Copilot app rebuilds pull request view to render a 2,200-file diff smoothly

GitHub rebuilt the pull request view in the GitHub Copilot app to keep review fast on very large diffs, testing it on an open source pull request with 2,200 files, over a million changed lines, and more than 400 inline review comments.

The core difficulty is that review comment heights can only be measured at render time, which breaks the fixed-geometry virtualization used for code-only diffs.

GitHub split the document height into a deterministic code domain and a separately measured domain for comment blocks.