FIX: Scroll padding on top for anchor links

[choldgraf]

• closes Anchor links have incorrect offset from top of screen #667

The failing test is a lighthouse performance check, which is notoriously flaky and I think can be ignored

[choldgraf]

@mathbunnyru how does this look to you? Maybe @consideRatio is looking at this as well?

you can see a demo here:

https://sphinx-book-theme--669.org.readthedocs.build/en/669/

[mathbunnyru]

Thank you for such a quick response @choldgraf!

Now there is some extra text above the anchor.
https://sphinx-book-theme--669.org.readthedocs.build/en/669/notebooks.html#code-blocks-and-image-outputs

[choldgraf]

Hmmm good point - it is a bit tricky to get just right, because if the top padding is too small then we will accidentally highlight the wrong section and the original problem will come back. I reduced the padding at the top in the latest commit to try to strike a balance. WDYT?

[mathbunnyru]

I think it looks better now, but I still see the text from the previous part. I guess it's impossible to have both things.

[choldgraf]

OK how's this? It means that if we have two headers directly after one another, we might accidentally highlight the second, but maybe that's not such a big deal and also not common.

[mathbunnyru]

Now it looks great to me 👍

[choldgraf]

OK I cleaned up a few other points that @mathbunnyru noted - will merge this if tests are happy since it's fixing a few bugs