In the screenshot I've set a spare mouse button to select items to compare. But this doesn't work.

This is related to non keyboard buttons in inventories.

Yes, to get that to work probably means we have to fix so inventory keybinds use the Minecraft vanilla method. Which apparently has been broken.

When fixing this, we also need to fix same key keybinds not working, even when they are not used in the same context. (Space keybind in inventory or a screen makes jump not work)