From 1f13e8b59619fad1ebd3d87a70d105a9a3b4867d Mon Sep 17 00:00:00 2001
From: Oriol Brufau <obrufau@igalia.com>
Date: Wed, 2 Apr 2025 05:10:04 -0700
Subject: [PATCH] layout: Allow collapsing bottom margins with any indefinite
 block size (#36278)

According to CSS2, the bottom margin of a block container can collapse
with the bottom margin of its last in-flow child if `height` computes to
`auto`.

However, according to CSS Sizing, that was "legacy spec prose" and
should be interpreted as "behaves as `auto`".

Therefore, cyclic percentages and intrinsic keywords like `min-content`
shouldn't prevent margin collapse, because they behave as `auto`.

This change aligns Servo with Gecko and Blink, but diverges from WebKit.

https://www.w3.org/TR/CSS22/box.html#collapsing-margins
https://www.w3.org/TR/css-sizing/#behave-auto

Testing:
-
`tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-001.html`
-
`tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-002.html`
-
`tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-003.html`
-
`tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-004.html`
-
`tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-005.html`

Signed-off-by: Oriol Brufau <obrufau@igalia.com>
---
 components/layout_2020/flow/mod.rs            |  2 +-
 tests/wpt/meta/MANIFEST.json                  | 65 +++++++++++++++++++
 ...llapse-with-indefinite-block-size-001.html | 23 +++++++
 ...llapse-with-indefinite-block-size-002.html | 23 +++++++
 ...llapse-with-indefinite-block-size-003.html | 23 +++++++
 ...llapse-with-indefinite-block-size-004.html | 23 +++++++
 ...llapse-with-indefinite-block-size-005.html | 23 +++++++
 7 files changed, 181 insertions(+), 1 deletion(-)
 create mode 100644 tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-001.html
 create mode 100644 tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-002.html
 create mode 100644 tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-003.html
 create mode 100644 tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-004.html
 create mode 100644 tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-005.html

