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Feature: Custom gutter columns for per-line metadata (blame, annotations)

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
35/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Quiet
Tech stack
typescript
Domain
frontend

Research direction

Start by reading the contributing guidelines, then inspect the File component and how annotations and renderHoverUtility currently work. The proposal leaves the API choice open between a callback and multiple gutters, so first clarify that design before implementation. Done means persistent per-line metadata can appear alongside code without requiring a separately synchronized column.

Written by the indexing model from the issue text.

Description

diffs enhancement
Prerequisites
Proposal

Add support for custom gutter columns that display per-line metadata alongside code content.

This could be exposed as a renderGutter callback or a gutters prop:

// Option A: Simple callback
<File
  file={file}
  renderGutter={(lineNumber: number) => (
    <span className="blame">{blameData[lineNumber].author}</span>
  )}
/>

// Option B: Multiple gutters
<File
  file={file}
  gutters={[
    { id: 'blame', width: 180, render: (line) => <BlameInfo line={line} /> }
  ]}
/>

This would enable displaying git blame information (author, commit hash, date), inline linting/CI
annotations, and custom line markers.

Motivation and context

When building code review or repository browsing tools, users often need to see git blame data alongside each line. Currently there's no way to render persistent per-line content in a gutter column:

  • Annotations render as blocks below lines, not inline
  • renderHoverUtility only shows on hover, not persistently
  • unsafeCSS can style lines but cannot inject text content

The workaround is building a parallel column outside the component with synchronized scrolling, which is complex due to Shadow DOM encapsulation.

This is a common feature in code editors (VS Code, GitHub, GitLab) and would expand the library's use cases for code analysis and review tools.

Dominant language
TypeScript
Stars
6.2k
Forks
226
Avg merge
1d 12h
Merged PRs (30d)
34

Getting set up

This project ships no dev container, Dockerfile or contributing guide, so setting up is up to you: start from its README, and see our first-contribution guide for the general steps.

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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