Xlib supports many mouse buttons, though there are 9 standard buttons, and they aren't arranged like other mouse APIs. Using only 5 buttons was preventing the use of buttons besides left/right/middle click and the scroll wheel. Here's what all the standard buttons are:
1. left button
2. middle button (pressing the scroll wheel)
3. right button
4. turn scroll wheel up
5. turn scroll wheel down
6. push scroll wheel left
7. push scroll wheel right
8. 4th button (aka browser backward button)
9. 5th button (aka browser forward button)
The remaining button indices are non-standard and device-specific, and technically far more than 32 are supported, but this seems like a reasonable limit to avoid cluttering the list with tons of useless mouse buttons. What mouse has more than 32 buttons anyways?
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