Just a suggestion to maybe consider making the Transposition dialog sticky so it will remember the last used settings. Perhaps a checkbox somewhere to allow it to remember the settings would be even better. I’ve found myself applying the exact same transposition many times and it would be convenient to save a few seconds by having it remember the previously applied transposition.
There seems to be a slight bit of confusion created by the interface too by having “Quality:” as a higher dropdown than “Interval:” We obviously think “transpose down a minor third,” with the quality first, but “minor” isn’t able to be selected in the Quality dropdown until “third” is selected even though minor is higher visually in the dialog box. By having “unison” as the default (which no one is using or they wouldn’t be in the Transposition dialog box) “major” and “minor” are obviously excluded as options in the visually higher dropdown, which is a bit confusing for someone looking to select “minor third.” Just my $0.02.
The “Calculate Interval” box is really brilliant for younger students BTW.