I'm managing keybinds with ClothConfig for a couple of reasons (no clashes with vanilla keybinds and modifiers are required), and it seems that ModifierKeyCode::matchesCurrentMouse will only return true if the mouse key pressed was left, right or middle click on the mouse. Because of this, I wouldn't have the option of assigning a keybind to my mouse's button 4 or button 5 if I tried. GLFW has mouse keys going from 1-8, and they work in vanilla Minecraft as normal keybinds so this should be possible.

This is where said code is defined :^)

Note: I'm obviously not setting buttons 4 and 5 as defaults for anything, but they should be options for players to set for themselves nonetheless.

Should be a bug, might change that piece of code to just get it from GLFW.

in v2-unstable for 1.16, might backport someday.