Treat indefinite percentages as auto for margin collapse (#32059)

The top and bottom margins of an element can collapse through if its
height is auto or zero. Indefinite percentages behave as auto, so they
shouldn't prevent the margins from collapsing.

Co-authored-by: Martin Robinson <mrobinson@igalia.com>

components/layout_2020/flow/mod.rs

@@ -13,7 +13,7 @@ use style::computed_values::clear::T as Clear;
 use style::computed_values::float::T as Float;
 use style::logical_geometry::WritingMode;
 use style::properties::ComputedValues;
-use style::values::computed::{Length, LengthOrAuto};
+use style::values::computed::{Length, LengthOrAuto, Size};
 use style::values::specified::{Display, TextAlignKeyword};
 use style::Zero;
 
@@ -131,13 +131,12 @@ impl BlockLevelBox {
             return false;
         }
 
-        let min_inline_size = style
+        let min_size = style
             .content_min_box_size(containing_block, &pbm)
-            .auto_is(Length::zero)
-            .inline;
-        let max_inline_size = style.content_max_box_size(containing_block, &pbm).inline;
-        let inline_size = style
-            .content_box_size(containing_block, &pbm)
+            .auto_is(Length::zero);
+        let max_size = style.content_max_box_size(containing_block, &pbm);
+        let prefered_size = style.content_box_size(containing_block, &pbm);
+        let inline_size = prefered_size
             .inline
             .auto_is(|| {
                 let margin_inline_start = pbm.margin.inline_start.auto_is(Au::zero);
@@ -148,10 +147,10 @@ impl BlockLevelBox {
                     margin_inline_end)
                     .into()
             })
-            .clamp_between_extremums(min_inline_size, max_inline_size);
-
-        // The block size is irrelevant here.
-        let block_size = AuOrAuto::Auto;
+            .clamp_between_extremums(min_size.inline, max_size.inline);
+        let block_size = prefered_size
+            .block
+            .map(|size| Au::from(size.clamp_between_extremums(min_size.block, max_size.block)));
 
         let containing_block_for_children = ContainingBlock {
             inline_size: inline_size.into(),
@@ -167,11 +166,10 @@ impl BlockLevelBox {
             return false;
         }
 
-        let block_size_zero =
-            style.content_block_size().is_definitely_zero() || style.content_block_size().is_auto();
-        let min_block_size_zero =
-            style.min_block_size().is_definitely_zero() || style.min_block_size().is_auto();
-        if !min_block_size_zero || !block_size_zero || pbm.padding_border_sums.block != Au::zero() {
+        if !block_size_is_zero_or_auto(style.content_block_size(), containing_block) ||
+            !block_size_is_zero_or_auto(style.min_block_size(), containing_block) ||
+            pbm.padding_border_sums.block != Au::zero()
+        {
             return false;
         }
 
@@ -755,12 +753,11 @@ fn layout_in_flow_non_replaced_block_level_same_formatting_context(
         content_block_size += collapsible_margins_in_children.end.solve().into();
     }
 
-    let computed_min_block_size = style.min_block_size();
     block_margins_collapsed_with_children.collapsed_through = collapsible_margins_in_children
         .collapsed_through &&
         pbm.padding_border_sums.block == Au::zero() &&
-        (computed_block_size.is_definitely_zero() || computed_block_size.is_auto()) &&
-        (computed_min_block_size.is_definitely_zero() || computed_min_block_size.is_auto());
+        block_size_is_zero_or_auto(computed_block_size, containing_block) &&
+        block_size_is_zero_or_auto(style.min_block_size(), containing_block);
 
     let block_size = containing_block_for_children.block_size.auto_is(|| {
         content_block_size
@@ -1675,3 +1672,14 @@ impl PlacementState {
         )
     }
 }
+
+fn block_size_is_zero_or_auto(size: &Size, containing_block: &ContainingBlock) -> bool {
+    match size {
+        Size::Auto => true,
+        Size::LengthPercentage(ref lp) => {
+            // TODO: Should this resolve definite percentages? Blink does it, Gecko and WebKit don't.
+            lp.is_definitely_zero() ||
+                (lp.0.has_percentage() && containing_block.block_size.is_auto())
+        },
+    }
+}

tests/wpt/meta/css/CSS2/normal-flow/margin-collapse-through-percentage-height-block.html.ini

@@ -1,2 +0,0 @@
-[margin-collapse-through-percentage-height-block.html]
-  expected: FAIL