Hi,

On recent versions on SDL I am seeing failures to display a fullscreen window. Before I proceed, a few notes on my hardware setup (as it's possibly relevant):

• I am on a Ubuntu 16.04 system (uname -a says 4.15.0-151-generic #157-Ubuntu SMP Fri Jul 9 23:07:57 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux), with x11, running i3 as the window manager
• I have two physical monitors, one of which has a 2560x1080 resolution, and the other has a 1920x1080 resolution

Below is a minimal reproducer example that fails for me with current main (please forgive the haphazard error handling, I do not program C often if at all these days):

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

int main(int argc, char **argv)
{
    if (SDL_Init(SDL_INIT_VIDEO) < 0)
    {
        printf("SDL failed to initialise with error %s\n", SDL_GetError());
        return EXIT_FAILURE;
    }

    SDL_Window *window = NULL;

    window = SDL_CreateWindow("test", 0, 0, 2560, 1080, SDL_WINDOW_SHOWN | SDL_WINDOW_FULLSCREEN);

    if (window == NULL)
    {
        printf("Window was not created: %s\n", SDL_GetError());
        return EXIT_FAILURE;
    }

    SDL_Renderer *renderer = SDL_CreateRenderer(window, -1, 0);

    SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
    SDL_RenderClear(renderer);
    SDL_RenderPresent(renderer);

    int quit = 0;

    SDL_Event e;

    while (!quit) {
        while (SDL_PollEvent(&e) != 0) {
            if (e.type == SDL_QUIT) {
                quit = 1;
            }
        }
    }

    return EXIT_SUCCESS;
}

This on my system fails with

X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  7 (RRSetScreenSize)
  Serial number of failed request:  256
  Current serial number in output stream:  257

I have managed to trace this back to 16e3bfe; commenting out the call to X11_XRRSetScreenSize prevents the crash to desktop.

The reason why I stated that this is possibly related to my monitor configuration is that upon some digging and instrumenting the call above with some log statements I have managed to fish out the following:

INFO: DisplayWidth = 4480, DisplayWidthMM = 1518
INFO: DisplayHeight = 1080, DisplayHeightMM = 366
INFO: about to XRRSetScreenSize (display = 55c9cd09e0f0, window = 254, w = 2560, h = 1080, mmWidth = 867, mmHeight = 366)

Note that DisplayWidth and DisplayHeight say 4480x1080, so it's the total resolution of both my screens. xrandr reports: (somewhat truncated)

Screen 0: minimum 8 x 8, current 4480 x 1080, maximum 16384 x 16384
eDP-1-1 connected 1920x1080+2560+0 (normal left inverted right x axis y axis) 344mm x 194mm
HDMI-1-1 connected 2560x1080+0+0 (normal left inverted right x axis y axis) 673mm x 284mm

I am not sure where xlib got the 1518x366 measurements from, and since I do not routinely develop anything for x11 I have no idea what the proper API would be for fetching the single screen's size.

If I can provide any further information or test possible patches/PRs to resolve this please let me know.