[examples/paste-html] Pasting rendered HTML can crash or insert with the wrong structure locally

[wudidewo12]

Description
A clear and concise description of what the bug is.

Recording
Screenshots attached below:
Local actual behavior:

Live reference expected behavior:

Video attached below:

CleanShot.2026-05-08.at.01.15.29.mp4

Sandbox
A link to a sandbox where the error can be reproduced. (You can start from the base sandbox here: https://codesandbox.io/s/slate-reproductions-c7gyg or refer to the Slate website too.)

Steps
To reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expectation
A clear and concise description of what you expected to happen. (Often it's helpful to test out the behavior of other editors like Google Docs, Medium, Notion, etc. to see how they handle the same issue.)

Environment

• Slate Version: [e.g. 0.59]
• Operating System: [e.g. iOS]
• Browser: [e.g. Chrome, Safari]
• TypeScript Version: [e.g. 3.9.7 - required only if it's a TypeScript issue]

Context
Add any other context about the problem here. (The fastest way to have an issue fixed is to create a pull request with working, tested code and we'll help merge it. Slate is solving a pretty complex problem, and we can't do it without active contributors, so thank you so much for your help!)

[zbeyens]

🟢 95-100% confidence

Phase	🧪 Tests	🌐 Browser
Reproduced	✅ Attached video shows rendered content cut/paste causing broken structure and a runtime error	✅ /examples/paste-html flow identified from the video
Verified	🟢 Video-derived paste-html row is green on current v2 07d79634	✅ No runtime error; block structure is preserved

🌐 Browser Check

• Open http://localhost:3100/examples/paste-html.
• Copy rendered Slate content from the editor.
• Paste it back into the editor.
• Confirm the internal Slate fragment path preserves block structure before generic HTML import.

✅ Outcome

• Current v2 keeps copied rendered Slate content as an internal fragment.
• The paste-html route preserves block structure and does not hit the reported runtime error in the verified flow.

🏗️ Design

• Chosen owner: clipboard serialization and paste-html import ordering.
• Why not quick patch: the current video-derived browser proof is already green.
• Why not broader change: the issue body only provides the attached video as repro input, and that flow is covered.

🧪 Verified

• cd /Users/zbeyens/git/slate-v2 && PLAYWRIGHT_RETRIES=0 PLAYWRIGHT_BASE_URL=http://localhost:3100 bun playwright playwright/integration/examples/plaintext.test.ts playwright/integration/examples/mentions.test.ts playwright/integration/examples/paste-html.test.ts playwright/integration/examples/iframe.test.ts --project=chromium --grep "keyboard undo restores caret after middle-line typing|keyboard undo restores partial selected text replacement|keyboard undo restores select-all replacement content|copies and pastes a selected mention without crashing|cuts a selected mention without crashing|pastes copied rendered Slate content as an internal fragment before HTML import|applies parent toolbar formatting to selected iframe text"
• cd /Users/zbeyens/git/slate-v2 && bun check