diff --git a/components/layout_2020/flow/mod.rs b/components/layout_2020/flow/mod.rs
index 1aee5748c60..da8f5c9f16a 100644
--- a/components/layout_2020/flow/mod.rs
+++ b/components/layout_2020/flow/mod.rs
@@ -1012,7 +1012,7 @@ fn layout_in_flow_non_replaced_block_level_same_formatting_context(
     let end_margin_can_collapse_with_children = collapsed_through ||
         (pbm.padding.block_end == Au::zero() &&
             pbm.border.block_end == Au::zero() &&
-            computed_block_size.is_auto());
+            !containing_block_for_children.size.block.is_definite());
     if end_margin_can_collapse_with_children {
         block_margins_collapsed_with_children
             .end
diff --git a/tests/wpt/meta/MANIFEST.json b/tests/wpt/meta/MANIFEST.json
index c6ce4dfd85b..437f621c427 100644
--- a/tests/wpt/meta/MANIFEST.json
+++ b/tests/wpt/meta/MANIFEST.json
@@ -248765,6 +248765,71 @@
        {}
       ]
      ],
+     "margin-collapse-with-indefinite-block-size-001.html": [
+      "47d2a0a043d32ba6220c1dd6a26859908a19bf6e",
+      [
+       null,
+       [
+        [
+         "/css/reference/ref-filled-green-100px-square.xht",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
+     "margin-collapse-with-indefinite-block-size-002.html": [
+      "eb401c92b5e9475129f39be9980f47f42cb2bfa0",
+      [
+       null,
+       [
+        [
+         "/css/reference/ref-filled-green-100px-square.xht",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
+     "margin-collapse-with-indefinite-block-size-003.html": [
+      "51f16f68e6998de4956a5f0615b49406fcacb02e",
+      [
+       null,
+       [
+        [
+         "/css/reference/ref-filled-green-100px-square.xht",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
+     "margin-collapse-with-indefinite-block-size-004.html": [
+      "c393560d6c6ce799943db09468ba8e5cb0e90a5c",
+      [
+       null,
+       [
+        [
+         "/css/reference/ref-filled-green-100px-square.xht",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
+     "margin-collapse-with-indefinite-block-size-005.html": [
+      "27814994aaac7ead76f67d051e8744323e8f4247",
+      [
+       null,
+       [
+        [
+         "/css/reference/ref-filled-green-100px-square.xht",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
      "max-content-input-001.html": [
       "aabbea015748f9276718e2ba9728efc375c8910b",
       [
diff --git a/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-001.html b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-001.html
new file mode 100644
index 00000000000..47d2a0a043d
--- /dev/null
+++ b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-001.html
@@ -0,0 +1,23 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel="author" title="Oriol Brufau" href="obrufau@igalia.com">
+<link rel="help" href="https://drafts.csswg.org/css2/#collapsing-margins">
+<link rel="help" href="https://drafts.csswg.org/css-sizing-3/#behave-auto">
+<link rel="help" href="https://drafts.csswg.org/css-sizing-3/#cyclic-percentage-contribution">
+<link rel="match" href="../reference/ref-filled-green-100px-square.xht">
+<meta name="assert" content="
+  According to CSS2, the bottom margin of a block container can collapse
+  with the bottom margin of its last in-flow child if `height` computes
+  to `auto`.
+
+  However, according to CSS Sizing, that was 'legacy spec prose' and
+  should be interpreted as 'behaves as `auto`'.
+
+  Therefore, a cyclic percentage in the preferred block size property
+  shouldn't prevent margin collapse, because it behaves as `auto`.
+">
+
+<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
+<div style="width: 100px; height: 100%; background: red">
+  <div style="height: 100px; margin-bottom: 100px; background: green"></div>
+</div>
diff --git a/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-002.html b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-002.html
new file mode 100644
index 00000000000..eb401c92b5e
--- /dev/null
+++ b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-002.html
@@ -0,0 +1,23 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel="author" title="Oriol Brufau" href="obrufau@igalia.com">
+<link rel="help" href="https://drafts.csswg.org/css2/#collapsing-margins">
+<link rel="help" href="https://drafts.csswg.org/css-sizing-3/#behave-auto">
+<link rel="help" href="https://drafts.csswg.org/css-sizing-3/#valdef-width-min-content">
+<link rel="match" href="../reference/ref-filled-green-100px-square.xht">
+<meta name="assert" content="
+  According to CSS2, the bottom margin of a block container can collapse
+  with the bottom margin of its last in-flow child if `height` computes
+  to `auto`.
+
+  However, according to CSS Sizing, that was 'legacy spec prose' and
+  should be interpreted as 'behaves as `auto`'.
+
+  Therefore, a `min-content` value in the preferred block size property
+  shouldn't prevent margin collapse, because it behaves as `auto`.
+">
+
+<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
+<div style="width: 100px; height: min-content; background: red">
+  <div style="height: 100px; margin-bottom: 100px; background: green"></div>
+</div>
diff --git a/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-003.html b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-003.html
new file mode 100644
index 00000000000..51f16f68e69
--- /dev/null
+++ b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-003.html
@@ -0,0 +1,23 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel="author" title="Oriol Brufau" href="obrufau@igalia.com">
+<link rel="help" href="https://drafts.csswg.org/css2/#collapsing-margins">
+<link rel="help" href="https://drafts.csswg.org/css-sizing-3/#behave-auto">
+<link rel="help" href="https://drafts.csswg.org/css-sizing-3/#valdef-width-max-content">
+<link rel="match" href="../reference/ref-filled-green-100px-square.xht">
+<meta name="assert" content="
+  According to CSS2, the bottom margin of a block container can collapse
+  with the bottom margin of its last in-flow child if `height` computes
+  to `auto`.
+
+  However, according to CSS Sizing, that was 'legacy spec prose' and
+  should be interpreted as 'behaves as `auto`'.
+
+  Therefore, a `max-content` value in the preferred block size property
+  shouldn't prevent margin collapse, because it behaves as `auto`.
+">
+
+<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
+<div style="width: 100px; height: max-content; background: red">
+  <div style="height: 100px; margin-bottom: 100px; background: green"></div>
+</div>
diff --git a/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-004.html b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-004.html
new file mode 100644
index 00000000000..c393560d6c6
--- /dev/null
+++ b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-004.html
@@ -0,0 +1,23 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel="author" title="Oriol Brufau" href="obrufau@igalia.com">
+<link rel="help" href="https://drafts.csswg.org/css2/#collapsing-margins">
+<link rel="help" href="https://drafts.csswg.org/css-sizing-3/#behave-auto">
+<link rel="help" href="https://drafts.csswg.org/css-sizing-4/#valdef-width-fit-content">
+<link rel="match" href="../reference/ref-filled-green-100px-square.xht">
+<meta name="assert" content="
+  According to CSS2, the bottom margin of a block container can collapse
+  with the bottom margin of its last in-flow child if `height` computes
+  to `auto`.
+
+  However, according to CSS Sizing, that was 'legacy spec prose' and
+  should be interpreted as 'behaves as `auto`'.
+
+  Therefore, a `fit-content` value in the preferred block size property
+  shouldn't prevent margin collapse, because it behaves as `auto`.
+">
+
+<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
+<div style="width: 100px; height: fit-content; background: red">
+  <div style="height: 100px; margin-bottom: 100px; background: green"></div>
+</div>
diff --git a/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-005.html b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-005.html
new file mode 100644
index 00000000000..27814994aaa
--- /dev/null
+++ b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-005.html
@@ -0,0 +1,23 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel="author" title="Oriol Brufau" href="obrufau@igalia.com">
+<link rel="help" href="https://drafts.csswg.org/css2/#collapsing-margins">
+<link rel="help" href="https://drafts.csswg.org/css-sizing-3/#behave-auto">
+<link rel="help" href="https://drafts.csswg.org/css-sizing-4/#stretch-fit-sizing">
+<link rel="match" href="../reference/ref-filled-green-100px-square.xht">
+<meta name="assert" content="
+  According to CSS2, the bottom margin of a block container can collapse
+  with the bottom margin of its last in-flow child if `height` computes
+  to `auto`.
+
+  However, according to CSS Sizing, that was 'legacy spec prose' and
+  should be interpreted as 'behaves as `auto`'.
+
+  Therefore, an indefinite `stretch` in the preferred block size property
+  shouldn't prevent margin collapse, because it behaves as `auto`.
+">
+
+<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
+<div style="width: 100px; height: stretch; background: red">
+  <div style="height: 100px; margin-bottom: 100px; background: green"></div>
+</div>