Yes; I only have one other Qt app off the top of my head installed currently (probably others I don't remember); namely, QtIrreco...

And you're right, the only custom-mapped button that works there is the same one that works for me in TxPad "[", for which the fn-key button is remapped to a dead instead of standard key.

Annoying, this is, and I have some Qt people to complain to no doubt. xkb is pretty standard on Linuxes, no?

Anyway, thank you for taking the time to investigate my issue. Much appreciated.