layout_2020: Use ArcRefCell to track hoisted fragments

This avoids the use of lookup tables for containing blocks when
constructing the stacking context tree.

This seems to catch some laid-out hoisted fragments that were otherwise
dropped in the previous design. The changes cause one new test to pass
and one to fail. Visual examination of the failing tests reveals that
it's a progression (list markers are appearing when they were previously
not rendered).

components/layout_2020/fragments.rs

@@ -7,7 +7,6 @@ use crate::geom::flow_relative::{Rect, Sides};
 use crate::geom::{PhysicalPoint, PhysicalRect};
 #[cfg(debug_assertions)]
 use crate::layout_debug;
-use crate::positioned::HoistedFragmentId;
 use gfx::font::FontMetrics as GfxFontMetrics;
 use gfx::text::glyph::GlyphStore;
 use gfx_traits::print_tree::PrintTree;
@@ -16,6 +15,7 @@ use serde::ser::{Serialize, Serializer};
 use servo_arc::Arc as ServoArc;
 use std::sync::Arc;
 use style::computed_values::overflow_x::T as ComputedOverflow;
+use style::computed_values::position::T as ComputedPosition;
 use style::dom::OpaqueNode;
 use style::logical_geometry::WritingMode;
 use style::properties::ComputedValues;
@@ -34,7 +34,10 @@ pub(crate) enum Fragment {
 }
 
 #[derive(Serialize)]
-pub(crate) struct AbsoluteOrFixedPositionedFragment(pub HoistedFragmentId);
+pub(crate) struct AbsoluteOrFixedPositionedFragment {
+    pub position: ComputedPosition,
+    pub hoisted_fragment: ArcRefCell<Option<ArcRefCell<Fragment>>>,
+}
 
 #[derive(Serialize)]
 pub(crate) struct BoxFragment {
@@ -57,11 +60,6 @@ pub(crate) struct BoxFragment {
 
     /// The scrollable overflow of this box fragment.
     pub scrollable_overflow_from_children: PhysicalRect<Length>,
-
-    /// If this fragment was laid out from a hoisted box, this id corresponds to the id stored in
-    /// the AbsoluteOrFixedPositionedFragment left as a placeholder in the tree position of the
-    /// box.
-    pub hoisted_fragment_id: Option<HoistedFragmentId>,
 }
 
 #[derive(Serialize)]
@@ -177,20 +175,6 @@ impl Fragment {
         }
     }
 
-    pub fn is_hoisted(&self) -> bool {
-        match self {
-            Fragment::Box(fragment) if fragment.hoisted_fragment_id.is_some() => true,
-            _ => false,
-        }
-    }
-
-    pub fn hoisted_fragment_id(&self) -> Option<&HoistedFragmentId> {
-        match self {
-            Fragment::Box(fragment) => fragment.hoisted_fragment_id.as_ref(),
-            _ => None,
-        }
-    }
-
     pub(crate) fn find<T>(
         &self,
         containing_block: &PhysicalRect<Length>,
@@ -228,7 +212,7 @@ impl Fragment {
 
 impl AbsoluteOrFixedPositionedFragment {
     pub fn print(&self, tree: &mut PrintTree) {
-        tree.add_item(format!("AbsoluteOrFixedPositionedFragment({:?})", self.0));
+        tree.add_item(format!("AbsoluteOrFixedPositionedFragment"));
     }
 }
 
@@ -292,7 +276,6 @@ impl BoxFragment {
         border: Sides<Length>,
         margin: Sides<Length>,
         block_margins_collapsed_with_children: CollapsedBlockMargins,
-        hoisted_fragment_id: Option<HoistedFragmentId>,
     ) -> BoxFragment {
         // FIXME(mrobinson, bug 25564): We should be using the containing block
         // here to properly convert scrollable overflow to physical geometry.
@@ -315,7 +298,6 @@ impl BoxFragment {
             margin,
             block_margins_collapsed_with_children,
             scrollable_overflow_from_children,
-            hoisted_fragment_id,
         }
     }
 
@@ -354,8 +336,7 @@ impl BoxFragment {
                 \nborder rect={:?}\
                 \nscrollable_overflow={:?}\
                 \noverflow={:?} / {:?}\
-                \nstyle={:p}\
-                \nhoisted_id={:?}",
+                \nstyle={:p}",
             self.content_rect,
             self.padding_rect(),
             self.border_rect(),
@@ -363,7 +344,6 @@ impl BoxFragment {
             self.style.get_box().overflow_x,
             self.style.get_box().overflow_y,
             self.style,
-            self.hoisted_fragment_id,
         ));
 
         for child in &self.children {