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Created attachment 4326 [details]
Red cross shows where cursor is located before alt-tabbing

On Linux if you have an active SDL window and the mouse is within the bounds of the window SDL_GetWindowsFlags reports SDL_WINDOW_MOUSE_FOCUS.  However if while the mouse is within bounds the user alt-tabs to another applications window which overlaps the SDL window (including the current cursor position) the SDL_GetWindowsFlags continues to report SDL_WINDOW_MOUSE_FOCUS.  

It then doesn't update itself until the cursor leaves the bounds of the other application, and crosses into the bounds of the visible portion of the SDL Window.

Using the included C code (it's fairly rudimentary as I usually access SDL from a .NET context via a C# wrapper), it shows white when the window reports Mouse Focus, and Black when it does not.  The picture I have attached shows with a red cross where the cursor is before alt tabbing from the active SDL window to in this case a terminal.

I have confirmed this is still an issue on the latest commit to default, running on XUbuntu 18.04 with the reported video driver being X11.


#include <stdio.h>
#include <SDL2/SDL.h>

int main(int argc, char *argv[])
{
    if (SDL_Init(SDL_INIT_VIDEO|SDL_INIT_AUDIO) != 0) {
      SDL_Log("Unable to initialize SDL: %s", SDL_GetError());
      return 1;
    }

    printf("Current: %s\n", SDL_GetCurrentVideoDriver());
    SDL_Window *window = SDL_CreateWindow("Test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 480, 0);
    SDL_Surface *surface = NULL;
    surface = SDL_GetWindowSurface(window);
    SDL_bool poll = SDL_TRUE;    
    while (poll) {
      SDL_Event event;
      while (SDL_PollEvent(&event)) {
        if (event.type == SDL_QUIT) {
          poll = SDL_FALSE;
          break;
        }  
      }

      if (SDL_GetWindowFlags(window) & SDL_WINDOW_MOUSE_FOCUS) {
        SDL_FillRect(surface, NULL, SDL_MapRGB(surface->format, 0xFF, 0xFF, 0xFF));    
      } else {
        SDL_FillRect(surface, NULL, SDL_MapRGB(surface->format, 0x00, 0x00, 0x00));
      }
      SDL_UpdateWindowSurface(window);
      SDL_Delay(500);
    }

    SDL_DestroyWindow(window);

    SDL_Quit();
    return 0;
}