Enable add a cell button

[tuncbkose]

Closes #6852
Fixes #6389

[github-actions]

👈 Launch a Binder on branch tuncbkose/notebook/add-cell

[jtpio]

Thanks!

Not sure why the UI test fails on Firefox only but I can have a look.

[tuncbkose]

The "click to add a cell" button seems to be visible by default in the videos of failing tests. I am not immediately sure why this would be the case.

[jtpio]

That is indeed odd. It works fine in a normal Firefox.

[jtpio]

Rebasing onto the latest main to grab the changes from #6874 and see if they might help fix the UI tests issue.

[jtpio]

see if they might help fix the UI tests issue.

Looks like it doesn't. I'll be investigating this as it is reproducible when running Playwright locally.

[jtpio]

Strangely running the Playwright tests locally with the --headed flag seems to be working fine:

[jtpio]

This could be an issue between the headless and headful modes in Firefox. Linking to microsoft/playwright#4295 as a potential starting point for investigation.

[jtpio]

Adding the workaround for Firefox helps make the tests more stable.

I opened #6876 to investigate more separately and to avoid blocking this PR.

[jtpio]

Thanks!