fix(theme-classic): fix tab focus bug in dropdown (#8697)

[kagankan]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Fixed #8697 bug.

It only worked if the dropdown was immediately followed by an a element, so I changed the implementation to use the focusin event so that it would also work with the button element.

Test Plan

Before

Pressing the Tab key on the last item will cause it to lose focus.

Docusaurus-before-cut.mp4

After

Pressing the Tab key on the last item shifts focus to the theme switcher button.

Docusaurus-after-cut.mp4

Test links

Deploy preview: https://deploy-preview-8699--docusaurus-2.netlify.app/tests/pages/navbar-dropdown-tests

Related issues/PRs

Fixed #8697

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 86	🟢 97	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 80	🟢 100	🟢 100	🟢 100	🟢 90	Report

[slorber]

Thanks 👍

Looks like a great fix, happy to get rid of this onKeyDown logic

We need your FB CLA signature to be able to merge it (the bot might take a few hours once its signed)

[kagankan]

@slorber

Thanks for your reply!

I just signed the CLA.

[slorber]

Thanks, will wait for the bot and merge :)

There's a little Prettier error to fix as well:

Run yarn format:diff
  yarn format:diff
  shell: /usr/bin/bash -e {0}
yarn run v1.[2](https://github.com/facebook/docusaurus/actions/runs/4248986676/jobs/7393690158#step:7:2)2.19
$ prettier --list-different .
website/_dogfooding/_pages tests/navbar-dropdown-tests.mdx

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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[kagankan]

@slorber
Sorry I forget to format!
Thank you for fixing and merging.
I'm glad to contribute to this project!🙌

[slorber]

np, thanks for the fix ;)