IDE Window: Editor Options Display Gutter
Lazarus IDE has the feature called "Overview gutter". It is the thin vertical bar near the vertical scrollbar of the source editor. It can be enabled/disabled in the IDE Options dialog: "Editor / Display / Show overview gutter" checkbox.
- modified lines: green/yellow
- gutter marks: breakpoints (red), bookmarks (blue), compile errors (orange)
- interface/implementation/finalization sections: shades of grey
- current visible page: shade of grey
If you click on a mark, you will go to the relevant point in the text.