In order to keep the instrument list simple and tidy, I think having the instrument list as a drop down menu would help.
What I mean is that the instrument list will be retracted at first when the sequencer is opened, it'll show the normal categories such as "Piano, Percussion (MIDI), Percussion (Classic), Guitar" and etc.
These categories will be clickable and when they are clicked, it would extend, showing the instruments under the said category.
Once again, this would make more space for the sequencer if the instrument selector is left open.
What I mean is that the instrument list will be retracted at first when the sequencer is opened, it'll show the normal categories such as "Piano, Percussion (MIDI), Percussion (Classic), Guitar" and etc.
These categories will be clickable and when they are clicked, it would extend, showing the instruments under the said category.
Once again, this would make more space for the sequencer if the instrument selector is left open.