Index autoexpansion makes index cluttered and unusable after prolonged reading.
Submitted by Ryder Lewis
When scrolling through a document, the sidebar index' categories expand automatically when a page corresponding to an index category is reached. When browsing a thoroughly-bookmarked PDF, so many categories expand, that the index becomes very long and cluttered, decreasing the index' usefulness and defeating the purpose of a hierarchical index. After prolonged browsing, the index is fully expanded; in a well-bookmarked PDF, this requires frequently scrolling and visually scanning the index in order to find a shortcut that would be easy to find otherwise.
My suggestion is to expand an index category only when the little triangle button beside the category is clicked. For what it's worth, this is also Acrobat's default behaviour.