[WebParagraph] Fixing edge cases for wrapping text (with newlines)

[Rusino]

Summary

Fixes newline (\n) handling and caret/selection geometry in WebParagraph to match SkParagraph behavior, resolving misplaced cursor issues in multiline text fields on the web.

Fixes #188320

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Background

In Flutter text editing (RenderEditable), caret positioning, line navigation, and selection rely directly on Paragraph query APIs (getPositionForOffset, getBoxesForRange, getLineBoundary, getLineNumberAt, and getGlyphInfoAt).

Previously, WebParagraph diverged from SkParagraph in several key newline edge cases:

1. Physical vs. Logical Width: Browser Canvas/DOM text measurement treats \n as whitespace with positive advance width. In SkParagraph, newlines occupy zero physical width while retaining their logical text ranges.
2. Trailing Newlines: Text ending in a newline (e.g. "Hello\n") must generate an additional empty line where the cursor resides after pressing Enter.
3. Query Parity: Hit-testing and boundary queries on lines ending with or consisting entirely of newlines returned incorrect offsets or collapsed bounds.

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Changes

• Explicit Hard Line Break Tracking (wrapper.dart):

  • Separated hard line breaks (\n) from collapsible whitespace in _LineBuilder.
  • Added support for generating the trailing empty line when text terminates with a hard line break.

• Logical vs. Physical Text Ranges (layout.dart, paragraph.dart):

  • Introduced physicalTextRange on TextBlock to exclude \n from physical geometry calculations (bounds, selection rects, line widths) while preserving textRange for logical text operations.
  • Adjusted getGlyphInfoAt to report zero layout width for \n and synthesize end-of-line glyph bounds for newlines outside visual blocks.
  • Aligned getPositionForOffset and getLineBoundary with SkParagraph hit-testing rules, correctly handling empty trailing lines and cluster boundary affinity.
  • Implemented SkParagraph-matching binary search in getLineNumberAt.

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Testing

• Added test/webparagraph/paragraph_edge_cases_test.dart (newline wrapping and empty line creation).
• Added test/webparagraph/paragraph_selection_caret_test.dart (caret hit-testing, affinities, and selection bounding boxes across multiline text).
• Added test/webparagraph/paragraph_line_metrics_test.dart (line geometry and width validation).
• Updated existing webparagraph unit tests to ensure 1:1 parity with SkParagraph.

[gemini-code-assist]

Code Review

This pull request updates the web paragraph layout and wrapping logic to explicitly track and handle hard line breaks and empty lines, and optimizes line boundary queries using binary search. The review feedback identifies a missing return statement in TextLayout that results in dead code, a potential StateError when accessing visual blocks on empty lines, leftover debugging code that introduces performance overhead, and redundant control flow statements in TextWrapper.

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Changes reported for pull request #189858 at sha 645ae6a

[Rusino]

I updated tests to use generic Paragraph API so I can always test CanvasKit on them (as a source of truth).

[mdebbar]