at the requested size. If it can’t do it, the closest appropriate size may be chosen instead. A windowed-mode window does not change the active display mode. A couple examples of that internal resizing behaviour are: • On macOS if the window is resizable and it would cover the dock area, the OS shrinks the size to fit. • On iOS, there’s no concept of floating app-specified window bounds, so the size request is used as an orientation (landscape/portrait) hint and SDL’s code adjusts the true size of the window to fill the screen. This is probably close to what I’d expect on other platforms that don’t support app-created floating windows (aside from the orientation part) - such as KMSDRM, I suppose. On iOS, the OS menu bar is left visible when windowed mode is used.

• Creating a window with SDL_WINDOW_FULLSCREEN_DESKTOP (or switching a window into that state via SDL_SetWindowFullscreen) changes the window’s dimensions to fill the monitor’s dimensions based on the existing display mode that was active before SDL touched it. It’s designed to avoid using a custom display mode, although transitioning from exclusive fullscreen to fullscreen-desktop may change the active display mode if exclusive fullscreen had set a custom one. In other words, it always makes sure the display mode retrieved via SDL_GetDesktopDisplayMode is active, and then makes sure the window fills the screen.

• Exclusive fullscreen (SDL_WINDOW_FULLSCREEN) finds the closest match to the requested display mode parameters and changes the monitor’s active display mode, while also making the window cover the entire screen at that resolution.

• SDL_SetWindowDisplayMode only affects windows that have SDL_WINDOW_FULLSCREEN active (exclusive fullscreen). If that window isn’t in exclusive fullscreen mode, then it doesn’t change the display’s active mode when it’s called. But making that window exclusive fullscreen via SDL_SetWindowFullscreen after calling SDL_SetWindowDisplayMode will change the active display mode to the one previously set in SetWindowDisplayMode.

I believe if you make a backend set the active display mode in windowed mode, it could cause a lot of apps using SDL to break because assumptions made in their code might be invalidated.