In modern Editors like sublime, vscode, intellij, there is a mini-map of the whole file, allowing a quick visual search.
Making any channel clicked in high priority for display, perhaps displacing a previous channel in the minimap, would help with context for next/prev edge skips.
A solid rectangle isn't a deal breaker if the current view region within the minimap is clear. The context within the capture is still clear, even if there isn't much "structure" in the map display.
A minimap would resolve a major problem I have: zooming in enough to read bubble text means that I lose big picture context, especially when I use the "skip to edge" feature.
Almost always I'm dealing with bursty I2C or Async serial activity in the context of slow moving other activity (CC lines and VBus state changes in my current use case). Seeing the slow changes in some channels and blocks of activity in others would provide great context. Zooming the minimap would probably help a lot too by giving a big picture view of local context rather than an overview of the entire recording.
Being able to select the channels used in the minimap would make it much more useful. That probably resolves the "solid rectangle" issue - any signals that are solid can be removed (or for bonus points, don't appear by default).
If the data is dense, I think that the minimap will look like a solid rectangle. Would showing timing markers and ranges over the scrollbar section be helpful?