LibWeb: Return slot ids instead of shells from layout FC callbacks

[kalenikaliaksandr]

The formatting-context callbacks handed raw Layout::Node pointers back into Rust in three places (static position containing block, anchor lookup, and the list item marker in the list item facts), forcing the Rust side to reconstruct arena slot ids by reading the NodeData pointer at a byte displacement inside the C++ object. That displacement had to be computed at runtime and defended by a vtable-sampling verification walk over the layout tree, all resting on the assumption that every layout node class keeps its NodeData member at one process-wide offset.

Slot ids are already the canonical cross-boundary node reference: the topology synchronization writes them into NodeData, and Node::slot_id() is a single field read carrying the packed index and generation. Return slot ids from the three callbacks directly, which lets the displacement field, the verification walk, and the Rust-side unsafe pointer arithmetic all be deleted with no replacement. The anchor lookup's bool-plus-out-pointer shape collapses into returning an invalid slot id for "not found", and the marker link in the list item facts still reads the C++ WeakPtr at fetch time, so a detached marker stays absent rather than becoming a stale slot reference.

[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: b3701bf6-676a-474c-9bdc-9667b1e6c5e6

📥 Commits

Reviewing files that changed from the base of the PR and between 22e1013 and d17dfb6.

📒 Files selected for processing (8)

• Libraries/LibWeb/Layout/LayoutRustBridge.cpp
• Libraries/LibWeb/Layout/LayoutRustBridge.h
• Libraries/LibWeb/Layout/Node.cpp
• Libraries/LibWeb/Layout/Node.h
• Libraries/LibWeb/Rust/src/layout/formatting_context.rs
• Libraries/LibWeb/Rust/src/layout/layout_node_arena.rs
• Libraries/LibWeb/Rust/src/layout/layout_state.rs
• Libraries/LibWeb/Rust/src/layout/node_facts.rs

💤 Files with no reviewable changes (3)

• Libraries/LibWeb/Rust/src/layout/layout_node_arena.rs
• Libraries/LibWeb/Layout/Node.cpp
• Libraries/LibWeb/Layout/Node.h

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📝 Walkthrough

Walkthrough

Formatting-context callbacks and list-item marker facts now use arena-backed NodeSlotId values across the C++/Rust layout FFI. Obsolete pointer conversion and node displacement helpers were removed.

Changes

NodeSlotId FFI migration

Layer / File(s)	Summary
Arena-backed node addressing Libraries/LibWeb/Layout/Node.*, Libraries/LibWeb/Rust/src/layout/layout_node_arena.rs, Libraries/LibWeb/Rust/src/layout/formatting_context.rs	Node slot identifiers and arena slot indexes replace displacement and data-reference slot helpers; Rust accesses NodeData directly through arena nodes.
Formatting callback contract Libraries/LibWeb/Layout/LayoutRustBridge.*, Libraries/LibWeb/Rust/src/layout/formatting_context.rs	Formatting callbacks no longer accept a subtree root, and static-position and anchor lookup callbacks return NodeSlotId values with INVALID for failure.
List-item marker slots Libraries/LibWeb/Layout/LayoutRustBridge.cpp, Libraries/LibWeb/Rust/src/layout/node_facts.rs, Libraries/LibWeb/Rust/src/layout/layout_state.rs	List-item marker facts store typed NodeSlotId values, default to INVALID, and return the stored marker node directly.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant LayoutRustBridge
  participant FfiLayoutFcCallbacks
  participant AbsposEngine
  participant LayoutNodeArena
  LayoutRustBridge->>FfiLayoutFcCallbacks: provide NodeSlotId callbacks
  AbsposEngine->>FfiLayoutFcCallbacks: request static containing block
  FfiLayoutFcCallbacks-->>AbsposEngine: return NodeSlotId
  AbsposEngine->>FfiLayoutFcCallbacks: perform anchor lookup
  FfiLayoutFcCallbacks->>LayoutNodeArena: resolve NodeData by Node
  FfiLayoutFcCallbacks-->>AbsposEngine: return NodeSlotId or INVALID

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Possibly related PRs

• LadybirdBrowser/ladybird#10870: Introduces shared arena-backed NodeSlotId layout addressing used by this FFI migration.
• LadybirdBrowser/ladybird#10886: Updates layout state and node abstractions around arena slot indexes.
• LadybirdBrowser/ladybird#10901: Adds the formatting-context FFI plumbing whose callback contracts are updated here.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description accurately matches the changes: it describes the slot-id callback migration, anchor lookup API change, and removal of unsafe displacement logic.
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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