Could it be that OP was talking about actual "part markers" see attached?

I see no point of hiding that navigator bar "with markers" (in MusicStudent image), the width of the upper panel will not change, as you have items that need that "width".

If Op is worried about "valuable real estate." a good idea would be to be able to hide upper "radio" button bar, as I believe 80% of current users are not using it and it is useless (outdated) in its present form because of Utility tracks.

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markers.jpg (94.26 KB, 83 downloads)