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Grid Lanes: stacking-axis content distribution and self-alignment #1275

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Grid Lanes: stacking-axis content distribution and self-alignment (sp…
nicoburns Oct 8, 2026
dad1597
Reference #1275 in the changelog
nicoburns Oct 8, 2026
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2 changes: 1 addition & 1 deletion CHANGELOG.md
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### Added

- Initial support for CSS Grid Level 3 [Grid Lanes](https://drafts.csswg.org/css-grid-3/) behind the new `grid_lanes` feature (#1270). `Display::GridLanes` lays items out in a waterfall along the stacking axis of a container whose tracks are defined in the other axis (`GridLanesDirection`; `Style::grid_lanes_direction` / `GridContainerStyle::grid_lanes_direction()`), with the auto-placement tie threshold set by `Style::fit_tolerance` / `GridContainerStyle::fit_tolerance()` (a `LengthPercentage`; `infinite` is `LengthPercentage::length(f32::INFINITY)`). The new trait getters have default implementations. Auto-placed items contribute to intrinsic grid-axis track sizes through the spec's virtual-item optimization (#1272). Containers whose `grid_auto_flow` is dense backfill items into skipped spaces (#1274). Stacking-axis alignment and absolutely positioned children follow in later releases
- Initial support for CSS Grid Level 3 [Grid Lanes](https://drafts.csswg.org/css-grid-3/) behind the new `grid_lanes` feature (#1270). `Display::GridLanes` lays items out in a waterfall along the stacking axis of a container whose tracks are defined in the other axis (`GridLanesDirection`; `Style::grid_lanes_direction` / `GridContainerStyle::grid_lanes_direction()`), with the auto-placement tie threshold set by `Style::fit_tolerance` / `GridContainerStyle::fit_tolerance()` (a `LengthPercentage`; `infinite` is `LengthPercentage::length(f32::INFINITY)`). The new trait getters have default implementations. Auto-placed items contribute to intrinsic grid-axis track sizes through the spec's virtual-item optimization (#1272). Containers whose `grid_auto_flow` is dense backfill items into skipped spaces (#1274). `align_content`/`justify_content` in the stacking axis moves the stacking range as a whole, and items adjacent to a gap (the last item in a track, or one followed by a spanning item) align within their margin box plus that gap with `align_self`/`justify_self`; baseline alignment in the stacking axis falls back to `start` (#1275). Absolutely positioned children follow in a later release
- `CoreStyle::is_replaced()` (defaulting to `false`; `Style::item_is_replaced`) now identifies [replaced elements](https://html.spec.whatwg.org/multipage/rendering.html#replaced-elements) for absolute positioning: only these (and tables) are exempt from being stretched between their insets under `normal` alignment. `CoreStyle::is_compressible_replaced()` now has its css-sizing meaning of [compressible replaced element](https://drafts.csswg.org/css-sizing-3/#compressible) — replaced elements *plus* form controls with an intrinsic size (`input`, `select`, `textarea`, `progress`, `meter`) — and is what block layout consults when deciding that an auto-width block-level box takes its intrinsic width rather than being stretched (matching browsers, where a block-level `<input>` keeps its intrinsic width but a `position: absolute; left: 0; right: 0` one is stretched). `Style` gains `item_is_compressible_replaced` for the form-control case; `item_is_replaced: true` continues to imply both.
- `AlignSelf::is_stretch_or_normal` returns whether an alignment's keyword is `Stretch` or `Normal`
- `AlignItems`, `AlignItemsKeyword` and `AlignmentSafety` now implement `Default` (`AlignItems::NORMAL`, `AlignItemsKeyword::Normal` and `AlignmentSafety::Default`)
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171 changes: 137 additions & 34 deletions src/compute/grid/lanes.rs
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20 changes: 20 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_align_content_center.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: align-content centers the stacking range
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-template-columns: 40px 40px 40px; width: 120px; height: 100px; align-content: center;">
<div style="height: 30px;"></div>
<div style="height: 20px;"></div>
<div style="height: 10px;"></div>
<div style="height: 20px;"></div>
</div>

</body>
</html>
20 changes: 20 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_align_content_end.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: align-content end shifts the stacking range to the end edge
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-template-columns: 40px 40px 40px; width: 120px; height: 100px; align-content: end;">
<div style="height: 30px;"></div>
<div style="height: 20px;"></div>
<div style="height: 10px;"></div>
<div style="height: 20px;"></div>
</div>

</body>
</html>
20 changes: 20 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_align_content_safe_end.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: safe end alignment falls back to start when the stacking range overflows
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-template-columns: 40px 40px 40px; width: 120px; height: 40px; align-content: safe end;">
<div style="height: 30px;"></div>
<div style="height: 20px;"></div>
<div style="height: 10px;"></div>
<div style="height: 40px;"></div>
</div>

