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>

components/layout_2020/fragment_tree/fragment.rs

@@ -10,14 +10,16 @@ use gfx_traits::print_tree::PrintTree;
 use msg::constellation_msg::{BrowsingContextId, PipelineId};
 use serde::Serialize;
 use servo_arc::Arc as ServoArc;
-use style::logical_geometry::WritingMode;
 use style::properties::ComputedValues;
 use style::values::computed::Length;
 use style::values::specified::text::TextDecorationLine;
 use style::Zero;
 use webrender_api::{FontInstanceKey, ImageKey};
 
-use super::{BaseFragment, BoxFragment, ContainingBlockManager, HoistedSharedFragment, Tag};
+use super::{
+    BaseFragment, BoxFragment, ContainingBlockManager, HoistedSharedFragment, PositioningFragment,
+    Tag,
+};
 use crate::cell::ArcRefCell;
 use crate::geom::{LogicalRect, LogicalSides, PhysicalRect};
 use crate::style_ext::ComputedValuesExt;
@@ -31,7 +33,7 @@ pub(crate) enum Fragment {
     /// the [SequentialLayoutState] of their float containing block formatting
     /// context.
     Float(BoxFragment),
-    Anonymous(AnonymousFragment),
+    Positioning(PositioningFragment),
     /// Absolute and fixed position fragments are hoisted up so that they
     /// are children of the BoxFragment that establishes their containing
     /// blocks, so that they can be laid out properly. When this happens
@@ -63,18 +65,6 @@ pub(crate) struct CollapsedMargin {
     min_negative: Length,
 }
 
-/// Can contain child fragments with relative coordinates, but does not contribute to painting itself.
-#[derive(Serialize)]
-pub(crate) struct AnonymousFragment {
-    pub base: BaseFragment,
-    pub rect: LogicalRect<Length>,
-    pub children: Vec<ArcRefCell<Fragment>>,
-    pub mode: WritingMode,
-
-    /// The scrollable overflow of this anonymous fragment's children.
-    pub scrollable_overflow: PhysicalRect<Length>,
-}
-
 #[derive(Serialize)]
 pub(crate) struct TextFragment {
     pub base: BaseFragment,
@@ -119,7 +109,7 @@ impl Fragment {
             Fragment::Box(fragment) => &fragment.base,
             Fragment::Text(fragment) => &fragment.base,
             Fragment::AbsoluteOrFixedPositioned(_) => return None,
-            Fragment::Anonymous(fragment) => &fragment.base,
+            Fragment::Positioning(fragment) => &fragment.base,
             Fragment::Image(fragment) => &fragment.base,
             Fragment::IFrame(fragment) => &fragment.base,
             Fragment::Float(fragment) => &fragment.base,
@@ -141,7 +131,7 @@ impl Fragment {
             Fragment::AbsoluteOrFixedPositioned(_) => {
                 tree.add_item("AbsoluteOrFixedPositioned".to_string());
             },
-            Fragment::Anonymous(fragment) => fragment.print(tree),
+            Fragment::Positioning(fragment) => fragment.print(tree),
             Fragment::Text(fragment) => fragment.print(tree),
             Fragment::Image(fragment) => fragment.print(tree),
             Fragment::IFrame(fragment) => fragment.print(tree),
@@ -166,7 +156,7 @@ impl Fragment {
                 fragment.scrollable_overflow_for_parent(containing_block)
             },
             Fragment::AbsoluteOrFixedPositioned(_) => PhysicalRect::zero(),
-            Fragment::Anonymous(fragment) => fragment.scrollable_overflow,
+            Fragment::Positioning(fragment) => fragment.scrollable_overflow,
             Fragment::Text(fragment) => fragment
                 .rect
                 .to_physical(fragment.parent_style.writing_mode, containing_block),
@@ -219,10 +209,10 @@ impl Fragment {
                     .iter()
                     .find_map(|child| child.borrow().find(&new_manager, level + 1, process_func))
             },
-            Fragment::Anonymous(fragment) => {
+            Fragment::Positioning(fragment) => {
                 let content_rect = fragment
                     .rect
-                    .to_physical(fragment.mode, containing_block)
+                    .to_physical(fragment.writing_mode, containing_block)
                     .translate(containing_block.origin.to_vector());
                 let new_manager = manager.new_for_non_absolute_descendants(&content_rect);
                 fragment
@@ -235,43 +225,6 @@ impl Fragment {
     }
 }
 
-impl AnonymousFragment {
-    pub fn new(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()),
-            )
-        });
-        AnonymousFragment {
-            base: BaseFragment::anonymous(),
-            rect,
-            children: children.into_iter().map(ArcRefCell::new).collect(),
-            mode,
-            scrollable_overflow,
-        }
-    }
-
-    pub fn print(&self, tree: &mut PrintTree) {
-        tree.new_level(format!(
-            "Anonymous\
-                \nrect={:?}\
-                \nscrollable_overflow={:?}",
-            self.rect, self.scrollable_overflow
-        ));
-
-        for child in &self.children {
-            child.borrow().print(tree);
-        }
-        tree.end_level();
-    }
-}
-
 impl TextFragment {
     pub fn print(&self, tree: &mut PrintTree) {
         tree.add_item(format!(