This topic is more appropriate for the **Plugin Development ** section.

To answer your question:
If Compact Navigation Bar setting is OFF the Navigation bar shows direct representation of the files (line-by-line). This means that if the files have a lot of lines (more than your screen’s vertical resolution) you will see a vertical scroll bar in the Navigation bar as it cannot preview all lines on a single screen.
If Compact Navigation Bar setting is ON it will try to reduce the consecutive equally marked lines so the Nav bar preview fits on a single screen. If you have a lot of lines in the file and there are a lot of differences (IOW - there are not many consecutive equally marked lines) the Nav bar preview might again show vertical scroller as it might not be able to fit all differences on single screen in this case.