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Bug 3067

Summary: [X11] relative mouse value is jumpy when pointer change window
Product: SDL Reporter: Cengiz <yaakuro>
Component: eventsAssignee: Ryan C. Gordon <icculus>
Status: ASSIGNED --- QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: icculus, sezeroz
Version: 2.0.3   
Hardware: x86_64   
OS: Linux   
Attachments: relative mouse value is jumpy when pointer change window

Description Cengiz 2015-07-23 06:09:56 UTC 
Created attachment 2223 [details]
relative mouse value is jumpy when pointer change window

Hi there, 

hi icculus. I think you know already about this. Here the issue in short. When the mouse pointer leaves a window and enters another window the xrel, yrel values jumps. Here a print out:

WindowID: 2, abs=(319, 64) rel=(320, -5)
WindowID: 2, abs=(318, 64) rel=(-1, 0)
WindowID: 2, abs=(319, 64) rel=(1, 0)
WindowID: 2, abs=(319, 64) rel=(1, 0)
WindowID: 3, abs=(0, 64) rel=(-320, 0)
WindowID: 3, abs=(1, 64) rel=(1, 0)
WindowID: 3, abs=(2, 64) rel=(1, 0)
WindowID: 3, abs=(3, 64) rel=(1, 0)
WindowID: 3, abs=(2, 64) rel=(-1, 0)
WindowID: 3, abs=(1, 64) rel=(-1, 0)
WindowID: 3, abs=(0, 64) rel=(-1, 0)
WindowID: 3, abs=(0, 64) rel=(-1, 0)
WindowID: 2, abs=(319, 62) rel=(320, -2)
WindowID: 2, abs=(318, 62) rel=(-1, 0)
WindowID: 2, abs=(317, 62) rel=(-1, 0)
WindowID: 2, abs=(318, 62) rel=(1, 0)
WindowID: 2, abs=(319, 62) rel=(1, 0)
WindowID: 2, abs=(319, 62) rel=(1, 0)
WindowID: 3, abs=(0, 64) rel=(-320, 2)
WindowID: 3, abs=(1, 64) rel=(1, 0)
WindowID: 3, abs=(2, 64) rel=(1, 0)

As you can see the jump happens when pointer changes the window. This happens for the y direction too.

Here my test code:

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

int main(int argc, char **argv)
{
	SDL_Init(SDL_INIT_EVERYTHING);
	SDL_Window* window1 = SDL_CreateWindow("Window 1", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,320, 200, 0);
	SDL_Window* window2 = SDL_CreateWindow("Window 2", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 320, 200, 0);

	while(true) {
		SDL_Event e;
		if(SDL_PollEvent(&e)) {
			if(e.type == SDL_QUIT) {
                break;
            } else if(e.type == SDL_KEYDOWN) {
                break;
            } else if(e.type == SDL_MOUSEMOTION) {
                printf("WindowID: %d, abs=(%d, %d) rel=(%d, %d)\n", e.window.windowID, (int)e.motion.x, (int)e.motion.y, (int)e.motion.xrel, (int)e.motion.yrel);
            }
		}
	}
	SDL_DestroyWindow(window1);
    SDL_DestroyWindow(window2);
	SDL_Quit();
	return 0;
}

Just move one window away from the other and move the pointer from one window to the other.
In the attachment I put a fix. Not sure if that is the best solution but it works. Entering a window should reset the last_x, last_y variables of the mouse pointer.


Cengiz (yaakuro)
Comment 1 Ryan C. Gordon 2016-01-08 06:24:10 UTC 
This patch is now https://hg.libsdl.org/SDL/rev/e1f07cbb3c86, thanks!

--ryan.
Comment 2 Ryan C. Gordon 2016-01-08 06:26:11 UTC 
This patch appears to solve the problem for X11, but I have a suspicion this bug hits other platforms, too. 

I'll look into resetting last_x/last_y in SDL_OnWindowEnter() before I resolve this bug fully, but we appear good to go in the place we're specifically seeing this issue (multi-window UnrealEd on Linux).

--ryan.