Use this function to capture the mouse and to track input outside an SDL window.
int SDL_CaptureMouse(SDL_bool enabled)
whether or not to enable capturing
Returns 0 on success or -1 if not supported; call SDL_GetError() for more information.
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Capturing enables your app to obtain mouse events globally, instead of just within your window. Not all video targets support this function. When capturing is enabled, the current window will get all mouse events, but unlike relative mode, no change is made to the cursor and it is not restrained to your window.
This function may also deny mouse input to other windows--both those in your application and others on the system--so you should use this function sparingly, and in small bursts. For example, you might want to track the mouse while the user is dragging something, until the user releases a mouse button. It is not recommended that you capture the mouse for long periods of time, such as the entire time your app is running.
While captured, mouse events still report coordinates relative to the current (foreground) window, but those coordinates may be outside the bounds of the window (including negative values). Capturing is only allowed for the foreground window. If the window loses focus while capturing, the capture will be disabled automatically.
While capturing is enabled, the current window will have the SDL_WINDOW_MOUSE_CAPTURE flag set.
This function is available since SDL 2.0.4.