Created attachment 175079 [details]
Illustration of the idea

Here is a suggestion for enhancing the GTK scrollbar - allow the application to highlight certain regions with specified colors. This would be very useful in allowing apps to show the existence and location of things in the document that are outside the current scroll view such as:
- search matches (e.g. in evince, gedit, abiword)
- matching parentheses (gedit, abiword)
- concurrent edits by another user (gedit has a pluging; abiword, gimp, inkscape could be made concurrent)
- recent changes to the content being edited

and probably other uses.

An illustration of the idea is attached.