The Discourse mobile site is pretty good in most aspects but many months ago an update caused the navbar dropdown to not work anymore. If you try to click it, it simply navigates you to the categories page. It should work like on SpongeHome and the docs, where you click it once then the dropdown is there until you choose an option or click elsewhere.
Phone: Nexus 6P running latest Android N
Browser: Chrome Dev 55 and Chrome Stable 53