Rendering huge pull requests in the GitHub Copilot app

GitHub Copilot’s pull request view was rebuilt to handle exceptionally large reviews, including a tested open-source pull request with 2,200 files, more than one million changed lines, and over 400 inline comments. While code diffs can use conventional virtualization because line heights are predictable, review comments have dynamic heights affected by markdown, expandable content, reply composers, images, and resizing, requiring a separate layout system. The implementation divides deterministic code geometry from dynamically measured comment blocks, uses viewport-scoped and scroll-aware measurement passes, and applies identity-based scroll anchoring to prevent layout corrections from disorienting users. The system streams diff structure early, defers expensive work such as syntax highlighting and markdown rendering, and caches recently viewed diffs to improve return navigation. Engineers also added permanent instrumentation, automated end-to-end tests, and unattended profiling workflows to detect issues such as blank spaces, observer leaks, excessive rendering, and scroll jank under realistic load.

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