I wonder if it would be possible to have some kind of “page marks” be visible on the vertical scroll bar on the right? Or maybe simply bookmarks? A mock up is seen below:
Of course because there are not enough room for all bookmarks (may happen!) the vertical bar could be divided into, say, 5 chunks and display the latest bookmark in each chunk. When the slider would be on top of the chunk, the bookmark would become invisible.
Main functionality would be to click the bookmark or the text and jump to the bookmarked message.