layout: Add support for table rows, columns, rowgroups and colgroups (#31341)

This adds support for table rows, columns, rowgroups and colgroups.
There are few additions here:

1. The createion of fragments, which allows script queries and hit
   testing to work properly. These fragments are empty as all cells are
   still direct descendants of the table fragment.
2. Properly handling size information from tracks and track groups as
   well as frustrating rules about reordering rowgroups.
3. Painting a background seemlessly across track groups and groups. This
   is a thing that isn't done in legacy layout (nor WebKit)!

Co-authored-by: Oriol Brufau <obrufau@igalia.com>

1 files changed, 100 insertions, 0 deletions

diff --git a/components/layout_2020/fragment_tree/positioning_fragment.rs b/components/layout_2020/fragment_tree/positioning_fragment.rs
new file mode 100644
index 00000000000..4a629b31698
--- /dev/null
+++ b/components/layout_2020/fragment_tree/positioning_fragment.rs
@@ -0,0 +1,100 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use gfx_traits::print_tree::PrintTree;
+use serde::Serialize;
+use servo_arc::Arc as ServoArc;
+use style::logical_geometry::WritingMode;
+use style::properties::ComputedValues;
+use style::values::computed::Length;
+
+use super::{BaseFragment, BaseFragmentInfo, Fragment};
+use crate::cell::ArcRefCell;
+use crate::geom::{LogicalRect, PhysicalRect};
+
+/// Can contain child fragments with relative coordinates, but does not contribute to painting
+/// itself. [`PositioningFragments`] may be completely anonymous, or just non-painting Fragments
+/// generated by boxes.
+#[derive(Serialize)]
+pub(crate) struct PositioningFragment {
+    pub base: BaseFragment,
+    pub rect: LogicalRect<Length>,
+    pub children: Vec<ArcRefCell<Fragment>>,
+    pub writing_mode: WritingMode,
+
+    /// The scrollable overflow of this anonymous fragment's children.
+    pub scrollable_overflow: PhysicalRect<Length>,
+
+    /// If this fragment was created with a style, the style of the fragment.
+    #[serde(skip_serializing)]
+    pub style: Option<ServoArc<ComputedValues>>,
+}
+
+impl PositioningFragment {
+    pub fn new_anonymous(
+        rect: LogicalRect<Length>,
+        children: Vec<Fragment>,
+        mode: WritingMode,
+    ) -> Self {
+        Self::new_with_base_fragment(BaseFragment::anonymous(), None, rect, children, mode)
+    }
+
+    pub fn new_empty(
+        base_fragment_info: BaseFragmentInfo,
+        rect: LogicalRect<Length>,
+        style: ServoArc<ComputedValues>,
+    ) -> Self {
+        let writing_mode = style.writing_mode;
+        Self::new_with_base_fragment(
+            base_fragment_info.into(),
+            Some(style),
+            rect,
+            Vec::new(),
+            writing_mode,
+        )
+    }
+
+    fn new_with_base_fragment(
+        base: BaseFragment,
+        style: Option<ServoArc<ComputedValues>>,
+        rect: LogicalRect<Length>,
+        children: Vec<Fragment>,
+        mode: WritingMode,
+    ) -> Self {
+        // FIXME(mrobinson, bug 25564): We should be using the containing block
+        // here to properly convert scrollable overflow to physical geometry.
+        let containing_block = PhysicalRect::zero();
+        let content_origin = rect.start_corner.to_physical(mode);
+        let scrollable_overflow = children.iter().fold(PhysicalRect::zero(), |acc, child| {
+            acc.union(
+                &child
+                    .scrollable_overflow(&containing_block)
+                    .translate(content_origin.to_vector()),
+            )
+        });
+        PositioningFragment {
+            base,
+            style,
+            rect,
+            children: children.into_iter().map(ArcRefCell::new).collect(),
+            writing_mode: mode,
+            scrollable_overflow,
+        }
+    }
+
+    pub fn print(&self, tree: &mut PrintTree) {
+        tree.new_level(format!(
+            "PositioningFragment\
+                \nbase={:?}\
+                \nrect={:?}\
+                \nscrollable_overflow={:?}",
+            self.base, self.rect, self.scrollable_overflow
+        ));
+
+        for child in &self.children {
+            child.borrow().print(tree);
+        }
+        tree.end_level();
+    }
+}