LibWeb: Key per-pass layout state by arena slot index

[kalenikaliaksandr]

The separate per-document index used for per-pass layout state grows sparse between full passes. That forces periodic full relayouts through the dense-range veto even though each live arena slot is already unique and free-list allocation keeps active indices compact between passes.

[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: a5bb2892-f330-465b-bc08-7c0be5eabe37

📥 Commits

Reviewing files that changed from the base of the PR and between e94da4a and b397098.

📒 Files selected for processing (11)

• Libraries/LibWeb/DOM/Document.cpp
• Libraries/LibWeb/DOM/Document.h
• Libraries/LibWeb/Layout/LayoutState.cpp
• Libraries/LibWeb/Layout/LayoutState.h
• Libraries/LibWeb/Layout/Node.cpp
• Libraries/LibWeb/Layout/Node.h
• Libraries/LibWeb/Layout/Rust/src/layout_node_arena.rs
• Libraries/LibWeb/Layout/Rust/src/node_data.rs
• Tests/LibWeb/Text/expected/abspos-relayout-arena-slot-churn.txt
• Tests/LibWeb/Text/expected/abspos-relayout-index-compaction.txt
• Tests/LibWeb/Text/input/abspos-relayout-arena-slot-churn.html

💤 Files with no reviewable changes (6)

• Libraries/LibWeb/Layout/Node.cpp
• Tests/LibWeb/Text/expected/abspos-relayout-index-compaction.txt
• Libraries/LibWeb/Layout/LayoutState.h
• Libraries/LibWeb/DOM/Document.cpp
• Libraries/LibWeb/Layout/Rust/src/node_data.rs
• Libraries/LibWeb/DOM/Document.h

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📝 Walkthrough

Walkthrough

Layout node indexing is removed in favor of arena slot indices. Layout-state storage and absolute-position ordering use arena slots and tree order, while partial relayout no longer bails out on dense-range growth. Related tests now require partial passes without full-layout passes.

Changes

Arena-slot layout state migration

Layer / File(s)	Summary
Arena identity contract Libraries/LibWeb/Layout/Rust/src/node_data.rs, Libraries/LibWeb/Layout/Node.*, Libraries/LibWeb/DOM/Document.h, Libraries/LibWeb/Layout/Rust/src/layout_node_arena.rs	Removes layout-index fields, accessors, allocation helpers, and bookkeeping, while exposing each layout node’s arena slot index and updating related assertions and tests.
Layout state and relayout migration Libraries/LibWeb/Layout/LayoutState.*, Libraries/LibWeb/DOM/Document.cpp	Uses arena slots for UsedValues, orders contained absolute-position children by tree order, removes explicit store capacity setup, and simplifies full-layout bookkeeping and partial-relayout eligibility.
Relayout regression expectations Tests/LibWeb/Text/input/abspos-relayout-arena-slot-churn.html, Tests/LibWeb/Text/expected/abspos-relayout-arena-slot-churn.txt	Updates the arena-slot churn test to require partial relayout on every iteration with no full passes and records the resulting metadata.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• LadybirdBrowser/ladybird#10824: Touches the same Document.cpp layout-tree rebuild entrypoints.
• LadybirdBrowser/ladybird#10870: Introduces the NodeArena slot identifiers used by this migration.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description matches the changeset by explaining the move from per-document layout indices to arena slot indexing and dense-range behavior.
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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