Problem

A followup to #28453, as @anntzer didn't have such a mouse. @tacaswell promised to look into it.

Proposed solution

From #28453 (comment):

Looking at it again I note that I don't try to report back/forward for macos just because I don't have such a mouse to test, but perhaps mpl_buttons() in _macosx.m just needs some additional entries for 1<<3 and 1<<4? (see https://developer.apple.com/documentation/appkit/nsevent/1527943-pressedmousebuttons for the docs)