</body>
</html>
20 changes: 20 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_align_content_space_around.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: distributed content alignment falls back in the stacking axis
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-template-columns: 40px 40px 40px; width: 120px; height: 100px; align-content: space-around;">
<div style="height: 30px;"></div>
<div style="height: 20px;"></div>
<div style="height: 10px;"></div>
<div style="height: 20px;"></div>
</div>

</body>
</html>
21 changes: 21 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_align_items_center.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: align-items center within the gap below the last items in each track
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-template-columns: 40px 40px 40px; width: 120px; height: 100px; align-items: center;">
<div style="height: 30px;"></div>
<div style="height: 10px;"></div>
<div style="height: 20px;"></div>
<div style="height: 20px;"></div>
<div style="height: 10px;"></div>
</div>

</body>
</html>
21 changes: 21 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_align_items_end.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: align-items end moves items next to a gap to the end of the gap
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-template-columns: 40px 40px 40px; width: 120px; height: 100px; align-items: end;">
<div style="height: 30px;"></div>
<div style="height: 10px;"></div>
<div style="height: 20px;"></div>
<div style="height: 10px; grid-column: span 3;"></div>
<div style="height: 15px;"></div>
</div>

</body>
</html>
21 changes: 21 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_align_items_gap.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: the stacking-axis gutter is excluded from the alignment container
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-template-columns: 40px 40px 40px; width: 120px; height: 100px; row-gap: 6px; align-items: end;">
<div style="height: 30px;"></div>
<div style="height: 10px;"></div>
<div style="height: 20px;"></div>
<div style="height: 10px; grid-column: span 3;"></div>
<div style="height: 15px;"></div>
</div>

</body>
</html>
22 changes: 22 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_align_items_normal.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: normal alignment stretches auto-height items into the adjacent gap
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-template-columns: 40px 40px 40px; width: 120px; height: 100px;">
<div style="height: 30px;"></div>
<div style="display: block; min-height: 10px;"></div>
<div style="height: 20px;"></div>
<div style="height: 10px; grid-column: span 3;"></div>
<div style="display: block; min-height: 10px;"></div>
<div style="height: 15px;"></div>
</div>

</body>
</html>
21 changes: 21 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_align_items_rtl.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: justify-items end in a row-direction rtl container
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-lanes-direction: row; grid-template-rows: 30px 30px 30px; height: 90px; width: 120px; direction: rtl; justify-items: end;">
<div style="width: 30px;"></div>
<div style="width: 10px;"></div>
<div style="width: 20px;"></div>
<div style="width: 10px; grid-row: span 3;"></div>
<div style="width: 15px;"></div>
</div>

</body>
</html>
22 changes: 22 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_align_items_stretch.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: align-items stretch grows auto-height items into the adjacent gap
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-template-columns: 40px 40px 40px; width: 120px; height: 100px; align-items: stretch;">
<div style="height: 30px;"></div>
<div style="display: block; min-height: 10px;"></div>
<div style="height: 20px;"></div>
<div style="height: 10px; grid-column: span 3;"></div>
<div style="display: block; min-height: 10px;"></div>
<div style="height: 15px;"></div>
</div>

</body>
</html>
21 changes: 21 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_align_self_mixed.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: align-self overrides align-items per item in the stacking axis
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-template-columns: 40px 40px 40px; width: 120px; height: 100px; align-items: start;">
<div style="height: 30px;"></div>
<div style="height: 10px; align-self: end;"></div>
<div style="height: 20px; align-self: center;"></div>
<div style="height: 10px; grid-column: span 3;"></div>
<div style="height: 15px; align-self: end;"></div>
</div>

</body>
</html>
22 changes: 22 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_dense_align_items_end.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: dense-packed items take part in stacking-axis alignment
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-template-columns: 40px 40px 40px; width: 120px; height: 100px; grid-auto-flow: dense; grid-lanes-pack: dense; align-items: end;">
<div style="height: 30px;"></div>
<div style="height: 10px;"></div>
<div style="height: 20px;"></div>
<div style="height: 10px; grid-column: span 2;"></div>
<div style="height: 10px;"></div>
<div style="height: 15px;"></div>
</div>

</body>
</html>
20 changes: 20 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_justify_content_center_row.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: justify-content centers the stacking range in a row-direction container
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-lanes-direction: row; grid-template-rows: 30px 30px 30px; height: 90px; width: 120px; justify-content: center;">
<div style="width: 30px;"></div>
<div style="width: 20px;"></div>
<div style="width: 10px;"></div>
<div style="width: 20px;"></div>
</div>

</body>
</html>
21 changes: 21 additions & 0 deletions test_fixtures/grid_lanes/grid_lanes_justify_items_end_row.html
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<!DOCTYPE html>
<html lang="en">
<head>
<script src="../../scripts/gentest/test_helper.js"></script>
<link rel="stylesheet" type="text/css" href="../../scripts/gentest/test_base_style.css">
<title>
Grid Lanes: justify-items end in a row-direction container
</title>
</head>
<body>

