Attempt to get the X1 and X2 button state on X11 by using the current event state instead of direct X11 query.
diff --git a/src/video/x11/SDL_x11mouse.c b/src/video/x11/SDL_x11mouse.c
index ecc6c7c..ed29288 100644
--- a/src/video/x11/SDL_x11mouse.c
+++ b/src/video/x11/SDL_x11mouse.c
@@ -406,6 +406,8 @@ X11_GetGlobalMouseState(int *x, int *y)
buttons |= (mask & Button1Mask) ? SDL_BUTTON_LMASK : 0;
buttons |= (mask & Button2Mask) ? SDL_BUTTON_MMASK : 0;
buttons |= (mask & Button3Mask) ? SDL_BUTTON_RMASK : 0;
+ /* Use the SDL state for the extended buttons - it's better than nothing */
+ buttons |= (SDL_GetMouseState(NULL, NULL) & (SDL_BUTTON_X1MASK|SDL_BUTTON_X2MASK));
/* SDL_DisplayData->x,y point to screen origin, and adding them to mouse coordinates relative to root window doesn't do the right thing
* (observed on dual monitor setup with primary display being the rightmost one - mouse was offset to the right).
*