Well, it looks like reverting to Stable seemed to at least partially fix it for me. The keyboard is still higher on the screen, but at least it fits to the screen width correctly. Thanks!
I couldn’t find any mention of anyone else with this issue around the web or on valve’s GitHub issue tracker, so I assumed it was something dumb I’d done rather than a bug.
Second Mihon. It’s an all-in-one downloader and reader, and supports reading offline.