layout_2020: Add 'establishes_block_formatting_context' method to 'ComputedValuesExt'

This method checks whether the style of a normal block would establish
a block formatting context.

components/layout_2020/style_ext.rs

@@ -11,6 +11,7 @@ use style::computed_values::transform_style::T as ComputedTransformStyle;
 use style::logical_geometry::WritingMode;
 use style::properties::longhands::backface_visibility::computed_value::T as BackfaceVisiblity;
 use style::properties::longhands::box_sizing::computed_value::T as BoxSizing;
+use style::properties::longhands::column_span::computed_value::T as ColumnSpan;
 use style::properties::ComputedValues;
 use style::values::computed::image::Image as ComputedImageLayer;
 use style::values::computed::{Length, LengthPercentage};
@@ -112,6 +113,7 @@ pub(crate) trait ComputedValuesExt {
     ) -> flow_relative::Sides<LengthPercentageOrAuto<'_>>;
     fn has_transform_or_perspective(&self) -> bool;
     fn effective_z_index(&self) -> i32;
+    fn establishes_block_formatting_context(&self) -> bool;
     fn establishes_stacking_context(&self) -> bool;
     fn establishes_containing_block_for_absolute_descendants(&self) -> bool;
     fn establishes_containing_block_for_all_descendants(&self) -> bool;
@@ -365,6 +367,25 @@ impl ComputedValuesExt for ComputedValues {
         }
     }
 
+    /// Return true if this style is a normal block and establishes
+    /// a new block formatting context.
+    fn establishes_block_formatting_context(&self) -> bool {
+        if self.get_box().overflow_x.is_scrollable() {
+            return true;
+        }
+
+        if self.get_column().is_multicol() {
+            return true;
+        }
+
+        if self.get_column().column_span == ColumnSpan::All {
+            return true;
+        }
+
+        // TODO: We need to handle CSS Contain here.
+        false
+    }
+
     /// Returns true if this fragment establishes a new stacking context and false otherwise.
     fn establishes_stacking_context(&self) -> bool {
         let effects = self.get_effects();