Resolve a rendered legend fit-content inline size before layout - #11047
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kalenikaliaksandr merged 1 commit intoAug 9, 2026
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A fieldset's rendered legend with 'width: auto' uses its fit-content inline size. That size was applied as an override written to the legend's box after its layout returned, so the legend's children were laid out against a provisional stretch size. The fit-content rule now resolves inside the shared block-level inline-size resolution, before the legend's children are laid out, so they resolve against the used inline size. Covered by a new regression test.
📝 WalkthroughWalkthroughRendered fieldset legends with ChangesFieldset legend sizing
Estimated code review effort: 3 (Moderate) | ~20 minutes Sequence Diagram(s)sequenceDiagram
participant FieldsetLayout
participant RootInlineMetrics
participant RenderedLegend
FieldsetLayout->>RootInlineMetrics: Resolve root inline metrics
RootInlineMetrics->>RenderedLegend: Detect auto computed width
RootInlineMetrics->>RenderedLegend: Calculate fit-content inline size
Possibly related PRs
🚥 Pre-merge checks | ✅ 3✅ Passed checks (3 passed)
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Actionable comments posted: 2
🤖 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.
Inline comments:
In `@Libraries/LibWeb/Rust/src/layout/block_formatting_context.rs`:
- Around line 519-537: Remove the early return in the block sizing flow and
store the fieldset rendered legend’s fit-content result from the
`style.width().is_auto()` branch as the common block-sizing input. Ensure that
value continues through the existing automatic margin resolution and
`min-width`/`max-width` calculations in the block sizing path, including the
logic around `SizingAxis::Inline` and the later sizing passes.
In
`@Tests/LibWeb/Text/input/fieldset-legend-independent-formatting-context-fit-content.html`:
- Line 7: Update the regression test’s child content and measurement logic:
replace the fixed-size .box setup with size-dependent or wrapping content, and
record each child’s rectangles in addition to the legend rectangles for every
formatting-context case covered by the test. Ensure the assertions observe child
layout at the legend’s available inline size rather than only the legend
geometry.
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📒 Files selected for processing (3)
Libraries/LibWeb/Rust/src/layout/block_formatting_context.rsTests/LibWeb/Text/expected/fieldset-legend-independent-formatting-context-fit-content.txtTests/LibWeb/Text/input/fieldset-legend-independent-formatting-context-fit-content.html
Comment on lines
+519
to
+537
| // https://html.spec.whatwg.org/multipage/rendering.html#the-fieldset-and-legend-elements | ||
| // A rendered legend with a computed inline size of auto uses the | ||
| // fit-content size, resolved here so the legend's children are laid | ||
| // out against the used size rather than a provisional stretch size. | ||
| if style.width().is_auto() { | ||
| let container = self.containing_block(node); | ||
| if !container.is_invalid() | ||
| && self.facts(container).is_fieldset_box() | ||
| && self.facts(container).rendered_legend() == node | ||
| { | ||
| return Some(sizing.calculate_fit_content_size( | ||
| node, | ||
| SizingAxis::Inline, | ||
| remaining_available_space, | ||
| constraints, | ||
| )); | ||
| } | ||
| } | ||
|
|
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🎯 Functional Correctness | 🟠 Major | ⚡ Quick win
Keep fit-content sizing on the common block-sizing path.
The early return bypasses automatic margin resolution and the min-width/max-width passes at Lines 661-694. For example, max-width: 20px will not constrain an 80px legend, and margin-inline: auto will remain unresolved.
Store the fit-content value as the common path’s input, then apply the existing margin and min/max calculations.
🤖 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/Rust/src/layout/block_formatting_context.rs` around lines
519 - 537, Remove the early return in the block sizing flow and store the
fieldset rendered legend’s fit-content result from the `style.width().is_auto()`
branch as the common block-sizing input. Ensure that value continues through the
existing automatic margin resolution and `min-width`/`max-width` calculations in
the block sizing path, including the logic around `SizingAxis::Inline` and the
later sizing passes.
| body { margin: 0; } | ||
| fieldset { margin: 0; border: 2px solid black; padding: 10px; width: 300px; } | ||
| legend { padding: 0; } | ||
| .box { width: 40px; height: 16px; background: papayawhip; } |
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🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win
Make the regression test observe child layout.
The fixed 40px children do not depend on the legend’s available inline size. The test also records only the legend rectangles. A regression that lays out children at the provisional fieldset width can therefore pass.
Add size-dependent or wrapping child content, and record the child rectangles for each formatting context.
Also applies to: 27-30
🤖 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
`@Tests/LibWeb/Text/input/fieldset-legend-independent-formatting-context-fit-content.html`
at line 7, Update the regression test’s child content and measurement logic:
replace the fixed-size .box setup with size-dependent or wrapping content, and
record each child’s rectangles in addition to the legend rectangles for every
formatting-context case covered by the test. Ensure the assertions observe child
layout at the legend’s available inline size rather than only the legend
geometry.
kalenikaliaksandr
merged commit Aug 9, 2026
abdb9c5
into
LadybirdBrowser:master
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A fieldset's rendered legend with 'width: auto' uses its fit-content inline size. That size was applied as an override written to the legend's box after its layout returned, so the legend's children were laid out against a provisional stretch size. The fit-content rule now resolves inside the shared block-level inline-size resolution, before the legend's children are laid out, so they resolve against the used inline size. Covered by a new regression test.