Hello XQuartz-dev, I noticed that XQuartz translates any button that isn't the left and right mouse button to act as button 2. While this correctly handles the middle mouse button, I believe it incorrectly handles additional buttons that appear on some mice (not necessarily Apple-branded ones). These mouse buttons normally appear as buttons 8 and higher under X11 on recent Linux and BSD distributions. This patch modifies the code translating the mouse buttons to continue mapping the middle mouse button to 2, while mapping any additional buttons to 8 and higher (skipping over buttons 4 to 7 as they are used for scrolling). I have attached the patch, and have already opened a merge request for it: https://gitlab.freedesktop.org/xorg/xserver/merge_requests/239 I would appreciate any feedback or testing. One possible issue I might need help with addressing is whether a maximum number of additional buttons to handle should be set. Thanks, Christopher A. Chavez