The problem it solves is efficiently calculating the height of some wrapped text on a web page, without actually rendering that text to the page first (very expensive).

It does that by pre-calculating the width/height of individual segments - think words - and caching those. Then it implements the full algorithm for how browsers construct text strings by line-wrapping those segments using custom code.

This is absurdly hard because of the many different types of wrapping and characters (hyphenation, emoji, Chinese, etc) that need to be taken into account - plus the fact that different browsers (in particular Safari) have slight differences in their rendering algorithms.

It tests the resulting library against real browsers using a wide variety of long text documents, see https://github.com/chenglou/pretext/tree/main/corpora and https://github.com/chenglou/pretext/blob/main/pages/accuracy...

This is incredibly impressive, many of this things have been missing for forever! I remember the first time I couldn't figure out how do a proper responsive accordion, it was with bootstrap 1, released in 2011 !! Today it's still not properly solved (until now?).

Many of thing things belong in css no in js, but this has been the pattern with so many things in the web

1) web needs evolve into more complex needs 2) hacky js/css implementation and workarounds 3) gets implemented as css standard

This is a not so hacky step 2. Really impressive,

I would have thunk that if this was actually possible someone would have done it already, apparently not, at some point I really want to understand what's the real insight in the library, their https://github.com/chenglou/pretext/blob/main/RESEARCH.md is interesting, they seem to have just done the hard work, of browser discrepancies to the last detail of what does an emoji measure in each browser, hope this is not a maintenance nightmare.

All in all this will push the web forward no doubt.

> This was achieved through showing Claude Code and Codex the browsers ground truth, and have them measure & iterate against those at every significant container width, running over weeks

https://x.com/_chenglou/status/2037715226838343871?s=20

There was another comment about using Autoresearch probably for this but I might be misremembering

This is something I've been thinking for ages and would love to add to Ensō (enso.sonnet.io), purely because it would allow me to apply better caret transitions between the lines of text.

(I'm not gonna do that because I'm trying to keep it simple, but it's a strong temptation)

Now a CSS tangent: regarding the accordion example from the site (https://chenglou.me/pretext/accordion), this can be solved with pure CSS (and then perhaps a JS fallback) using the `interpolate-size` property.

https://www.joshwcomeau.com/snippets/html/interpolate-size/

Regarding the text bubbles problem (https://chenglou.me/pretext/bubbles), you can use `text-wrap: balance | pretty` to achieve the same result.

(`balance` IIRC evens out the # of lines)

Pretext makes this easier. Just pass the text and text properties (font, color, size, etc) into a pure JS API and it layouts the content into given viewport dimension.

Earlier you'll have to either use measureText or ship harbuzz to browser somehow. I guess pretext is not a technical breakthrough, just the right things assembled to make layouting as a pure JS API.

I have one question though: how is this different from Skia-wasm / Canvaskit? Skia already has sophisticated API to layout multiline text and it also is a pure algorithmic API.

Edit: example: https://files.catbox.moe/4w3um0.png

It works, and still runs quite successfully in production, but there are still off-by-one hacks where I have no idea why they work. The iterative line generation feature here is huge.

That's why I've added Graphics.Text (https://docs.sciter.com/docs/Graphics/Text) in Sciter.

Graphics.Text is basically a detached <p> element that can be rendered on canvas with all CSS bells and whistles.

Maybe for this we need a new web "Search" API instead of JS. Not sure it can be done otherwise without browser's help.

https://drafts.css-houdini.org/font-metrics-api-1/

RIP eae@.

Problem: DOM-based text measurement (getBoundingClientRect, offsetHeight) forces synchronous layout reflow. When components independently measure text, each measurement triggers a reflow of the entire document. This creates read/write interleaving that can cost 30ms+ per frame for 500 text blocks.

Solution: two-phase measurement centered around canvas measureText.

prepare(text, font) — segments text via Intl.Segmenter, measures each word via canvas, caches widths, and does one cached DOM calibration read per font when emoji correction is needed. Call once when text first appears.

layout(prepared, maxWidth, lineHeight) — walks cached word widths with pure arithmetic to count lines and compute height. Call on every resize. ~0.0002ms per text.

https://github.com/chenglou/pretext/blob/main/src/layout.ts

wasted generation.