[mejo-]

CollaborativeEditor serialized the whole document to markdown on every editor update to emit update:content, even when no listener uses the result. On large documents this blocks every keystroke, especially in Chromium where prosemirror-markdown's serializer is quadratic in the output size.

Emit the markdown as a getter so it is only serialized when a listener reads it.

Benchmark on a 500 kB markdown document (headings, paragraphs, lists with marks), headless browsers against a local instance, median per step, before -> after:

Chromium:

• typing (cursor at start): 981 ms -> 67 ms
• typing (cursor at end): 933 ms -> 34 ms
• toggle bold on a word: 930 ms -> 50 ms
• editor update handlers: ~870 ms -> <1 ms

Firefox:

• typing (cursor at start): 125 ms -> 76 ms
• typing (cursor at end): 55 ms -> 47 ms
• toggle bold on a word: 67 ms -> 88 ms (within noise)
• editor update handlers: ~9 ms -> 1 ms

Large tables are not affected by this change; their cost lies elsewhere.

Related to #908

Assisted-by: ClaudeCode:claude-opus-5-5

I also opened https://code.haverbeke.berlin/prosemirror/prosemirror-markdown/pulls/137 in prosemirror-markdown upstream that significantly improves serialization performance, especially in Chrome-based browsers.

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI tools
• The AI-generated content was reviewed, comprehended and tested by a human

[mejo-]

/backport to stable35

[mejo-]

/backport to stable34

[backportbot]

The backport to stable34 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable34
git pull origin stable34

# Create the new backport branch
git checkout -b backport/9525/stable34

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 1ad52013

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/9525/stable34

Error: Failed to check for changes with origin/stable34: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[mejo-]

Manual backport to stable34: #9543