LibWeb: Skip empty atomic blocks during intrinsic layout

[awesomekling]

Size empty atomic block roots directly in the parent measurement instead of constructing child formatting-context runs during min-content and max-content layout. Root sizing continues to resolve box model metrics, constraints, and automatic block size.

This removes tens of thousands of empty measurement runs from StyleBench while preserving coverage through the layout cache scenario matrix.

Benchmark    Old Score     New Score       Score Improvement  Old Total Time (ms)    New Total Time (ms)      Speedup
-----------  ------------  ------------  -------------------  ---------------------  ---------------------  ---------
StyleBench   63.22 ± 5.46  65.25 ± 4.86                1.032  4.02 ± 0.31            3.91 ± 0.27                1.029

[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: 3d08e89b-a8f8-48b6-a88f-18743bfe403c

📥 Commits

Reviewing files that changed from the base of the PR and between dee0370 and 8a5d831.

📒 Files selected for processing (1)

• Libraries/LibWeb/Rust/src/layout/formatting_context.rs

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📝 Walkthrough

Walkthrough

During intrinsic sizing, empty atomic block children use a direct sizing path when independent context execution is not required. The parent dimensions the child, resolves its automatic block size to zero, and returns Skipped. This avoids constructing and running an independent formatting context for the empty child.

Possibly related PRs

• LadybirdBrowser/ladybird#10949: Relocated the SizingContext implementation used by intrinsic sizing.
• LadybirdBrowser/ladybird#10971: Updated child-layout handling for skipped and independent roots during intrinsic layout.
• LadybirdBrowser/ladybird#11146: Introduced empty atomic block handling extended by this change.

Suggested reviewers: kalenikaliaksandr

Merge Risk: ⚪ Minimal · up to 8a5d8

This localized layout optimization has no actionable merge-blocking risk remaining and is merge-ready after normal checks and review.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description directly explains the intrinsic layout optimization and its reported StyleBench performance improvement.
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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