The 2.0 Editor can support hundreds of files per Pen, not to mention folders mixed in which don't count toward that total. This can be pretty weighty on the DOM and thus adversely affect performance. Especially as each file/folder has children, a variety of event listeners, are drag and drop enabled, etc. "Virtualizing" data like…

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CodePen's 2.0 Editor supports hundreds of files per Pen, which can create heavy DOM load and hurt performance. Instead of full virtualization, the team uses IntersectionObserver to detect visibility of file/folder items, freezing off-screen elements and fading them in and out — a lightweight but effective optimization for large file lists with event listeners and drag-and-drop support.

424: File List Optimization