X11 mouse position has subtle issues

[niconii]

Here's a YouTube video showcasing the below code. driver is the current video driver, mouse is the position from the latest mouse motion event, and gmouse is the position from SDL_GetGlobalMouseState.

It's hard to pin down exactly what's wrong, but you can see instances where mouse seems to lag behind gmouse, instances where the mouse cursor visibly moves a pixel but this isn't reflected in SDL, and so on.

As best as I can tell, SDL_VIDEO_DRIVER=wayland doesn't have these issues.

#include <SDL3/SDL.h>

#define SDL_MAIN_USE_CALLBACKS
#include <SDL3/SDL_main.h>

SDL_Window *window;
SDL_Renderer *renderer;

float mouse_x = 0;
float mouse_y = 0;
float gmouse_x = 0;
float gmouse_y = 0;

SDL_AppResult SDL_AppInit(void **appstate, int argc, char **argv) {
  SDL_Init(SDL_INIT_VIDEO);
  SDL_CreateWindowAndRenderer("Test Window", 320, 240, 0, &window, &renderer);
  
  return SDL_APP_CONTINUE;
}

SDL_AppResult SDL_AppIterate(void *appstate) {
  char *text = NULL;
  
  SDL_SetRenderDrawColor(renderer, 0, 0, 255, 255);
  SDL_RenderClear(renderer);

  SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);

  SDL_asprintf(&text, "driver %s", SDL_GetCurrentVideoDriver());
  SDL_RenderDebugText(renderer, 0, 0*8, text);
  SDL_free(text);

  SDL_asprintf(&text, " mouse x=%11.6f y=%11.6f", mouse_x, mouse_y);
  SDL_RenderDebugText(renderer, 0, 1*8, text);
  SDL_free(text);

  SDL_GetGlobalMouseState(&gmouse_x, &gmouse_y);
  SDL_asprintf(&text, "gmouse x=%11.6f y=%11.6f", gmouse_x, gmouse_y);
  SDL_RenderDebugText(renderer, 0, 2*8, text);
  SDL_free(text);

  SDL_RenderPresent(renderer);

  return SDL_APP_CONTINUE;
}

SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event) {
  switch (event->type) {
    case SDL_EVENT_QUIT: {
      return SDL_APP_SUCCESS;
    }

    case SDL_EVENT_MOUSE_MOTION: {
      mouse_x = event->motion.x;
      mouse_y = event->motion.y;
      break;
    }
  }

  return SDL_APP_CONTINUE;
}

void SDL_AppQuit(void *appstate, SDL_AppResult result) {
  SDL_DestroyRenderer(renderer);
  SDL_DestroyWindow(window);
}

[Kontrabant]

For clarification, are you seeing this behavior on a real X11 session, when running via XWayland, or both?

[niconii]

I've tested this in GNOME on Arch Linux, in both Wayland and X11. It happens on both a real X11 session and Xwayland, but doesn't happen running natively on Wayland. @12Me21 and @y-ack were both able to reproduce this on their computers. @y-ack has also confirmed this doesn't happen on Windows.

I'd like to provide more videos, but while recording I discovered a bug in OBS preventing me from accurately recording the mouse position, so now I'm investigating that too. In the meantime, adding this code to SDL_AppIterate before SDL_RenderPresent makes the problem a bit easier to see:

  SDL_SetRenderDrawColor(renderer, 0, 255, 0, 128);
  SDL_RenderLine(renderer, SDL_floorf(mouse_x), 0, SDL_floorf(mouse_x), 240);
  SDL_RenderLine(renderer, 0, SDL_floorf(mouse_y), 320, SDL_floorf(mouse_y));

[Kontrabant]

This seems like a mismatch between the coordinates provided by XInput and XQueryPointer. XInput does provide the root coordinates in motion events, so that should be used if a window has focus to avoid possible relative/global mismatches.

Can you test this branch and see if it fixes your issue: https://github.com/Kontrabant/SDL/tree/x11_coordinate_mismatch

[niconii]

Before this branch, mouse seemed to lag behind by one motion, but gmouse was correct.

With this branch, mouse and gmouse both lag behind by one motion.

So, this doesn't fix the issue. However, this suggests to me that the problem lies with the Xinput code specifically, and indeed, when I build without Xinput support, the bug does not occur.

[niconii]

SDL/src/video/x11/SDL_x11xinput2.c

Lines 451 to 454 in 5608bf5

	if (xev->deviceid != xev->sourceid) {
	// Discard events from "Master" devices to avoid duplicates.
	break;
	}

Commenting out these lines fixes the bug, though I don't know why.