GitHub rebuilt the Copilot app diff view to open a million-line PR.

Rendering splits into deterministic code geometry in typed arrays and lazily measured comment-block geometry, so a resized comment doesn't force recalculating the whole coordinate system. Per-block ResizeObservers created feedback loops, replaced with a single idle- and scroll-gated measurement pass anchored to the user's focus point (GitHub). Diff structure streams from the backend before content. Reusable for anyone virtualizing long lists with variable-height rows, which is most people who've tried and given up.