This script adds or substitutes an accurate /* section link */ (which appears as →section link) in the summary as you edit a page (or every time you hit "Show changes" when editing an old revision). You have to have live preview enabled (Preferences → Editing → Show preview without reloading the page), or it won't work well.
That is, it checks what section the modified parts of the source belong to and updates the summary accordingly. If no section heading precedes the first modified line, or the modified parts belong to no one particular section (i.e. the source has no headings or multiple highest-level sections have been modified), it adds no section link (and removes any existing one).
Whenever the summary is changed by the script, an undo button appears next to the summary box. Clicking it will restore the previous section link.
If you have live preview disabled, it won't work well because every time you show a preview or diff the script will think the new version of the text is the original.
Show preview without reloading the page), or it won't work well.