SDL: SDL_GetGlobalMouseState returns wrong mouse position when using x11vnc

Hi,

When using x11vnc to stream the content of the screen on Ubuntu 22.04, SDL_GetGlobalMouseState will not return the actual current mouse position, but the position where the last mouse click occurred.

I have reported this bug first on ImGui, which has an SDL2 backend: https://github.com/ocornut/imgui/issues/7217

I confirmed that the current code of X11_GetGlobalMouseState is almost the same as SDL 2.0.20 (the version shipped by Ubuntu 22.04; it only has an ifdef check for XInput2 instead of a runtime check): https://github.com/libsdl-org/SDL/blob/5b3ee51c6ca94f803a4ca77ce0dce08ee0426f80/src/video/x11/SDL_x11mouse.c#L419

I can’t say for certain if XInput2 is enabled in the version of SDL2 shipped with Ubuntu, but if it is, then I believe this behavior could be explained this way:

• The position is only updated if global_mouse_changed is true.
• It is set to true in SDL_x11xinput2.c if, e.g., XI_RawMotion or XI_RawButtonRelease events are encountered by XInput2.
• The XI_RawMotion event is not triggered by x11vnc, but XI_Motion. global_mouse_changed is not set to true by this event.

This is also supported by this source: https://stackoverflow.com/questions/62448181/how-do-i-monitor-mouse-movement-events-in-all-windows-not-just-one-on-x11

Set mask to receive XI_RawMotion events. Because it’s raw, XWarpPointer() events are not included, you can use XI_Motion instead.

If x11vnc uses XWarpPointer (which is highly likely, as the mouse position is transferred via the network), XI_RawMotion will never trigger, and as SDL doesn’t set the variable to true for XI_Motion events, it will think the mouse position never changed. Only when a XI_RawButton* event arrives, it will finally update the mouse position.

The solution should be for SDL to also set global_mouse_changed to true when encountering XI_Motion events. This should be a bug in SDL2 and not in x11vnc, as it should be perfectly legal for x11vnc to call XWarpPointer, which triggers a XI_Motion event, which in turn is not handled correctly by SDL.