Make volta mouse drops apply only to first staff by default, unless user holds control to drop to a specific staff
Currently, dragging and dropping a volta onto a measure will cause the volta to be applied to the specific stave under the mouse when mouse released. The most common & practical case when applying voltas is that they are applied to the top staff.
Applying a volta to a staff other than the first staff causes problems because MuseScore only checks for the presence of a volta on the first staff when applying voltas to part scores when generating parts, and also applies voltas dropped on the main score to previously-generated part scores only when they were dropped onto the first staff of the main score. (This is problematic because at this point the staff in the main score would differ from the part score as they wouldn't have the same voltas.)
I suggested in https://musescore.org/en/comment/929227 that "I think should voltas should always drop to first staff unless user holds control." Marc seems to agree in that he certainly wouldn't mind if holding Ctrl were necessary to make dropping to non-first staves.
I've discussed with marc that I think voltas should be applied to the first staffand he agrees that at least: https://musescore.org/en/comment/929227
Note I'm only doing this change to Mouse drops. I haven't decided whether to make this behavior change to be applied for double-clicking palette elements to a selection too.