**display-index** is a low‑level library used by Atom’s text‑buffer to map between screen coordinates (rows/columns as shown after tab expansion, folds, soft‑wraps, etc.) and the underlying buffer coordinates. It stores the screen‑line layout in a balanced binary tree and provides fast iterators for lines and tokens, enabling the editor to translate positions efficiently for features like folding and soft‑wrapping. The package is intended for developers working on Atom or any text‑editing tool that needs high‑performance screen‑to‑buffer coordinate translation.