Previously, I prototyped a "minimap / quest tracker" UI -- roughly, a fancy scrollbar gutter like a lot of IDEs have -- in Differential, but we ultimately didn't ship it. It was removed in D18043.
I think this is more strongly motivated now, at least in some form:
See PHI1662. See PHI1644. Both of these issues discuss symbol navigation within changesets, which I think likely requires this element exist to be any good.