How GitHub Rebuilt the Copilot App to Render a Million-Line Pull Request
mariorod1 · x · 2026-09-25
GitHub's engineering blog details how the team rebuilt the diff surface in the Copilot app to smoothly open an open-source PR with 2,200 changed files, over a million changed lines, and 400+ inline comments.
The problem
- Large diffs are a solved rendering problem: virtualize rows, keep mounted DOM small, exploit fixed line heights.
- Comments are the hard part — their height depends on markdown wrapping, expandable sections, reply boxes, and image loading, all known only at render time.
Three challenges
- Measurement: unknown comment heights break the fixed-row virtualization design, requiring async measurement and layout compensation.
- Data pipeline: a fast diff surface is worthless if the data feeding it stalls.
- (Full 14-minute article continues beyond the excerpt.)
Directly useful for engineers building virtual lists, review tooling, or rendering very large documents.
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