Auto merge of #29856 - Loirooriol:improve-margin-collapse, r=mrobinson

Improve margin collapse in layout-2020

According to https://drafts.csswg.org/css2/#collapsing-margins, bottom
margins should only collapse with the last child if `height` is `auto`.
Also, the note mentions `min-height: 0`, but the normative text doesn't
have such requirement, so I'm dropping it, matching WebKit.

The previous logic is moved into the case of collapsing the top and
bottom margins of the same element, since this can happen either with
`height: auto` or `height: 0`, and requires `min-height: 0`.

---

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #29858
- [X] There are tests for these changes

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3 files changed, 6 insertions, 9 deletions

diff --git a/components/layout_2020/flow/mod.rs b/components/layout_2020/flow/mod.rs
index fcd2c2b1837..e23c8db33e8 100644
--- a/components/layout_2020/flow/mod.rs
+++ b/components/layout_2020/flow/mod.rs
@@ -483,8 +483,7 @@ fn layout_in_flow_non_replaced_block_level(
     let end_margin_can_collapse_with_children = block_is_same_formatting_context &&
         pbm.padding.block_end == Length::zero() &&
         pbm.border.block_end == Length::zero() &&
-        block_size.auto_is(|| Length::zero()) == Length::zero() &&
-        min_box_size.block == Length::zero();
+        block_size == LengthOrAuto::Auto;
 
     let mut clearance = Length::zero();
     let parent_containing_block_position_info;
@@ -570,9 +569,11 @@ fn layout_in_flow_non_replaced_block_level(
                 content_block_size += collapsible_margins_in_children.end.solve();
             }
             block_margins_collapsed_with_children.collapsed_through =
-                start_margin_can_collapse_with_children &&
-                    end_margin_can_collapse_with_children &&
-                    collapsible_margins_in_children.collapsed_through;
+                collapsible_margins_in_children.collapsed_through &&
+                    block_is_same_formatting_context &&
+                    pbm.padding_border_sums.block == Length::zero() &&
+                    block_size.auto_is(|| Length::zero()) == Length::zero() &&
+                    min_box_size.block == Length::zero();
         },
         NonReplacedContents::EstablishesAnIndependentFormattingContext(non_replaced) => {
             let independent_layout = non_replaced.layout(
diff --git a/tests/wpt/metadata-layout-2020/css/CSS2/margin-padding-clear/margin-collapse-025.xht.ini b/tests/wpt/metadata-layout-2020/css/CSS2/margin-padding-clear/margin-collapse-025.xht.ini
deleted file mode 100644
index 41403961633..00000000000
--- a/tests/wpt/metadata-layout-2020/css/CSS2/margin-padding-clear/margin-collapse-025.xht.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[margin-collapse-025.xht]
-  expected: FAIL
diff --git a/tests/wpt/metadata-layout-2020/css/CSS2/margin-padding-clear/margin-collapse-037.xht.ini b/tests/wpt/metadata-layout-2020/css/CSS2/margin-padding-clear/margin-collapse-037.xht.ini
deleted file mode 100644
index 049aa133c56..00000000000
--- a/tests/wpt/metadata-layout-2020/css/CSS2/margin-padding-clear/margin-collapse-037.xht.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[margin-collapse-037.xht]
-  expected: FAIL