Turns out we can’t easily add a horizontal scroll here, because that involves hiding overflowed content, and the gear menu requires overflow to be visible.
@pmusaraj did make an improvement that scales the toolbar better at smaller sizes:
https://github.com/discourse/discourse/commit/e3164671698aeef94cf2e142903a30e4688e9dfe
…but at some point we’ll need to figure out a way to support more buttons. Maybe we need to turn the gear icon into a … like on posts and hide more things there by default.