SynEdit now has annotated Scrollbars.
In this screenshot you can see in the scrollbar:
the position of the multiple carets (full blueish line)
lines with errors (full red line)
the position of the bookmarks (mini bluish marks on the left)
line modification info (on the right side)
alpha-blended slider
This feature is on the multicaret branch.
Zoom In/Out/Reset functionality has also been implemented.
You can still vote on this poll to influence future development.
See the technical details.
See also SynEdit now supports mulit-caret, multi-selection editing - I made this - Delphi-PRAXiS [en].