fix(v2): keep mobile TOC after hydration

[lex111]

Motivation

After we merged the new mobile UX, anchor links in headings broke. The reason for this is that during prerendering we render mobile TOC to avoid layout shift, but after hydration mobile TOC is removed from DOM on desktop. However, this is wrong because we hide mobile TOC on desktop with CSS. Therefore, we need to always render mobile TOC, then anchor links will work again and we still won't have FOUC.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Preview.

Test mobile TOC, then open any anchor link in new tab.

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

[netlify]

✔️ [V2]

🔨 Explore the source changes: 1737ce2

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/6100976fde6889000744bb72

😎 Browse the preview: https://deploy-preview-5229--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	97
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5229--docusaurus-2.netlify.app/

[github-actions]

Size Change: -6 B (0%)

Total Size: 859 kB

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	59.2 kB	0 B
website/build/assets/css/styles.********.css	92.8 kB	0 B
website/build/assets/js/main.********.js	463 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	62.4 kB	0 B
website/build/blog/index.html	26.4 kB	0 B
website/build/docs/index.html	40.3 kB	-3 B (0%)
website/build/docs/installation/index.html	47.9 kB	-3 B (0%)
website/build/index.html	27.3 kB	0 B
website/build/tests/docs/index.html	20.9 kB	0 B
website/build/tests/docs/standalone/index.html	18.8 kB	0 B

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[slorber]

Thanks, it seems to fix the issue so I'm going to merge this for now.

However, this is wrong because we hide mobile TOC on desktop with CSS. Therefore, we need to always render mobile TOC, then anchor links will work again and we still won't have FOUC.

However I don't agree with that. Rendering the mobile TOC shouldn't need necessary for anchor links to work.

It may be necessary on first load to prevent fouc but for subsequent navigations, not rendering it improves perf as React does not need to insert/update a mobile toc that won't even be displayed.

As it's just a micro-optimization, we can do without for now, and it's better to have good anchor link behavior than this optim.