Implement 'empty-cells' for layout 2020 (#32331)

https://drafts.csswg.org/css-tables/#empty-cell-rendering

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

components/layout_2020/display_list/mod.rs

@@ -32,7 +32,7 @@ use crate::context::LayoutContext;
 use crate::display_list::conversions::ToWebRender;
 use crate::display_list::stacking_context::StackingContextSection;
 use crate::fragment_tree::{
-    BackgroundMode, BoxFragment, Fragment, FragmentTree, Tag, TextFragment,
+    BackgroundMode, BoxFragment, Fragment, FragmentFlags, FragmentTree, Tag, TextFragment,
 };
 use crate::geom::{LogicalRect, PhysicalPoint, PhysicalRect};
 use crate::replaced::IntrinsicSizes;
@@ -631,6 +631,14 @@ impl<'a> BuilderForBoxFragment<'a> {
             self.build_outline(builder);
         } else {
             self.build_hit_test(builder);
+            if self
+                .fragment
+                .base
+                .flags
+                .contains(FragmentFlags::DO_NOT_PAINT)
+            {
+                return;
+            }
             self.build_background(builder);
             self.build_box_shadow(builder);
             self.build_border(builder);

components/layout_2020/fragment_tree/base_fragment.rs

@@ -94,6 +94,9 @@ bitflags! {
         /// Whether or not this Fragment was created to contain a list item marker
         /// with a used value of `list-style-position: outside`.
         const IS_OUTSIDE_LIST_ITEM_MARKER = 0b00001000;
+        /// Avoid painting the fragment, this is used for empty table cells when 'empty-cells' is 'hide'.
+        /// This flag doesn't avoid hit-testing.
+        const DO_NOT_PAINT = 0b00010000;
     }
 }
 

components/layout_2020/table/layout.rs

@@ -10,6 +10,7 @@ use servo_arc::Arc;
 use style::computed_values::border_collapse::T as BorderCollapse;
 use style::logical_geometry::WritingMode;
 use style::properties::longhands::box_sizing::computed_value::T as BoxSizing;
+use style::properties::longhands::empty_cells::computed_value::T as EmptyCells;
 use style::properties::ComputedValues;
 use style::values::computed::{
     CSSPixelLength, Length, LengthPercentage as ComputedLengthPercentage, Percentage,
@@ -60,6 +61,15 @@ impl CellLayout {
     fn is_empty(&self) -> bool {
         self.layout.fragments.is_empty()
     }
+
+    /// Whether the cell is considered empty for the purpose of the 'empty-cells' property.
+    fn is_empty_for_empty_cells(&self) -> bool {
+        !self
+            .layout
+            .fragments
+            .iter()
+            .any(|fragment| !matches!(fragment, Fragment::AbsoluteOrFixedPositioned(_)))
+    }
 }
 
 /// Information stored during the layout of rows.
@@ -1616,6 +1626,7 @@ impl<'a> TableLayout<'a> {
             row_relative_cell_rect,
             row_baseline,
             positioning_context,
+            &self.table.style,
         );
         let column = self.table.columns.get(column_index);
         let column_group = column
@@ -2142,6 +2153,7 @@ impl TableSlotCell {
         cell_rect: LogicalRect<Au>,
         cell_baseline: Au,
         positioning_context: &mut PositioningContext,
+        table_style: &ComputedValues,
     ) -> BoxFragment {
         // This must be scoped to this function because it conflicts with euclid's Zero.
         use style::Zero as StyleZero;
@@ -2162,6 +2174,14 @@ impl TableSlotCell {
             },
         };
 
+        let mut base_fragment_info = self.base_fragment_info;
+        if self.style.get_inherited_table().empty_cells == EmptyCells::Hide &&
+            table_style.get_inherited_table().border_collapse != BorderCollapse::Collapse &&
+            layout.is_empty_for_empty_cells()
+        {
+            base_fragment_info.flags.insert(FragmentFlags::DO_NOT_PAINT);
+        }
+
         // Create an `AnonymousFragment` to move the cell contents to the cell baseline.
         let mut vertical_align_fragment_rect = cell_content_rect.clone();
         vertical_align_fragment_rect.start_corner = LogicalVec2 {
@@ -2190,7 +2210,7 @@ impl TableSlotCell {
         positioning_context.append(layout.positioning_context);
 
         BoxFragment::new(
-            self.base_fragment_info,
+            base_fragment_info,
             self.style.clone(),
             vec![Fragment::Positioning(vertical_align_fragment)],
             cell_content_rect,