Use the is_absolute_containing_block method everywhere

This is a better approach than relying on
contains_positioned_fragments, because in the future other properties
will create absolute containing blocks.

components/layout/block.rs

@@ -1989,12 +1989,12 @@ impl Flow for BlockFlow {
 
         // For relatively-positioned descendants, the containing block formed by a block is just
         // the content box. The containing block for absolutely-positioned descendants, on the
-        // other hand, is only established if we are positioned.
+        // other hand, is established in other circumstances (see `is_absolute_containing_block').
         let relative_offset =
             self.fragment.relative_position(&self.base
                                                  .early_absolute_position_info
                                                  .relative_containing_block_size);
-        if self.contains_positioned_fragments() {
+        if self.is_absolute_containing_block() {
             let border_box_origin = (self.fragment.border_box -
                 self.fragment.style.logical_border_width()).start;
             self.base
@@ -2013,14 +2013,15 @@ impl Flow for BlockFlow {
                                                  logical_border_width.inline_start,
                                                  logical_border_width.block_start);
                 let position = position.to_physical(self.base.writing_mode, container_size);
-                if self.contains_positioned_fragments() {
+
+                // Some blocks establish a stacking context, but not a containing block for
+                // absolutely positioned elements. An example of this might be a block that has
+                // `position: static` and `opacity` set. In these cases, absolutely-positioned
+                // children will not be positioned relative to us but will instead be positioned
+                // relative to our containing block.
+                if self.is_absolute_containing_block() {
                     position
                 } else {
-                    // We establish a stacking context but are not positioned. (This will happen
-                    // if, for example, the element has `position: static` but has `opacity` or
-                    // `transform` set.) In this case, absolutely-positioned children will not be
-                    // positioned relative to us but will instead be positioned relative to our
-                    // containing block.
                     position - self.base.stacking_relative_position
                 }
             } else {
@@ -2105,10 +2106,6 @@ impl Flow for BlockFlow {
         (self.fragment.border_box - self.fragment.style().logical_border_width()).size
     }
 
-    fn is_absolute_containing_block(&self) -> bool {
-        self.contains_positioned_fragments()
-    }
-
     fn update_late_computed_inline_position_if_necessary(&mut self, inline_position: Au) {
         if self.base.flags.contains(IS_ABSOLUTELY_POSITIONED) &&
                 self.fragment.style().logical_position().inline_start ==

components/layout/construct.rs

@@ -616,14 +616,12 @@ impl<'a, ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode>
         flow.finish();
 
         // Set up the absolute descendants.
-        let contains_positioned_fragments = flow.contains_positioned_fragments();
+        if flow.is_absolute_containing_block() {
-        let is_absolutely_positioned = flow::base(&*flow).flags.contains(IS_ABSOLUTELY_POSITIONED);
-        if contains_positioned_fragments {
             // This is the containing block for all the absolute descendants.
             flow.set_absolute_descendants(abs_descendants);
 
             abs_descendants = AbsoluteDescendants::new();
-            if is_absolutely_positioned {
+            if flow::base(&*flow).flags.contains(IS_ABSOLUTELY_POSITIONED) {
                 // This is now the only absolute flow in the subtree which hasn't yet
                 // reached its CB.
                 abs_descendants.push(flow.clone());
@@ -1060,16 +1058,13 @@ impl<'a, ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode>
 
         // The flow is done.
         flow.finish();
-        let contains_positioned_fragments = flow.contains_positioned_fragments();
+        if flow.is_absolute_containing_block() {
-        if contains_positioned_fragments {
             // This is the containing block for all the absolute descendants.
             flow.set_absolute_descendants(abs_descendants);
 
             abs_descendants = AbsoluteDescendants::new();
 
-            let is_absolutely_positioned =
+            if flow::base(&*flow).flags.contains(IS_ABSOLUTELY_POSITIONED) {
-                flow::base(&*flow).flags.contains(IS_ABSOLUTELY_POSITIONED);
-            if is_absolutely_positioned {
                 // This is now the only absolute flow in the subtree which hasn't yet
                 // reached its containing block.
                 abs_descendants.push(flow.clone());
@@ -1134,16 +1129,13 @@ impl<'a, ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode>
         legalizer.finish(&mut wrapper_flow);
         wrapper_flow.finish();
 
-        let contains_positioned_fragments = wrapper_flow.contains_positioned_fragments();
+        if wrapper_flow.is_absolute_containing_block() {
-        if contains_positioned_fragments {
             // This is the containing block for all the absolute descendants.
             wrapper_flow.set_absolute_descendants(abs_descendants);
 
             abs_descendants = AbsoluteDescendants::new();
 
-            let is_absolutely_positioned =
+            if flow::base(&*wrapper_flow).flags.contains(IS_ABSOLUTELY_POSITIONED) {
-                flow::base(&*wrapper_flow).flags.contains(IS_ABSOLUTELY_POSITIONED);
-            if is_absolutely_positioned {
                 // This is now the only absolute flow in the subtree which hasn't yet
                 // reached its containing block.
                 abs_descendants.push(wrapper_flow.clone());

components/layout/flow.rs

@@ -404,7 +404,7 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
 
     /// Returns true if this is an absolute containing block.
     fn is_absolute_containing_block(&self) -> bool {
-        false
+        self.contains_positioned_fragments()
     }
 
     /// Updates the inline position of a child flow during the assign-height traversal. At present,

components/layout/inline.rs

@@ -1486,7 +1486,7 @@ impl Flow for InlineFlow {
             indentation = Au(0)
         }
 
-        if self.contains_positioned_fragments() {
+        if self.is_absolute_containing_block() {
             // Assign block-sizes for all flows in this absolute flow tree.
             // This is preorder because the block-size of an absolute flow may depend on
             // the block-size of its containing block, which may also be an absolute flow.