Accidentals

FWIW, I have often wished that shift-8 would add a a true octave below the original note, including accidental. So hitting shift-8 while the cursor is on a Bb would always add a Bb, period, regardless of the key signature. It's incredibly rare I'd want it any other way. For other intervals, though, I have no particular expectations. But I think I'd be surprised if adding any other interval ever produced an accidental by default. Which is to say, I think the current behavior is fine for all intervals *except* the octave.

This does sort of relate to another common request, though, and that is what happens is, in the key of C, you enter a B, lower it to Bb, then enter another B. I think most people at first expect this to result in a single note with no accidental - which is to say, it would actually be a Bb, since the previous flat would still apply. Currently, of course, you actually get a B natural, with an explicit natural sign. If MuseScore were to some day support the style of note entry where typing B would enter a note with no explicit accidental (meaning typing B would give you Bb if a Bb existed earlier in the measure), I think I'd expect the "add interval" behavior to behave similarly. So if in that same situation I were to type D followed by shift-3, I'd get a note with no accidental: a Bb.