SDL_SetRelativeMouseMode causes mouse motion events when window not in focus

SDL Development
1

I’m not sure if this is a bug or expected behaviour, but when setting relative mouse mode I get mouse motion events even if SDL window does not have focus. Also initially I get the events with an invalid window id, until window gains focus.

Here is a minimal example:

#include <stdio.h>
#include "SDL.h"

int main(int argc, char const* argv[])
{
	SDL_Init(SDL_INIT_EVERYTHING);

	SDL_Window* window = SDL_CreateWindow("window", 0,0, 0,0, 0);

	SDL_SetRelativeMouseMode(SDL_TRUE);

	SDL_bool run = SDL_TRUE;
	while(run)
	{
		SDL_Event e;
		while(SDL_PollEvent(&e))
		{
			switch(e.type)
			{
				case SDL_MOUSEBUTTONDOWN:
					run = SDL_FALSE;
				break;

				case SDL_MOUSEMOTION:
					printf("%d\n", e.motion.windowID);
				break;
			}
		}
	}

	SDL_DestroyWindow(window);

	SDL_Quit();
	return 0;
}

In this particular case when I run the program the window does not have focus and moving the mouse I get motion events with window id 0 (invalid, SDL_GetWindowFromID returns NULL), then if I alt-tab to the window I get them with id 2(ok, returns the created window), after alt-tabbing again away from the window - still get id 2.

If that’s normal behaviour can I generally consider id 0 a special NULL id? Or should I always set the relative mouse mode after acquiring focus (waiting for SDL_WINDOWEVENT_FOCUS_GAINED)?

I’m testing on a linux system with X11, and I tried SDL versions 2.0.0, 2.0.7 and the default mercurial branch.