This board is pretty much the same as Trinket M0, with two notable differences: it has additional flash for the filesystem, and it uses more pins than Trinket M0 breaks out. That's why I needed a custom board definition for CircuitPython, which I made. And it seemed to work fine, for a while at least. I modified the library for the ST7735 display to optimize it a bit, and started to write a simple program testing the buttons, when I ran into a problem: every time a soft reset happened (either because I pressed ctrl+d, or because the board detected changed files and restarted), it would crash into "safe mode".
Of course, I assumed this must be the changes I made, so either the flash handling, or the fact that I don't have a DotStar LED connected on pins PA00 and PA01, but instead use them for buttons... It took me a couple of days since I first saw the problem with #PewPew FeatherWing to realize what is the real reason for it: the gamepad library that I added upstream some time ago!
A quick fix for that library to stop scanning keys on restart resolved the issue, and I have my key test working:
Next I need to write the tiles+sprites engine that I've been thinking about since at least two years...