perf(text): size the line numbers in one layout, not one per line

[andiwand]

A megabyte of plain text took 38 seconds to appear in a WebView, of which 37.6 seconds was spent inside one function.

The bug

text.js's updateLineNumberHeight interleaves a write and a read:

for (var i = 0; i < textCells.length && i < nrCells.length; ++i) {
  nrCells[i].style.height =
    textCells[i].getBoundingClientRect().height + "px";
}

Setting style.height invalidates layout; the getBoundingClientRect() on the next iteration forces it again. With one gutter cell per line that is one full layout of the entire document per line.

Every height is read first, then written. Same output, one layout.

Numbers

Measured in a WebView on a Pixel 9 Pro emulator, timing wall clock to a stable DOM and accumulating the time spent inside this function. 1 MB of plain text, 7,823 lines:

	ready	in updateLineNumberHeight
as served	38.2s	37.6s
batched	0.26s	6ms
gutter script removed entirely	0.25s	—

Batching is indistinguishable from not doing the work at all. Repeated back to back to rule out warm-up: 29.1s / 0.19s / 30.2s.

Cost is linear in line count from here, and what remains is the browser, not this function:

text	html	elements	fetch	parse + layout	ready
1 MB	1.2 MB	15,661	33ms	264ms	0.46s
10 MB	12 MB	155,229	71ms	1.5s	3.0s
25 MB	30 MB	386,343	192ms	6.7s	6.7s

Not in this PR

Two elements per line, half of them the gutter, costs roughly 4.6–5.4 KB of renderer memory each. At 50 MB and 100 MB the renderer is killed by the low-memory killer (2.7 GB RSS) and takes the app process with it — the engine itself is fine there, writing 121 MB of html in 2.5s. Replacing the parallel gutter column with a CSS counter would halve both the element count and the memory and remove this script; virtualising the line list is what would actually lift the ceiling. Neither is here.

Test

No test: this is a rendering performance fix with identical output, and the reference-output suite compares rendered pages. Verified by the measurements above.

🤖 Generated with Claude Code