Still small problems with the secondary sidebar TOC

[cmarqu]

The following URL: https://cocotb--4834.org.readthedocs.build/en/4834/building.html#scheduler (built from cocotb/cocotb#4834)
has been built with git+https://github.com/pydata/pydata-sphinx-theme@151623d27c775a2706de7bbc7e8102f31fb76af5 which is the git hash of the merged #2185 and should also contain the scrollspy fixes of #2119.

However, if you scroll up on that page so far that the "Regression Manager" heading has vanished at the bottom of the page, it is still highlighted even though "Cocotb" should now be highlighted.

[cmarqu]

On a tangent... Copying the behavior of https://sphinx-nefertiti.readthedocs.io/latest/users-guide/customization/header-links.html#header-links-with-dropdowns would be a slight behavioral change but maybe that's easier to get right?