[web_paragraph] Fix multi-span BiDi span order and region direction - #194980

flutter/flutter@c091098...5e805bf

2026-10-30 [email protected] Fix/shared thread merger expired lease (flutter/flutter#194200)
2026-10-30 [email protected] [FGP] Report the real error when applying kotlin-android fails (flutter/flutter#195080)
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2026-10-30 [email protected] [web] Keep semantics editing state while the text input connection is open (flutter/flutter#195141)
2026-10-30 [email protected] [web_paragraph] Fix multi-span BiDi span order and region direction (flutter/flutter#194980)
2026-10-30 [email protected] [web] Unskip web tests that pass on CanvasKit and Skwasm: accessibility_inspector_test, color_filter_test (flutter/flutter#195541)
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…lutter#194980)

Within an RTL `BidiRun`, logical spans progress from right to left, so
`TextLayout.addLine` must iterate `paragraph.spans` in reverse logical
order to append visual blocks from left to right.

Additionally, `TextSpan._getMetrics` previously measured each span in
isolation using the paragraph's default direction, causing leading or
isolated neutral characters in a span following an RTL span to be
measured as LTR (`level = 0`) instead of RTL (`level = 1` per UAX flutter#9 N1)
and embedded LTR spans inside an RTL paragraph to be measured as RTL.
`WebParagraph._resolveSpans` now keeps the builder-supplied
`_styleSpans` private, splits text spans at paragraph-level
`bidiRegions` boundaries, and exposes only the BiDi-resolved
`WebParagraph.spans` with each sub-span's resolved region direction so
`extractTextClusters` and `Canvas2D` metrics match paragraph-level BiDi
runs.

### Reviewer & AI Audit Context
- **Core Logic Hunks**: `+75 / -9` production lines in
`engine/src/flutter/lib/web_ui/lib/src/engine/web_paragraph/paragraph.dart`
and
`engine/src/flutter/lib/web_ui/lib/src/engine/web_paragraph/layout.dart`
(`WebParagraph._resolveSpans()`, `TextLayout.extractBidiRuns()`,
`TextLayout.addLine()`) and `+182 / -0` test lines in
`engine/src/flutter/lib/web_ui/test/webparagraph/paragraph_bidi_test.dart`.
- **C++ `SkParagraph` Parity Anchor**: Matches
`OneLineShaper::iterateThroughShapingRegions`
(`engine/src/flutter/third_party/skia/modules/skparagraph/src/OneLineShaper.cpp:556-578,
697`), which intersects paragraph-level `fBidiRegions` with style
`Block`s so each shaped run has a single UAX flutter#9 level, and
`TextLine::iterateThroughSingleRunByStyles`
(`engine/src/flutter/third_party/skia/modules/skparagraph/src/TextLine.cpp:1034-1035`),
which traverses style blocks in reverse logical order
(`run->leftToRight() ? fBlockRange.start + index : fBlockRange.end -
index - 1`) within RTL runs.
- **Callers & Precondition Proofs**:
- Exposing only the BiDi-resolved `WebParagraph.spans` (while keeping
builder-supplied `_styleSpans` private) guarantees that `layout.dart`,
`wrapper.dart`, and `painter.dart` cannot accidentally read unsplit
spans.
- Within each UAX flutter#9 `BidiRegion` sub-span, `Canvas2D`
`getTextClusters()` emits clusters in monotonic logical offset order
(`engine/src/flutter/lib/web_ui/lib/src/engine/web_paragraph/paragraph.dart:783`,
Ledger `L007`), and `TextBlock` / `TextPainter` whole-block and
per-cluster rendering translate by `spanShiftFromLineStart =
shiftFromLineStart - shiftFromSpanStart`
(`engine/src/flutter/lib/web_ui/lib/src/engine/web_paragraph/layout.dart:381,
1306`,
`engine/src/flutter/lib/web_ui/lib/src/engine/web_paragraph/painter.dart:356-360`),
which handles RTL sub-spans measured in `[-spanWidth, 0]`.
- **Cross-Phase Consumer Check (`Placeholders` / `Backgrounds` /
`Queries`)**:
- `PlaceholderSpan`s are preserved without `Canvas2D` text measurement
in `_resolveSpans()` and positioned in visual left-to-right order
alongside `TextBlock`s in `addLine()`.
- `TextPainter` backgrounds/decorations
(`engine/src/flutter/lib/web_ui/lib/src/engine/web_paragraph/painter.dart:214,
346`), `getBoxesForRange`
(`engine/src/flutter/lib/web_ui/lib/src/engine/web_paragraph/layout.dart:587`),
`getPositionForOffset`
(`engine/src/flutter/lib/web_ui/lib/src/engine/web_paragraph/layout.dart:774`),
and `getGlyphInfoAt`
(`engine/src/flutter/lib/web_ui/lib/src/engine/web_paragraph/layout.dart:888`)
all consume `line.visualBlocks` and `spanShiftFromLineStart`, so visual
left-to-right block ordering and sub-span direction propagate across
painting, selection boxes, and hit-testing.

### Tests
-
`engine/src/flutter/lib/web_ui/test/webparagraph/paragraph_bidi_test.dart`:
- `Paragraph BiDi order and layout within multi-span RTL bidi run` —
verifies left-to-right visual box ordering (`"BiDi "` -> `"שלום"` -> `"
& "` -> `"مرحبا بالعالم"` -> `" · Emoji ..."`),
`GlyphInfo.writingDirection` and cluster bounds, and
`getPositionForOffset` hit-testing across a span that crosses an
RTL->LTR boundary.
- `Paragraph BiDi isolated neutral span between RTL spans in LTR
paragraph` — verifies that an isolated neutral span (`" + "`) between
two RTL spans resolves to RTL (`level = 1`) and is positioned visually
between `"שלום"` and `"مرحبا"`.
- `Paragraph BiDi multi-span order in RTL paragraph` — verifies that two
RTL spans in an RTL paragraph (`"שלום "` followed by bold `"עולם"`) are
ordered visually from right to left (`second.right == first.left`).
- `Embedded multi-span LTR run inside RTL paragraph` — verifies that
`_resolveSpans` overrides the RTL paragraph direction to
`TextDirection.ltr` for an embedded multi-span LTR run (`"Hello"`, `" +
"`, `"World"`).
- `Multi-span RTL run with inline placeholder` — verifies that an inline
`PlaceholderSpan` between two RTL text spans is preserved by
`_resolveSpans` and positioned visually between them (`second` ->
placeholder -> `first`).
- **Full WebParagraph corpus (`--full`, `preferWebParagraph` +
`forceTestFonts` on `40c36fffd2ec`)**:
- Part 1 (`engine/src/flutter/lib/web_ui`): `17/17`
`engine/src/flutter/lib/web_ui/test/webparagraph` suites (`+214 ~5 -1`,
all 8 BiDi tests passing) + `62/62`
`engine/src/flutter/lib/web_ui/test/ui` suites (`+196 ~0 -44`).
- Part 2 (`packages/flutter`, `896/896` suites across `8/8` shards):
`+16478 ~1231 -812` (`0` compile errors or unfinished shards).

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

Assisted-by: Gemini 4 Argon (Medium)