Links that fill as the section each one points at moves through the screen.
Put each part of the page inside a Progress section and leave the Scroll progress links list empty. It finds them, names them and measures them.
First section
Each link fills as its own section travels through the window. The rail stays where it is while they go past.
Second section
A section holds its own contents, so the rail measures the real thing rather than a set of heights typed in by hand.
Third section
Nothing here carries an id anybody typed. The sections name themselves and the rail lists them.