PR 12590: local wrapping comparison

Measured wrapping is the best next experiment. Large-diff setup still needs work.

All five alternatives were prototyped locally, plus the original PR baseline. The measured version kept the existing renderer, used cheap column arithmetic for printable ASCII, and measured other text in one pass with boundary validation. Nothing was pushed.

Option	1,000 rows	10,000 rows	Cached draw	Observed result
Current PR	12.7 ms	122 ms	2.53 ms	Clips CJK
Measured + ASCII shortcut	39.0 ms	399 ms	2.49 ms	81/81 fitting and cluster checks passed
Unicode width estimate	73.6 ms	739 ms	2.39 ms	Clips emoji and combining text
Native Android layout	85.3 ms	835 ms	2.40 ms	Text fit checked; tab highlights need integration
ASCII wrap + scroll fallback	9.4 ms	87 ms	6.13 ms	Works as a compromise; Unicode stays unwrapped
Wrapping off	1.2 ms	12 ms	8.63 ms	Fast setup; loses wrapping

Layout medians on Android 15 x86_64 emulator: 80% ASCII, 10% CJK, 10% emoji; 36px monospace, 840px width. Three warm-ups, 15 shuffled samples for 1,000 rows, five for 10,000. Cached draw is software Canvas CPU time for a viewport over a separate Unicode-heavy dataset, not GPU frame completion or phone performance.

Your performance concern was justified. The initial measured implementation took 97 ms/1,000 rows. The ASCII shortcut reduced that to 39 ms, compared with 13 ms for the PR. On the separate Unicode-heavy 10,000-row view, setup still took 1.72 seconds versus 0.24 seconds for the PR.

Scrolling was not the main regression. After layout, measured wrapping took 2.49 ms per cached draw versus 2.53 ms for the baseline in the final run. No sampled Choreographer interval exceeded 25 ms; this does not measure GPU completion or the startup stall.

A tempting shortcut failed. Repeated native breakText calls took about 762 ms for one 20,000-unit CJK line and overflowed six Arabic cases. Measuring advances once reduced that long-line case to about 13 ms and passed the tested fitting cases.

Character-width estimates were a poor trade. They were slower than measured wrapping with the ASCII shortcut and still overflowed emoji/combining fixtures. Native StaticLayout fit the tested visible text, but needs its own highlight coordinates and syntax integration.

The fallback is possible, but changes the feature. Non-ASCII lines pan while wrapped ASCII stays fixed. Pixel comparisons confirmed that behavior. Wrapping disabled is the cheapest setup, but draws more long text per viewport.

Recommendation: continue with measured wrapping and the ASCII shortcut, then address large initial layouts and reuse unchanged row layouts. Keep horizontal scrolling disabled in full-wrap mode. Do not merge these prototypes as-is.

Scope: the actual PR Android canvas code was copied into a standalone local test app at f95e22ae670edd48bce3e840e47f44a412097f80. Nine fixtures × three fonts × three widths gave 81 cases per option; 36 native selection/pan checks passed. Native layout had two advance-width warnings; wider-bitmap checks found zero ink outside the viewport. This is not a full T3 integration pass. iOS, physical devices, syntax-token/font-style interactions, complete bidirectional text correctness, allocations, and the real comment editor remain unverified. The disabled helper's ~1 ms cost is experiment overhead. Source, APK, and raw evidence remain in /tmp/t3-wrap-lab.

Baseline canvas with clipped CJK text — Original PR: CJK runs extend beyond the right edge.

Measured wrapping with complete CJK lines — Measured prototype: the same CJK text fits, with line 2 preserved.

Other prototypes and raw measurements

Fallback after panning: ASCII stays fixed.

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