Comment 10 for bug 1633284

I should have mentioned that you can switch the layout without having Onboard running. The preferences dialog is a separate little program:
$ onboard-settings

I've been trying to come up with scenarios where the high resolution would cause overflows or something, but got nothing so far. It's still possible this is one contributing factor. On the other hand, it started suddenly, and I suppose you didn't change monitors when it happened. Also, while I don't have access to a 4k monitor, I did test similar high resolution (in x at least) setups with multiple monitors.

It'd be good if you could verify that the values read back are the ones you put in, e.g.

gsettings reset org.onboard.window.landscape width
or
gsettings set org.onboard.window.landscape width 123
then
gsettings get org.onboard.window.landscape width

Just like that, without running Onboard.

I tested the layout together with the svg files of Full Keyboard, but no change, nothing unexpected happened.

Btw. there is a hidden setting to turn on secondary labels:
gsettings set org.onboard.keyboard show-secondary-labels true

This only works with some layouts (that's why it's hidden), it does with Full Keyboard, though.

I may need to pepper onboard with more debug output and let you install it from source. I see no better option, currently.