[Prototype] Convert plain text/thin-wrapper paste to formatted HTML via markdown

[JiuqingSong]

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Summary

Adds a feature to PastePlugin that detects plain-text / markdown-style paste content and converts it to formatted HTML by default.

When content is pasted with the default source (not Word/Excel/PowerPoint/OneNote/etc.) and as a normal paste, the plugin now:

1. Detects whether the copied content is effectively plain text:
   • There must be non-empty plain text (clipboardData.text).
   • Either there is no HTML (rawHtml null/undefined/empty), or the HTML is a thin wrapper of the plain text — every element is a structural tag (DIV, P, BR, SPAN), no element carries attributes (no style, class, etc.), and the HTML text matches the plain text. Any real formatting element (<b>, <a>, <table>, styled spans…) disqualifies it.
2. Converts the plain text through convertMarkdownToContentModel, turns the resulting Content Model into a DOM tree via contentModelToDom, and replaces the contents of BeforePasteEvent.fragment in place — so the rest of the paste pipeline sees the formatted HTML instead of raw markdown.

This makes pasting markdown text produce formatted HTML (headings, lists, links, etc.) by default instead of literal markdown.

Scope / safety

• Gated to pasteType === normal, so "paste as plain text" (asPlainText) keeps raw text, and mergeFormat/asImage are untouched.
• Detection is conservative: it only triggers when the HTML genuinely adds no formatting.

Files changed

• lib/paste/Markdown/convertPastedTextToMarkdown.ts (new) — shouldConvertPastedTextToMarkdown (detection) and convertPastedTextToMarkdown (conversion + fragment replacement).
• lib/paste/PastePlugin.ts — added a default case to the paste-source switch that runs detection/conversion.
• package.json — added roosterjs-content-model-markdown dependency.
• Tests — new test/paste/Markdown/convertPastedTextToMarkdownTest.ts plus integration tests in ContentModelPastePluginTest.ts.

Verification

• yarn test:fast (filtered) — 29 passing
• yarn eslint — clean
• yarn b — build succeeds (confirms the new cross-package dependency resolves)

🤖 Generated with Claude Code
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[github-actions]

PR Preview Action v1.8.1
Preview removed because the pull request was closed.
2026-06-19 17:17 UTC