With SDL 2.0.12 under MS Windows, if the window is partially offscreen calling SDL_SetWindowGrab(w, SDL_TRUE) works, but subsequent call to SDL_SetWindowGrab(w, SDL_FALSE) does not work.

I tested this in both real program, and a small test app, where unlocking cursor worked perfectly while window is fully in desktop bounds, but did not work if it was at least few pixels outside.

For the reference, following code is enough to reproduce the issue:

#include <windows.h>
#include <SDL.h>

int WinMain(
    HINSTANCE hInstance,
    HINSTANCE hPrevInstance,
    LPSTR     lpCmdLine,
    int       nShowCmd)
{
    SDL_Init(SDL_INIT_VIDEO);
    SDL_Window* w = SDL_CreateWindow("", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 400, 0);

    bool grabbed = false;
    bool want_quit = false;
    while (!want_quit)
    {
        SDL_Event event;
        while (SDL_PollEvent(&event))
        {
            switch (event.type)
            {
            case SDL_QUIT: want_quit = true; break;
            case SDL_KEYDOWN:
                if (event.key.keysym.scancode == SDL_SCANCODE_SPACE)
                {
                    SDL_SetWindowGrab(w, static_cast<SDL_bool>(!grabbed));
                    grabbed = !grabbed;
                }
            }
        }
    }

    SDL_DestroyWindow(w);
    SDL_Quit();
    return 0;
}