[6.4.x] CI failures on 6.4.x branch

After reviewing the open PRs on the `6.4.x` branch, the selenium and javascript tests all fail. This is probably coming from the CI, and not from the respective PRs.

I am not sure what the status was before the creation of that 6.4.x branch. @Zsailer or @jtpio any thoughts on this?

So far we have the following recurring failures:

- [x] check_release (link_check) https://github.com/jupyter/notebook/pull/6320
- [ ] Linux JS Tests / build (macos and ubuntu, notebook): https://github.com/jupyter/notebook/pull/6354
- [ ] build windows 3.x `notebook/terminal/tests/test_terminals_api.py`

Start by reviewing the 6.4.x branch CI results and the recurring failures listed in the issue. Check PRs 6320 and 6354, then inspect notebook/terminal/tests/test_terminals_api.py and the Linux JS build failures across macOS and Ubuntu. Done means the listed CI failures are understood and the affected checks pass or have a documented cause.

Written by the indexing model from the issue text.