Marginalia

About

Add review comments to GitHub's rich markdown diffs.
 
A browser extension for Chrome and Firefox that lets you comment on markdown files in GitHub PR rich diff views — something GitHub doesn't natively support.
 
 
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✦ FEATURES
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⟨/⟩ Jump to Source Diff
Hover over any paragraph, heading, or block in the rich diff to see a source button. Click it to switch the file to source diff mode, where GitHub's native inline commenting works.
 
💬 Inline Comment Overlay
Hover to see a comment button that opens a comment form right there in the rich diff. The extension maps the rendered block back to the source line number and posts a real PR review comment via the GitHub API.
 
Both approaches can be used together (default) or individually — configurable in the extension popup.
 
 
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⟐ HOW TO USE
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1. Open any GitHub PR's "Files changed" tab
2. Find a markdown file (.md, .markdown, .mdx) showing the rich diff
3. Hover over any content block (paragraph, heading, list, code block, etc.)
4. Two buttons appear on the left:
     ⟨/⟩  Switch to source diff for native commenting
     💬  Open an inline comment form
5. Write your comment and hit Comment (or Cmd/Ctrl + Enter)
 
 
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⚙ HOW LINE MAPPING WORKS
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The extension fetches the file's source content from the PR's head branch and uses a text-matching heuristic to find which source lines correspond to each rendered block. It then maps those to diff positions for the GitHub API. The mapping isn't perfect for heavily edited files, but works well for typical docs/README changes.
 
 
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⌨ KEYBOARD SHORTCUTS
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Cmd/Ctrl + Enter → Submit comment
Escape → Cancel / close comment form
 
 
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⚠ LIMITATIONS
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• Line mapping is heuristic-based and may occasionally target a nearby line instead of the exact one
• Very large diffs (100+ files) may hit API rate limits
• The extension only activates on PR "Files changed" pages
• Rich diff must be enabled for the markdown file (GitHub's default for .md)
• Firefox: inline commenting requires manually granting api.github.com host permission (see Installation)