[andiwand]

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Refs #764.

Summary

This is level 1 of docs/design/lazy-loading.md, for text files. With a text file of 1 M lines (71 MB) in headless Chrome:

	first frame	layout	renderer RSS
before	154 s	91 s	6.3 GB
after	0.85 s	0.1 s	0.9 GB

• Chunks. The view writes the lines in chunks of 256. A chunk has content-visibility: hidden until text.js marks it near the viewport with an IntersectionObserver (one screen of margin). So the browser lays out a few chunks at a time. content-visibility: auto alone takes 13.5 s here, because Chrome lays out every auto chunk once on the first frame. Without the script, the chunks fall back to auto (<noscript>), and beforeprint shows every chunk.
• Line numbers. A line number is now a CSS counter in the line itself (::before, with empty alternative text for screen readers). So the gutter column and its ResizeObserver are gone. That observer measured every line and doubled the load. A wrapped line keeps its number beside it. A chunk has style containment, so the counter does not count on past it, and the first line of each chunk sets where it starts. After an edit that adds or removes lines, the editor sets the starts and the digits of the gutter again, and it removes a chunk that has no line left.
• Search. odr.search shows the chunk of a hit before it scrolls to it, as it opens a folded <details>. The find of the browser does not reach a far chunk. The design doc lists hidden="until-found" as an open question.
• The names are generic (odr-chunk, odr-chunk-near), so pdf and sheets can use them later.

The reference output changes for the five txt pages and text.css, text.js and search.js. The output repo main is at b0a3b91, and the pin is advanced. The gutter is now as wide as the digits of the last line, so the text moves by about a pixel.

Test

• html.a_text_file_writes_its_lines_in_chunks: a 300-line file. The test checks the number of digits, where each chunk counts from, and the short last chunk. All 2,529 tests pass.
• test/browser/plaintext: 54 checks, 0 failed. The fixture now has two chunks, so the checks cover an Enter, a join and a delete across the chunk boundary.
• Screenshots at the top, at the chunk boundary (lines 251–267), and in the middle of the 1 M-line file: the numbers are right and the lines have no gaps.

Review fixes

• An edit that went from 9 to 10 lines put the number 10 into a gutter of one digit. renumber now sets --odr-text-digits again. Two new checks in test/browser/plaintext cover it.
• The comments that said "the containment of a chunk hides a counter" now give the reason in the words of the design: style containment keeps the counter inside the chunk.
• This change touches only the edit path of text.js, so the rendered pages do not change. CI compares only output/, so the reference output stays at b0a3b91.