No I haven’t seen it in any source-code editor. It’s a feature I’ve always wanted in a source-code editor and haven’t been able to find it (if anyone knows a source-code editor with this feature please let me know). So I decided to ask for it as ‘feature request’ :) (between us I’ve asked for it in a couple of places :P ) But certainly I would be a loyal user of the editor which has this feature.
But to tell you the truth while studying other people’s code I used to do the same thing in a graphic program (code was in the top layer as text). That way I could add freehand annotations, zoom out/in for a big picture view, open multiple views, etc. And as far as it was studying other people’s code I wasn’t going to change the source code, so the rectangles didn’t have to change.