LibWeb: Hand layout a Rust-owned style container instead of a mirror

[kalenikaliaksandr]

Layout read style through a per-node side table in the node arena that mirrored all 23 group payload pointers, copied out of ComputedValues on every style application. The mirror was 184 bytes per node of duplicated pointers whose validity rested on the rewrite-on-set protocol alone: nothing owned the payloads on the Rust side, and the generation-keyed side table existed only to keep stale copies from being read after slot reuse.

Every built ComputedValues now lazily creates a style container on first request: a Rust-owned refcounted allocation, using the same header-before-payload scheme as the group payloads themselves, holding the group pointer array and retaining every group. NodeData.style points at the container's array, which the layout side reads in place as the payload array, so style application shrinks to publishing one pointer and the side table, its generation keying, and the per-set 23-pointer copy are deleted. The container is created after group adoption has settled (adopt_identical_group_payloads now asserts it runs first) and released with its ComputedValues, making payload ownership explicit where the mirror only had it by protocol. It also gives the style a single identity usable for future retention from paintables and for input-keyed layout caching.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 88a9a328-19c6-461f-a8bf-b52b16867c8a

📥 Commits

Reviewing files that changed from the base of the PR and between 6e068c8 and b72d5a3.

📒 Files selected for processing (9)

• Libraries/LibWeb/CSS/ComputedProperties.cpp
• Libraries/LibWeb/CSS/ComputedValues.cpp
• Libraries/LibWeb/CSS/ComputedValues.h
• Libraries/LibWeb/Layout/Node.cpp
• Libraries/LibWeb/Layout/Node.h
• Libraries/LibWeb/Rust/src/css/computed_values.rs
• Libraries/LibWeb/Rust/src/layout/formatting_context.rs
• Libraries/LibWeb/Rust/src/layout/layout_node_arena.rs
• Libraries/LibWeb/Rust/src/layout/node_data.rs

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📝 Walkthrough

Walkthrough

Changes

Style container migration

Layer / File(s)	Summary
Style container lifecycle Libraries/LibWeb/CSS/ComputedValues.*, Libraries/LibWeb/CSS/ComputedProperties.cpp, Libraries/LibWeb/Rust/src/css/computed_values.rs	ComputedValues lazily creates and caches a Rust style container. Rust retains and releases group payloads through reference counting.
Node style publication Libraries/LibWeb/Layout/Node.*	Node construction and computed-style updates publish the style container to node data.
Layout container consumption Libraries/LibWeb/Rust/src/layout/*	Layout reads payloads from the published node style pointer. The layout arena no longer maintains a payload side table.

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

Sequence Diagram(s)

sequenceDiagram
  participant ComputedValues
  participant NodeData
  participant LayoutNodeArena
  participant FormattingContext
  ComputedValues->>ComputedValues: create and cache style container
  ComputedValues->>NodeData: publish style container
  FormattingContext->>LayoutNodeArena: request style payloads
  LayoutNodeArena->>NodeData: read published style pointer
  NodeData-->>FormattingContext: return style payload array

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Possibly related PRs

• LadybirdBrowser/ladybird#10939: Replaces payload mirroring with a refcounted Rust style container across the same pathways.
• LadybirdBrowser/ladybird#10912: Continues the style-payload migration in ComputedValues, nodes, and the layout arena.
• LadybirdBrowser/ladybird#10870: Relates to layout-node style storage migration in Node.cpp and Rust layout data.

Suggested reviewers: awesomekling

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description accurately explains the style-container ownership change, removal of the per-node mirror, and related layout updates.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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