He just crawled through hell to fix the browser…

Cheng Lou, ex-React Core and Midjourney engineer, releases Pretext — a pure-TypeScript library that measures text without triggering browser layout reflows.

• Pretext bypasses browser layout reflow entirely, eliminating the most expensive operation in text-heavy UI rendering.
• Canvas API provides pixel-accurate string width measurements outside the DOM, with zero reflow cost.
• Line-height algorithm was trained iteratively: LLMs wrote line-break logic, tested against real browsers across languages, and iterated for weeks.
• Cheng Lou previously built React Motion, one of the most widely used animation libraries in the React ecosystem.
• Virtual lists of 10,000+ items now feasible without render-then-measure hacks or guessing heights.
• Pretext API is two calls: prepare (segments + caches widths) and layout (returns total height and line count).
• Demo app renders video frames as ASCII art by mapping per-character brightness — made possible only because Pretext knows exact character positions without touching the DOM.

2026-04-02 · Watch on YouTube