Actually check if root is absolute positioned

components/layout/block.rs

@@ -903,7 +903,7 @@ impl BlockFlow {
     ///   10.6.7.
     ///
     /// For absolute flows, we store the calculated content block-size for the flow. We defer the
-    /// calculation of the other values until a later traversal.
+    /// calculation of the other values until a later traversal at root flow.
     ///
     /// When `fragmentation_context` is given (not `None`), this should fit as much of the content
     /// as possible within the available block size.
@@ -1200,6 +1200,8 @@ impl BlockFlow {
                 .contains(FlowFlags::IS_ABSOLUTELY_POSITIONED)
             {
                 self.propagate_early_absolute_position_info_to_children();
+                // Return early until the absolute flow tree traversal at root flow.
+                // Assigning block size for absolute flow will happen in `traverse_absolute_flows` below.
                 return None;
             }
 

components/layout/flow.rs

@@ -557,6 +557,11 @@ pub trait MutableOwnedFlowUtils {
     /// Set this flow as the Containing Block for all the absolute descendants.
     fn set_absolute_descendants(&mut self, abs_descendants: AbsoluteDescendants);
 
+    /// Push absolute descendants to this flow.
+    ///
+    /// Set this flow as the Containing Block for the provided absolute descendants.
+    fn push_absolute_descendants(&mut self, abs_descendants: AbsoluteDescendants);
+
     /// Sets the flow as the containing block for all absolute descendants that have been marked
     /// as having reached their containing block. This is needed in order to handle cases like:
     ///
@@ -1364,6 +1369,26 @@ impl MutableOwnedFlowUtils for FlowRef {
         }
     }
 
+    /// Push absolute descendants for this flow.
+    ///
+    /// Set yourself as the Containing Block for the provided absolute descendants.
+    ///
+    /// This is called when retreiving layout root if it's not absolute positioned. We can't just
+    /// call `set_absolute_descendants` because it might contain other abs_descendants already.
+    /// We push descendants instead of replace it since it won't cause circular reference.
+    fn push_absolute_descendants(&mut self, mut abs_descendants: AbsoluteDescendants) {
+        let this = self.clone();
+        let base = FlowRef::deref_mut(self).mut_base();
+
+        for descendant_link in abs_descendants.descendant_links.iter_mut() {
+            debug_assert!(!descendant_link.has_reached_containing_block);
+            let descendant_base = FlowRef::deref_mut(&mut descendant_link.flow).mut_base();
+            descendant_base.absolute_cb.set(this.clone());
+        }
+
+        base.abs_descendants.push_descendants(abs_descendants);
+    }
+
     /// Sets the flow as the containing block for all absolute descendants that have been marked
     /// as having reached their containing block. This is needed in order to handle cases like:
     ///

components/layout_thread/lib.rs

@@ -44,7 +44,7 @@ use layout::context::RegisteredPainter;
 use layout::context::RegisteredPainters;
 use layout::display_list::items::WebRenderImageInfo;
 use layout::display_list::{IndexableText, ToLayout};
-use layout::flow::{Flow, GetBaseFlow, ImmutableFlowUtils, MutableOwnedFlowUtils};
+use layout::flow::{Flow, FlowFlags, GetBaseFlow, ImmutableFlowUtils, MutableOwnedFlowUtils};
 use layout::flow_ref::FlowRef;
 use layout::incremental::{RelayoutMode, SpecialRestyleDamage};
 use layout::layout_debug;
@@ -950,11 +950,20 @@ impl LayoutThread {
         let mut flow = match result {
             ConstructionResult::Flow(mut flow, abs_descendants) => {
                 // Note: Assuming that the root has display 'static' (as per
-                // CSS Section 9.3.1). Otherwise, if it were absolutely
+                // CSS Section 9.3.1). If it was absolutely positioned,
-                // positioned, it would return a reference to itself in
+                // it would return a reference to itself in `abs_descendants`
-                // `abs_descendants` and would lead to a circular reference.
+                // and would lead to a circular reference. Otherwise, we
-                // Set Root as CB for any remaining absolute descendants.
+                // set Root as CB and push remaining absolute descendants.
-                flow.set_absolute_descendants(abs_descendants);
+                if flow
+                    .base()
+                    .flags
+                    .contains(FlowFlags::IS_ABSOLUTELY_POSITIONED)
+                {
+                    flow.set_absolute_descendants(abs_descendants);
+                } else {
+                    flow.push_absolute_descendants(abs_descendants);
+                }
+
                 flow
             },
             _ => return None,