Phase 4 Markdown Execution

Goal

Complete the phase-4 markdown slice only. Add high-ROI non-React logic coverage for parser and serializer helpers plus incomplete-MDX fallback behavior.

Checklist

• [completed] Audit current markdown seams and existing tests
• [completed] Add focused helper specs for markdown lib utilities
• [completed] Add focused markdownToSlateNodesSafely behavior specs
• [completed] Fix any real runtime bug exposed by the new specs
• [completed] Run markdown package verification
• [completed] Record learnings and final results

Findings

• markdown already has broad deserializer/serializer coverage, but several helper seams are still only covered indirectly.
• High-ROI direct seams are:
  • stripMarkdown*
  • parseAttributes / propsToAttributes
  • tagRemarkPlugin / getRemarkPluginsWithoutMdx
  • getCustomMark
  • markdownToSlateNodesSafely
• markdownToSlateNodesSafely currently drops already-parsed content when the last complete block is void and the input ends with incomplete MDX. Example probe: "<hr /><u>" returns only a paragraph with "<u>", losing the horizontal rule.

Progress

• Re-read the phase-4 plan, task.mdc, testing.mdc, planning-with-files, tdd, and changeset guidance.
• Audited markdown package structure, existing specs, and helper/test harnesses.
• Probed live markdownToSlateNodesSafely output with a temporary Bun script to identify public behavior and confirm a likely fallback bug.
• Added focused markdown helper specs for:
  • stripMarkdown*
  • parseAttributes / propsToAttributes
  • tagRemarkPlugin / getRemarkPluginsWithoutMdx
  • getCustomMark
• Added a focused markdownToSlateNodesSafely suite covering:
  • normal markdown passthrough
  • incomplete MDX appended to a non-void block
  • no-complete-block fallback
  • void-block preservation
• Fixed markdownToSlateNodesSafely so incomplete-MDX fallback appends a new paragraph after a complete void block instead of dropping the already-parsed node.
• Added a patch changeset for @platejs/markdown.

Verification

• bun test packages/markdown/src/lib/deserializer/utils/stripMarkdown.spec.ts packages/markdown/src/lib/rules/utils/parseAttributes.spec.ts packages/markdown/src/lib/utils/getRemarkPluginsWithoutMdx.spec.ts packages/markdown/src/lib/serializer/utils/getCustomMark.spec.ts packages/markdown/src/lib/deserializer/utils/markdownToSlateNodesSafely.spec.tsx
• bun test packages/markdown/src
• bun run test:slowest -- --top 15 packages/markdown/src
• pnpm install
• pnpm turbo build --filter=./packages/markdown
  • failed before package code execution in rolldown / Node 20.12.1 with ERR_INVALID_ARG_VALUE from node:util.styleText
• NO_COLOR=1 pnpm turbo build --filter=./packages/markdown
  • failed with the same external tooling error
• pnpm lint:fix
• bun test packages/markdown/src (post-lint rerun)

Learnings

• package reality: markdownToSlateNodesSafely had a real content-loss bug on the incomplete-MDX fallback path when the fully parsed prefix ended in a void node.
• package reality: the safe fallback should preserve complete parsed output and append the fallback paragraph, not replace the whole result set.
• verification blocker: package build is currently blocked outside slice scope by a rolldown + Node 20.12.1 styleText call that passes ['underline', 'gray'] where Node expects a single format string.