Bug: pasting plain-text Markdown only converts inline marks — block elements (headings, dividers) have no paste rules

[marcusbellamyshaw-cell]

Bug

Pasting plain-text Markdown into the Portable Text editor only partially converts. Inline marks (**bold**, _italic_) convert correctly but block-level syntax (# Heading, ## Heading, --- dividers) pastes as literal text regardless of editor focus or paste method.

Pasting from an HTML source (e.g. copying a rendered response from a browser chat UI) works correctly because TipTap picks up the HTML from the clipboard and parses <h1>, <h2>, <hr> etc. as structured blocks. The problem is specifically with plain-text Markdown in the clipboard.

Root cause

There is no Markdown clipboard text parser in the editor. TipTap has two separate rule systems:

• Paste rules (markPasteRule) — fire on paste events. StarterKit's Bold, Italic, and Code marks have these, which is why inline formatting converts correctly.
• Input rules (textblockTypeInputRule) — fire on typed input only, never on paste. Heading and HorizontalRule only have input rules, so they never convert pasted content regardless of focus state.

Fix

Add a clipboardTextParser extension to PortableTextEditor.tsx that detects plain-text Markdown and routes it through the existing markdownToPortableText → portableTextToProsemirror pipeline already in the codebase (packages/core/src/client/portable-text.ts). This bypasses the input rule / paste rule distinction entirely and makes plain-text Markdown paste work consistently.

Impact

Any editor who writes content in Markdown and pastes from a plain-text source (document editor, AI tool text output, text file) hits this. Copying from a rendered HTML source such as a browser-based chat UI works as a workaround because the clipboard carries HTML, but this is non-obvious and easy to miss.