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Bug 2260

Summary: SDL_SetCursorGrab() is buggy on Windows
Product: SDL Reporter: BurnSpamAddress
Component: videoAssignee: Sam Lantinga <slouken>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: tomrijnbeek
Version: 2.0.1   
Hardware: x86   
OS: Windows 7   

Description BurnSpamAddress 2013-11-22 12:24:32 UTC 
Steps to reproduce:
1. Grab the cursor with SDL_SetCursorGrab()
2. Alt-tab away from the window
3. Click on the titlebar of the window

This will cause the window to disappear underneath the taskbar!

This appears to be a general issue with ClipCursor() on windows, i.e. I am getting the same behavior if I call ClipCursor() directly. Can anyone think of a good workaround?

The following code reproduces the issue on Windows 7 SP1 and Windows 8.1.

#include "SDL.h"
#include "SDL_main.h"

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

    auto wnd = SDL_CreateWindow(
        "Test",
        SDL_WINDOWPOS_CENTERED,
        SDL_WINDOWPOS_CENTERED,
        800,
        600,
        0);

    SDL_SetWindowGrab(wnd, SDL_TRUE);

    bool running = true;
    while (running)
    {
        SDL_Event e;
        if (SDL_PollEvent(&e))
        {
            switch (e.type)
            {
            case SDL_KEYDOWN:
            case SDL_WINDOWEVENT_CLOSE:
            case SDL_QUIT:
                running = false;
                break;
            }
        }
    }

    SDL_Quit();

    return 0;
}
Comment 1 BurnSpamAddress 2013-11-22 13:30:34 UTC 
Additional investigation indicates that this issue is not specific to SDL, but can be reproduced on native applications under the correct circumstances.

It is caused by a feedback loop between the ClipCursor function and the modal resize/move event loop that handles mouse-based sizing on Windows. The solution lies in SDL_windowsevents.c (WIN_WindowProc)

switch (msg)
{
case WM_ENTERSIZEMOVE:
case WM_ENTERMENULOOP:
    is_in_modal_loop = 1;
    if (cursor_grabbed) {
        ClipCursor(NULL);
    }
    break;

case WM_EXITSIZEMOVE:
case WM_EXITMENULOOP:
    is_in_modal_loop = 0;
    if (cursor_grabbed) {
        ClipCursor(&rect);
    }
    break;

case WM_WINDOWPOSCHANGED:
    if ((window_flags & SDL_WINDOW_INPUT_GRABBED) &&
        (window_flags & SDL_WINDOW_INPUT_FOCUS) &&
        !is_in_modal_loop) {
        ClipCursor(&rect);
    }
}

Apologies for not creating a proper patch, but I do not have access to hg at work.
Comment 2 Tom Rijnbeek 2014-03-22 21:06:18 UTC 
I am running OpenTK on top of the SDL2 Win32 backend (Windows 8) and I am running into the same issue. As I don't really have access to the SDL2 source, manually patching is not really an option. I made a video showing what exactly happens for our game that can be found at http://youtu.be/uvFzta6COYw
It would be much appreciated if this could be fixed.
Comment 3 Sam Lantinga 2014-03-23 16:49:50 UTC 
This should be fixed in 2.0.3, please let me know if you are still having issues with this.

Cheers!