fix(v2): evaluate code in live editor on client only

[lex111]

Motivation

Attempt to fix #2986 without much large changes.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Page with demo example - https://deploy-preview-4318--docusaurus-2.netlify.app/classic/examples/markdownPageExample#test

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.)

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Deploy preview for docusaurus-2 ready!

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⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	89
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4318--docusaurus-2.netlify.app/classic/

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Size Change: 0 B

Total Size: 532 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.1 kB	0 B
website/build/assets/js/main.********.js	359 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	60.3 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	25.4 kB	0 B

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[lex111]

@slorber this PR does not solve the problem that we currently performing twice rendering (in order to define the proper Prism theme on the client-side during the initial rendering).

For a more detailed I recommend you checkout these links:

• Description of bug with dark Prism theme (FOUC), for the solution of which it was necessary to create the isClient state in the root App whice causes re-renders: feat(v2): add support another Prism theme for dark mode #1984 (comment) (PR with implementation)
• feat(v2): injectHtmlTags API to inject head and/or body html tags #2057 (comment)
• fix(v2): render correct syntax theme for current website theme #2150

Well, perhaps there is a better solution for this case?

[slorber]

Perhaps there is a better solution, but this one seems to solve the problem without side effect for now so let's use that

[lex111]

Yes, my previous comment was a challenge for the future to make you aware of why the isClient state was added, which causes a side effect as double rendering.

[slorber]

thanks :) didn't know about these issues