GitHub rebuilt the Copilot app diff view to open a million-line PR with hundreds of inline comments

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GitHub's engineering post splits rendering into deterministic code geometry held in typed arrays and lazily measured comment-block geometry. That stops a resized comment from forcing a recalculation of the whole coordinate system. Per-block ResizeObservers caused feedback loops, so the team replaced them with a single idle- and scroll-gated measurement pass anchored to the user's focus point. The diff structure streams from the backend before content, and recently viewed diffs stay cached. Anyone virtualizing long lists with variable-height rows can reuse the pattern.