I think this is the simplest thing to try first… we’ll end up truncating a lot of tags and long category names, but the dropdowns can still be full width and show all the content.
Otherwise… do we give up on jamming everything into the tiny space above the keyboard? maybe the mobile topic composer becomes a real (fully scrollable) page? In that case we could have the toolbar, a more obvious link to previewing, etc… we’re already half-abandoning the overlay on iOS to avoid scrolling issues.