<div id="test-root" style="display: grid-lanes; grid-lanes-direction: row; grid-template-rows: 30px 30px 30px; height: 90px; width: 120px; justify-items: end;">
<div style="width: 30px;"></div>
<div style="width: 10px;"></div>
<div style="width: 20px;"></div>
<div style="width: 10px; grid-row: span 3;"></div>
<div style="width: 15px;"></div>
</div>

</body>
</html>
19 changes: 19 additions & 0 deletions tests/xml/grid_lanes/grid_lanes_align_content_center__border_box_ltr.xml
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<test name="grid_lanes_align_content_center__border_box_ltr" use-rounding="true">
<viewport width="max-content" height="max-content"/>
<input>
<div display="grid-lanes" direction="ltr" position="absolute" align-content="center" width="120px" height="100px" fit-tolerance="10px" grid-template-columns="40px 40px 40px">
<div direction="ltr" position="relative" height="30px"/>
<div direction="ltr" position="relative" height="20px"/>
<div direction="ltr" position="relative" height="10px"/>
<div direction="ltr" position="relative" height="20px"/>
</div>
</input>
<expectations>
<node x="0" y="0" width="120" height="100">
<node x="0" y="30" width="40" height="30"/>
<node x="40" y="30" width="40" height="20"/>
<node x="80" y="30" width="40" height="10"/>
<node x="40" y="50" width="40" height="20"/>
</node>
</expectations>
</test>
19 changes: 19 additions & 0 deletions tests/xml/grid_lanes/grid_lanes_align_content_center__border_box_rtl.xml
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<test name="grid_lanes_align_content_center__border_box_rtl" use-rounding="true">
<viewport width="max-content" height="max-content"/>
<input>
<div display="grid-lanes" direction="rtl" position="absolute" align-content="center" width="120px" height="100px" fit-tolerance="10px" grid-template-columns="40px 40px 40px">
<div direction="rtl" position="relative" height="30px"/>
<div direction="rtl" position="relative" height="20px"/>
<div direction="rtl" position="relative" height="10px"/>
<div direction="rtl" position="relative" height="20px"/>
</div>
</input>
<expectations>
<node x="0" y="0" width="120" height="100">
<node x="80" y="30" width="40" height="30"/>
<node x="40" y="30" width="40" height="20"/>
<node x="0" y="30" width="40" height="10"/>
<node x="40" y="50" width="40" height="20"/>
</node>
</expectations>
</test>
19 changes: 19 additions & 0 deletions tests/xml/grid_lanes/grid_lanes_align_content_center__content_box_ltr.xml
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<test name="grid_lanes_align_content_center__content_box_ltr" use-rounding="true">
<viewport width="max-content" height="max-content"/>
<input>
<div display="grid-lanes" box-sizing="content-box" direction="ltr" position="absolute" align-content="center" width="120px" height="100px" fit-tolerance="10px" grid-template-columns="40px 40px 40px">
<div box-sizing="content-box" direction="ltr" position="relative" height="30px"/>
<div box-sizing="content-box" direction="ltr" position="relative" height="20px"/>
<div box-sizing="content-box" direction="ltr" position="relative" height="10px"/>
<div box-sizing="content-box" direction="ltr" position="relative" height="20px"/>
</div>
</input>
<expectations>
<node x="0" y="0" width="120" height="100">
<node x="0" y="30" width="40" height="30"/>
<node x="40" y="30" width="40" height="20"/>
<node x="80" y="30" width="40" height="10"/>
<node x="40" y="50" width="40" height="20"/>
</node>
</expectations>
</test>
19 changes: 19 additions & 0 deletions tests/xml/grid_lanes/grid_lanes_align_content_center__content_box_rtl.xml
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<test name="grid_lanes_align_content_center__content_box_rtl" use-rounding="true">
<viewport width="max-content" height="max-content"/>
<input>
<div display="grid-lanes" box-sizing="content-box" direction="rtl" position="absolute" align-content="center" width="120px" height="100px" fit-tolerance="10px" grid-template-columns="40px 40px 40px">
<div box-sizing="content-box" direction="rtl" position="relative" height="30px"/>
<div box-sizing="content-box" direction="rtl" position="relative" height="20px"/>
<div box-sizing="content-box" direction="rtl" position="relative" height="10px"/>
<div box-sizing="content-box" direction="rtl" position="relative" height="20px"/>
</div>
</input>
<expectations>
<node x="0" y="0" width="120" height="100">
<node x="80" y="30" width="40" height="30"/>
<node x="40" y="30" width="40" height="20"/>
<node x="0" y="30" width="40" height="10"/>
<node x="40" y="50" width="40" height="20"/>
</node>
</expectations>
</test>
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