Native `draw_text` painted every run through `Canvas::draw_str`, which maps
to Skia's `drawSimpleText` — a plain cmap lookup with no bidi reordering and
no contextual glyph selection. Arabic therefore rendered in storage order
with isolated letterforms. CanvasKit's `drawText` has the same limitation on
web, and `drawScriptRun` there segmented a run by script before drawing the
segments left to right, so bidi was resolved inside each segment and then
undone across them.
The paragraph shaper was bypassed natively on purpose: jian's
`draw_text_paragraph` builds a fresh `FontCollection` on every call, which
is what produced the ~605ms chrome frames noted in crates/CLAUDE.md. That
cost is avoidable rather than inherent — `ParagraphBaseline` already keeps a
generation-guarded collection — so this caches the collection and routes
only the runs that genuinely need a shaper. Latin and CJK stay on the cmap
fast path, leaving the frame budget and their rendered output untouched.
Measurement moves with paint. `measure_text_*` summed isolated-glyph
advances, so wrap decisions and caret geometry were computed against widths
the painter never used and text overflowed its boxes. The routing predicate
lives in op-editor-core so the two hosts cannot disagree about which runs
are shaped, and so paint and measure within a host cannot drift apart.
Hebrew is deliberately out of scope and stays on the fast path.
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
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