LibWeb: Invalidate viewport-relative lengths nested in math functions

[awesomekling]

Style invalidation on viewport resize only touches elements whose computed values recorded a viewport metric dependency. Lengths resolved by the Rust calc simplification had no way to report that dependency back, so calc(50vh - 10px), min(50vh, ...), clamp() and anything reaching them through var() kept their stale pixel values after a resize until something else dirtied the element's style. On sites like YouTube that made a window resize appear to do nothing until a later unrelated invalidation moved everything at once.

Carry the resolution context's tracking flag across the FFI boundary so the Rust length absolutization records viewport (and viewport-dependent font metric) resolutions the same way the C++ path does. Adds a test resizing an iframe and checking each construct picks up the new viewport height.

[coderabbitai]

📝 Walkthrough

Walkthrough

Viewport-relative length resolution now propagates a dependency flag through the C++/Rust context, including font-relative and viewport-relative cases. A new iframe resize test verifies recalculation for direct lengths and lengths nested in CSS math functions.

Changes

Viewport dependency tracking

Layer / File(s)	Summary
Length-resolution dependency propagation Libraries/LibWeb/CSS/Length.h, Libraries/LibWeb/CSS/StyleComputeFFI.h, Libraries/LibWeb/CSS/Rust/src/style_compute.rs, Libraries/LibWeb/CSS/Rust/src/animation.rs	The resolution context exposes a nullable dependency flag, maps it across the C++/Rust boundary, and marks it when font-relative or viewport-relative metrics are resolved.
Iframe resize validation Tests/LibWeb/Text/input/css/viewport-relative-length-in-math-function-invalidation.html, Tests/LibWeb/Text/expected/css/viewport-relative-length-in-math-function-invalidation.txt	The test measures direct and nested viewport-relative lengths before and after iframe resizing, with updated expected values.

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

Possibly related PRs

• LadybirdBrowser/ladybird#10772: Both changes extend the Rust/C++ length-resolution context and viewport-related absolutization pipeline.
• LadybirdBrowser/ladybird#10861: Both changes use per-element length-resolution context data during style computation.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description matches the changeset, describing the FFI tracking flag update and the viewport-resize test.
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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[coderabbitai]

🧹 Nitpick comments (1)

Libraries/LibWeb/CSS/Rust/src/style_compute.rs (1)

56-68: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Exercise the new write-through path in a Rust unit test.

test_context() always supplies null_mut(), so the current tests do not execute record_viewport_relative_length_resolution()’s unsafe pointer write. Add a test with a local boolean pointer and verify both a viewport unit and a viewport-dependent font unit set it.

Also applies to: 3860-3860

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@Libraries/LibWeb/CSS/Rust/src/style_compute.rs` around lines 56 - 68, Add a
Rust unit test alongside the existing tests that creates a local boolean,
assigns its pointer to an FfiLengthResolutionContext derived from
test_context(), and verifies viewport-unit resolution sets the flag. Reset the
flag and resolve a viewport-dependent font unit, asserting that
record_viewport_relative_length_resolution() sets it there as well.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@Libraries/LibWeb/CSS/Rust/src/style_compute.rs`:
- Around line 56-68: Add a Rust unit test alongside the existing tests that
creates a local boolean, assigns its pointer to an FfiLengthResolutionContext
derived from test_context(), and verifies viewport-unit resolution sets the
flag. Reset the flag and resolve a viewport-dependent font unit, asserting that
record_viewport_relative_length_resolution() sets it there as well.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: a76f2efb-17af-4326-b953-a8be53ee5c17

📥 Commits

Reviewing files that changed from the base of the PR and between 27d11da and 0d754f1.

📒 Files selected for processing (6)

• Libraries/LibWeb/CSS/Length.h
• Libraries/LibWeb/CSS/Rust/src/animation.rs
• Libraries/LibWeb/CSS/Rust/src/style_compute.rs
• Libraries/LibWeb/CSS/StyleComputeFFI.h
• Tests/LibWeb/Text/expected/css/viewport-relative-length-in-math-function-invalidation.txt
• Tests/LibWeb/Text/input/css/viewport-relative-length-in-math-function-invalidation.html