The uphill climb of making diff lines performant

This post details the engineering effort to optimize the 'Files changed' tab performance in pull requests, particularly for large ones. Key contributions include: reducing the number of React components per diff line from 8 to 2, simplifying the component tree, moving complex state (comments, context menus) to conditionally rendered child components, adopting O(1) data access using JavaScript Maps for state lookups, restricting useEffect usage to top-level components, and implementing window virtualization for rendering only visible portions of massive pull requests. These changes resulted in significant reductions in total lines of code, unique component types, components rendered, DOM nodes, memory usage, and INP scores.