[sciurius]

If we can agree that transposition by steps is the way to go, this would simplify things a lot.
Personally, I still think that transposition is not part of the text display settings, it is more like the metronome, deserving its own dialog. To call the dialog a button next to the [A] (top bar) would be appropriate. For example a note symbol with two arrows, up / down.
The transposition dialog would then be quite small, so you can see the effect of each transposition step (currently most of the song is obscured by the text display settings dialog).
Just an up and a down button, an orig button. Default is to use sharps when transposing up, and flats when transposing down. Since this would require 11 steps to go from A to Bb, a 'swap sharp/flat button' would be handy. For other purposes as well.
A slider would be nice, but I think does not add much value anymore.