LibWeb: Advance the SVG current text position across text runs

[sideshowbarker]

Problem: All <tspan> elements without explicit positions rendered on top of each other, at the position where their parent <text> element starts.

Cause: SVG text layout never advanced the current text position by the advance of each rendered text run. The position handed to a text box’s children was the position before any of the box’s own glyphs got placed. And between sibling boxes, the position was clobbered with the bottom-right corner of the previous box’s ink bounding box. That differs from the glyph advance, and is wrong in y — since the pen must stay on the baseline. In addition, text-anchor was applied separately to every box’s own text run. So, anchoring a <text> whose content is split across <tspan>s shifted each run individually, instead of the whole text chunk.

Fix: Track the current text position as the spec defines it: each text run advances the position by its own advance; a <tspan> without x/y continues at the current position; and a <text> element always starts at its own absolute position — since x and y default to 0 for <text>. Apply text-anchor per text chunk: When a box starts a chunk (every <text>, and any <tspan> with an absolute position), measure the total advance of all runs in the chunk, and shift the chunk’s starting position once — instead of shifting every run separately. Fixes #10625.

[coderabbitai]

📝 Walkthrough

Walkthrough

SVG text layout now measures text chunks across descendants, applies chunk-level anchoring and positioning, and propagates the resulting position through Rust layout. New and updated tests validate tspan placement, rendering, and bounding rectangles.

Changes

SVG text positioning

Layer / File(s)	Summary
Chunk measurement and anchoring Libraries/LibWeb/Layout/LayoutRustBridge.cpp	Measures descendant text advances, detects chunks, applies positioning attributes and text-anchor, and generates paths from adjusted positions.
Text position propagation Libraries/LibWeb/Layout/LayoutRustBridge.cpp, Libraries/LibWeb/Rust/src/layout/svg_formatting_context.rs	Renames the returned position field to text_position_after and uses it for descendant and following text layout.
SVG text positioning validation Tests/LibWeb/Layout/expected/svg/svg-text-positioning.txt, Tests/LibWeb/Ref/..., Tests/LibWeb/Text/expected/...	Updates layout expectations and adds rendering coverage for tspan positioning, anchoring, and bounding rectangles.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant SVGTextElement
  participant LayoutRustBridge
  participant SVGFormattingContext
  SVGTextElement->>LayoutRustBridge: provide text positioning attributes
  LayoutRustBridge->>LayoutRustBridge: measure chunks and apply text-anchor
  LayoutRustBridge-->>SVGFormattingContext: return text_position_after
  SVGFormattingContext->>SVGFormattingContext: position descendant and following text

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Possibly related PRs

• LadybirdBrowser/ladybird#10901: Introduces related SVG text-positioning and path-result changes.
• LadybirdBrowser/ladybird#10907: Modifies related SVG text layout and chunk processing.

Suggested reviewers: kalenikaliaksandr

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address issue #10625 by advancing tspan text positions and preserving correct spacing across consecutive runs.
Out of Scope Changes check	✅ Passed	All code and test changes support the linked SVG tspan positioning and spacing objectives.
Description check	✅ Passed	The description clearly explains the SVG text layout changes, their cause, the fix, and the linked issue.

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[BenWiederhake]

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