Significant visual regression in collapse headings mode with Sphinx 4

[CAM-Gerlach]

I've noticed this a while ago, but didn't want to hold up the v0.2.0 release with it.

There's a significant visual regression with docstring display using Sphinx 4 (repro'd in 4.0.3 and 4.4.0; renders correctly in 3.5.3). The headings don't display as they did before, and collapse mode no longer works.

I tested all the way back to the earliest working commits using the fabfile.py collapse test and the difference between Sphinx 3 and Sphinx 4 was still present, in otherwise identical environments. From skimming the DOM, it looks like either the script that does it isn't firing at all, or it isn't finding the correct elements to modify.

@martinRenou , any change you or one of your colleagues could help fix this?

Running the numpy.sin test works as before.

python -I -bb -X dev -m pytest --open-browser -k numpy_module --asyncio-mode=auto -W default

gives:

Sphinx 3, numpy module test:

Sphinx 4, numpy module test:

Running the numpy.sin test works as before

python -I -bb -X dev -W error -m pytest --open-browser -k sin --asyncio-mode=auto

gives:

Sphinx 3, numpy.sin test:

Sphinx 4, numpy.sin test:

Running the collapse test, however, shows very different results.

python -I -bb -X dev -W error -m pytest --open-browser -k collapse --asyncio-mode=auto

gives:

Sphinx 3, collapse test:

Sphinx 4, collapse test: