Navigation highlight scrolls with page on Safari 15

On Safari 15, occasionally the navigation highlight rectangle gets out of alignment with the element it highlights:

The highlight rectangle appears to be fixed relative to the window and not the body content, so scrolling the page causes this to happen. This happens whenever the up/down arrow keys are pressed unless the page is already at the scrolling boundary.

Reproduce the issue in Safari 15 by using the up and down arrow keys and scrolling away from the boundary. Then trace the navigation highlight and the selected element's positioning; done means the rectangle remains aligned while the page scrolls. The payload names no source files or tests, so locate the relevant entry point from the navigation-highlight behavior.

Written by the indexing model from the issue text.