1 files changed, 9 insertions, 52 deletions

diff --git a/components/layout_2020/flow/root.rs b/components/layout_2020/flow/root.rs
index f44b0c88159..5006b65a48b 100644
--- a/components/layout_2020/flow/root.rs
+++ b/components/layout_2020/flow/root.rs
@@ -4,9 +4,7 @@
 
 use crate::cell::ArcRefCell;
 use crate::context::LayoutContext;
-use crate::display_list::stacking_context::{
-    ContainingBlock, ContainingBlockInfo, StackingContext, StackingContextBuildMode,
-};
+use crate::display_list::StackingContext;
 use crate::dom::{LayoutBox, NodeExt};
 use crate::dom_traversal::{iter_child_nodes, Contents, NodeAndStyleInfo};
 use crate::flexbox::FlexLevelBox;
@@ -38,7 +36,6 @@ use style::dom::OpaqueNode;
 use style::properties::ComputedValues;
 use style::values::computed::Length;
 use style_traits::CSSPixel;
-use webrender_api::{ClipId, SpaceAndClipInfo, SpatialId};
 
 #[derive(Serialize)]
 pub struct BoxTree {
@@ -60,7 +57,7 @@ pub struct FragmentTree {
     /// * The first fragment is generated by the root element.
     /// * There may be additional fragments generated by positioned boxes
     ///   that have the initial containing block.
-    root_fragments: Vec<ArcRefCell<Fragment>>,
+    pub(crate) root_fragments: Vec<ArcRefCell<Fragment>>,
 
     /// The scrollable overflow rectangle for the entire tree
     /// https://drafts.csswg.org/css-overflow/#scrollable
@@ -384,58 +381,18 @@ impl BoxTree {
 }
 
 impl FragmentTree {
-    pub fn build_display_list(&self, builder: &mut crate::display_list::DisplayListBuilder) {
-        let stacking_context = self.build_stacking_context_tree(builder);
-
+    pub(crate) fn build_display_list(
+        &self,
+        builder: &mut crate::display_list::DisplayListBuilder,
+        root_stacking_context: &StackingContext,
+    ) {
         // Paint the canvas’ background (if any) before/under everything else
-        stacking_context.build_canvas_background_display_list(
+        root_stacking_context.build_canvas_background_display_list(
             builder,
             self,
             &self.initial_containing_block,
         );
-
-        stacking_context.build_display_list(builder);
-    }
-
-    fn build_stacking_context_tree(
-        &self,
-        builder: &mut crate::display_list::DisplayListBuilder,
-    ) -> StackingContext {
-        let mut stacking_context = StackingContext::create_root(&builder.wr);
-        let pipeline_id = builder.wr.pipeline_id;
-        let cb_for_non_fixed_descendants = ContainingBlock::new(
-            &self.initial_containing_block,
-            SpaceAndClipInfo::root_scroll(pipeline_id),
-        );
-        let cb_for_fixed_descendants = ContainingBlock::new(
-            &self.initial_containing_block,
-            SpaceAndClipInfo {
-                spatial_id: SpatialId::root_reference_frame(pipeline_id),
-                clip_id: ClipId::root(pipeline_id),
-            },
-        );
-
-        for fragment in &self.root_fragments {
-            fragment.borrow().build_stacking_context_tree(
-                fragment,
-                &mut builder.wr,
-                // We need to specify all three containing blocks here, because absolute
-                // descdendants of the root cannot share the containing block we specify
-                // for fixed descendants. In this case, they need to have the spatial
-                // id of the root scroll frame, whereas fixed descendants need the
-                // spatial id of the root reference frame so that they do not scroll with
-                // page content.
-                &ContainingBlockInfo {
-                    for_non_absolute_descendants: &cb_for_non_fixed_descendants,
-                    for_absolute_descendants: Some(&cb_for_non_fixed_descendants),
-                    for_absolute_and_fixed_descendants: &cb_for_fixed_descendants,
-                },
-                &mut stacking_context,
-                StackingContextBuildMode::SkipHoisted,
-            );
-        }
-        stacking_context.sort();
-        stacking_context
+        root_stacking_context.build_display_list(builder);
     }
 
     pub fn print(&self) {