Right-hand navigation runs over page

[roaldarbol]

Describe the bug

context
If you have a very long list of headings and sub-headings (such as in some papers), it runs over the page, and you can't get to the bottom one even if you scroll to the end of document.

expectation
I expect the entire navigation to be visible (with scrolling)

bug
But I can't reach the bottom.

Reproduce the bug

1. Have document with many headings/subheadings
2. Run myst

List your environment

myst v1.1.47
Brave browser, Version 1.63.174 Chromium: 122.0.6261.128 (Official Build) (x86_64)
MacOS, Sonoma 14.4

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