Collapsible headings not working well

[cpita-work]

The collapsible headings feature described in:

https://jupyterlab.readthedocs.io/en/stable/user/toc.html

breaks when jupyterlab-myst is installed. For some headings it works, for many others it doesn't. I'm not sure about the pattern, but at least top-level headings don't show the collapse toggle next to them.