I discovered that Editorial does not play well with the keyboard and "safe area" layout in iPadOS 13, first public beta.
(I know I can't expect the first public beta of an operating system to play well with anything, but let's get this into the bugbase. Also, when I don't expect to be logging bugs, I don't keep rigorous track of what actions led up to them. I'm sorry.)
When I set an insertion point in an existing Editorial (.md) document, the keyboard appeared as expected. I tried selecting some text and using the three-finger Copy gesture, tapped elsewhere in the screen, and gave the three-finger gesture for Paste. As I recall this worked once.
At some point, the software keyboard disappeared, leaving the top-of-keyboard toolbar on-screen, sometimes at keyboard height, sometimes along the bottom. When at the bottom of the screen, the iPad would not respond to the swipe-from-bottom gesture to expose the Dock or dismiss the app.
The "Smart Cover" hardware keyboard was attached to the device, but folded. Other apps behaved consistently with the keyboard not being deployed.
One sign that this was more than just an excursion from the Safe Area was that the device did not respond to the Home button either (it's a 2016 iPad Pro). I eventually had to do a hard shutdown to restore functionality.