Musescore opens software multiple times
MuseScore 3 had all scores in the same window which forced them all to also use exactly the same set of soundfonts. This was never good, but tolerable when the only sounds available were soundfonts. not that MuseScore support VST instruments and Msue Sounds, it just wouldn’t not be practical to force all open scores to use the same sounds. hence, they are now in separate instances so you can have different sounds for different scores.
Not sure what you mean about pop-up windows.
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