4 files changed, 26 insertions, 12 deletions

diff --git a/components/layout/fragment_tree/base_fragment.rs b/components/layout/fragment_tree/base_fragment.rs
index 48d672a8547..0cf6ee511cb 100644
--- a/components/layout/fragment_tree/base_fragment.rs
+++ b/components/layout/fragment_tree/base_fragment.rs
@@ -132,11 +132,6 @@ impl Tag {
         Tag { node, pseudo }
     }
 
-    /// Returns true if this tag is for a pseudo element.
-    pub(crate) fn is_pseudo(&self) -> bool {
-        self.pseudo.is_some()
-    }
-
     pub(crate) fn to_display_list_fragment_id(self) -> u64 {
         combine_id_with_fragment_type(self.node.id(), self.pseudo.into())
     }
diff --git a/components/layout/fragment_tree/box_fragment.rs b/components/layout/fragment_tree/box_fragment.rs
index 0e83c0d71a6..e87826ec3ca 100644
--- a/components/layout/fragment_tree/box_fragment.rs
+++ b/components/layout/fragment_tree/box_fragment.rs
@@ -212,6 +212,10 @@ impl BoxFragment {
         )
     }
 
+    pub(crate) fn set_containing_block(&mut self, containing_block: &PhysicalRect<Au>) {
+        self.cumulative_containing_block_rect = *containing_block;
+    }
+
     pub fn offset_by_containing_block(&self, rect: &PhysicalRect<Au>) -> PhysicalRect<Au> {
         rect.translate(self.cumulative_containing_block_rect.origin.to_vector())
     }
@@ -405,8 +409,4 @@ impl BoxFragment {
             _ => CollapsedBlockMargins::zero(),
         }
     }
-
-    pub(crate) fn set_containing_block(&mut self, containing_block: &PhysicalRect<Au>) {
-        self.cumulative_containing_block_rect = *containing_block;
-    }
 }
diff --git a/components/layout/fragment_tree/fragment.rs b/components/layout/fragment_tree/fragment.rs
index ceccd1ec304..7708b0893ee 100644
--- a/components/layout/fragment_tree/fragment.rs
+++ b/components/layout/fragment_tree/fragment.rs
@@ -134,7 +134,9 @@ impl Fragment {
             Fragment::Float(float_fragment) => float_fragment
                 .borrow_mut()
                 .set_containing_block(containing_block),
-            Fragment::Positioning(_) => {},
+            Fragment::Positioning(positioning_fragment) => positioning_fragment
+                .borrow_mut()
+                .set_containing_block(containing_block),
             Fragment::AbsoluteOrFixedPositioned(hoisted_shared_fragment) => {
                 if let Some(ref fragment) = hoisted_shared_fragment.borrow().fragment {
                     fragment.set_containing_block(containing_block);
@@ -191,13 +193,16 @@ impl Fragment {
         }
     }
 
-    pub(crate) fn cumulative_content_box_rect(&self) -> Option<PhysicalRect<Au>> {
+    pub(crate) fn cumulative_border_box_rect(&self) -> Option<PhysicalRect<Au>> {
         match self {
             Fragment::Box(fragment) | Fragment::Float(fragment) => {
                 let fragment = fragment.borrow();
                 Some(fragment.offset_by_containing_block(&fragment.border_rect()))
             },
-            Fragment::Positioning(_) |
+            Fragment::Positioning(fragment) => {
+                let fragment = fragment.borrow();
+                Some(fragment.offset_by_containing_block(&fragment.rect))
+            },
             Fragment::Text(_) |
             Fragment::AbsoluteOrFixedPositioned(_) |
             Fragment::Image(_) |
diff --git a/components/layout/fragment_tree/positioning_fragment.rs b/components/layout/fragment_tree/positioning_fragment.rs
index 853caed6709..1fe968eb484 100644
--- a/components/layout/fragment_tree/positioning_fragment.rs
+++ b/components/layout/fragment_tree/positioning_fragment.rs
@@ -20,12 +20,17 @@ pub(crate) struct PositioningFragment {
     pub base: BaseFragment,
     pub rect: PhysicalRect<Au>,
     pub children: Vec<Fragment>,
+
     /// The scrollable overflow of this anonymous fragment's children.
     pub scrollable_overflow: PhysicalRect<Au>,
 
     /// If this fragment was created with a style, the style of the fragment.
     #[conditional_malloc_size_of]
     pub style: Option<ServoArc<ComputedValues>>,
+
+    /// This [`PositioningFragment`]'s containing block rectangle in coordinates relative to
+    /// the initial containing block, but not taking into account any transforms.
+    pub cumulative_containing_block_rect: PhysicalRect<Au>,
 }
 
 impl PositioningFragment {
@@ -61,9 +66,18 @@ impl PositioningFragment {
             rect,
             children,
             scrollable_overflow,
+            cumulative_containing_block_rect: PhysicalRect::zero(),
         })
     }
 
+    pub(crate) fn set_containing_block(&mut self, containing_block: &PhysicalRect<Au>) {
+        self.cumulative_containing_block_rect = *containing_block;
+    }
+
+    pub fn offset_by_containing_block(&self, rect: &PhysicalRect<Au>) -> PhysicalRect<Au> {
+        rect.translate(self.cumulative_containing_block_rect.origin.to_vector())
+    }
+
     pub fn print(&self, tree: &mut PrintTree) {
         tree.new_level(format!(
             "PositioningFragment\