Basically this (as far as I can tell): http://lists.ardour.org/pipermail/ardour-users-ardour.org/2015-November/027570.html
Except that I’m using gnome, not KDE.
The weird thing is that it worked normally for a short while when I was first testing it, and then I suddenly wasn’t able to input notes any more. After fiddling around a bit, I realized that all keyboard controls had stopped working. However, the “keyboard” inside the full editor view seems to be responding with the usual key press animations (see grayed out X here: https://imgur.com/a/xArMNVA).
Deleting and reinserting the plugin doesn’t help, neither does restarting Ardour. It works without issue in Carla.