Improve containing block creation for position:absolute flows

components/layout/block.rs

@@ -448,20 +448,16 @@ pub struct AbsoluteAssignBSizesTraversal<'a>(pub &'a SharedStyleContext<'a>);
 impl<'a> PreorderFlowTraversal for AbsoluteAssignBSizesTraversal<'a> {
     #[inline]
     fn process(&self, flow: &mut Flow) {
-        {
-            // The root of the absolute flow tree is definitely not absolutely
-            // positioned. Nothing to process here.
-            let flow: &Flow = flow;
-            if flow.contains_roots_of_absolute_flow_tree() {
-                return;
-            }
-            if !flow.is_block_like() {
-                return
-            }
+        if !flow.is_block_like() {
+            return
         }
 
+        // This flow might not be an absolutely positioned flow if it is the root of the tree.
         let block = flow.as_mut_block();
-        debug_assert!(block.base.flags.contains(IS_ABSOLUTELY_POSITIONED));
+        if !block.base.flags.contains(IS_ABSOLUTELY_POSITIONED) {
+            return;
+        }
+
         if !block.base.restyle_damage.intersects(REFLOW_OUT_OF_FLOW | REFLOW) {
             return
         }
@@ -2106,6 +2102,17 @@ impl Flow for BlockFlow {
         (self.fragment.border_box - self.fragment.style().logical_border_width()).size
     }
 
+    /// Returns true if this flow contains fragments that are roots of an absolute flow tree.
+    fn contains_roots_of_absolute_flow_tree(&self) -> bool {
+        self.contains_relatively_positioned_fragments() || self.is_root() ||
+        self.fragment.has_filter_transform_or_perspective()
+    }
+
+    /// Returns true if this is an absolute containing block.
+    fn is_absolute_containing_block(&self) -> bool {
+        self.contains_positioned_fragments() || self.fragment.has_filter_transform_or_perspective()
+    }
+
     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/flex.rs

@@ -981,6 +981,14 @@ impl Flow for FlexFlow {
         self.block_flow.compute_overflow()
     }
 
+    fn contains_roots_of_absolute_flow_tree(&self) -> bool {
+        self.block_flow.contains_roots_of_absolute_flow_tree()
+    }
+
+    fn is_absolute_containing_block(&self) -> bool {
+        self.block_flow.is_absolute_containing_block()
+    }
+
     fn generated_containing_block_size(&self, flow: OpaqueFlow) -> LogicalSize<Au> {
         self.block_flow.generated_containing_block_size(flow)
     }

components/layout/flow.rs

@@ -407,6 +407,11 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
         self.contains_positioned_fragments()
     }
 
+    /// Returns true if this flow contains fragments that are roots of an absolute flow tree.
+    fn contains_roots_of_absolute_flow_tree(&self) -> bool {
+        self.contains_relatively_positioned_fragments() || self.is_root()
+    }
+
     /// Updates the inline position of a child flow during the assign-height traversal. At present,
     /// this is only used for absolutely-positioned inline-blocks.
     fn update_late_computed_inline_position_if_necessary(&mut self, inline_position: Au);
@@ -502,9 +507,6 @@ pub trait ImmutableFlowUtils {
     /// Returns true if this flow is one of table-related flows.
     fn is_table_kind(self) -> bool;
 
-    /// Returns true if this flow contains fragments that are roots of an absolute flow tree.
-    fn contains_roots_of_absolute_flow_tree(&self) -> bool;
-
     /// Returns true if this flow has no children.
     fn is_leaf(self) -> bool;
 
@@ -1198,11 +1200,6 @@ impl<'a> ImmutableFlowUtils for &'a Flow {
         }
     }
 
-    /// Returns true if this flow contains fragments that are roots of an absolute flow tree.
-    fn contains_roots_of_absolute_flow_tree(&self) -> bool {
-        self.contains_relatively_positioned_fragments() || self.is_root()
-    }
-
     /// Returns true if this flow has no children.
     fn is_leaf(self) -> bool {
         base(self).children.is_empty()

components/layout/fragment.rs

@@ -2487,6 +2487,13 @@ impl Fragment {
                               stacking_relative_border_box.size.height - border_padding.vertical()))
     }
 
+    /// Returns true if this fragment has a filter, transform, or perspective property set.
+    pub fn has_filter_transform_or_perspective(&self) -> bool {
+           self.style().get_box().transform.0.is_some() ||
+           !self.style().get_effects().filter.0.is_empty() ||
+           self.style().get_box().perspective != Either::Second(values::None_)
+    }
+
     /// Returns true if this fragment establishes a new stacking context and false otherwise.
     pub fn establishes_stacking_context(&self) -> bool {
         // Text fragments shouldn't create stacking contexts.
@@ -2500,22 +2507,17 @@ impl Fragment {
         if self.style().get_effects().opacity != 1.0 {
             return true
         }
-        if !self.style().get_effects().filter.0.is_empty() {
-            return true
-        }
+
         if self.style().get_effects().mix_blend_mode != mix_blend_mode::T::normal {
             return true
         }
 
-        if self.style().get_box().transform.0.is_some() ||
-           self.style().get_box().transform_style == transform_style::T::preserve_3d ||
-           self.style().overrides_transform_style() {
-            return true
+        if self.has_filter_transform_or_perspective() {
+            return true;
         }
 
-        // TODO(mrobinson): Determine if this is necessary, since blocks with
-        // transformations already create stacking contexts.
-        if let Either::First(ref _length) = self.style().get_box().perspective {
+        if self.style().get_box().transform_style == transform_style::T::preserve_3d ||
+           self.style().overrides_transform_style() {
             return true
         }
 

components/layout/list_item.rs

@@ -127,6 +127,10 @@ impl Flow for ListItemFlow {
         self.block_flow.place_float_if_applicable()
     }
 
+    fn contains_roots_of_absolute_flow_tree(&self) -> bool {
+        self.block_flow.contains_roots_of_absolute_flow_tree()
+    }
+
     fn is_absolute_containing_block(&self) -> bool {
         self.block_flow.is_absolute_containing_block()
     }

components/layout/multicol.rs

@@ -201,6 +201,14 @@ impl Flow for MulticolFlow {
         self.block_flow.compute_overflow()
     }
 
+    fn contains_roots_of_absolute_flow_tree(&self) -> bool {
+        self.block_flow.contains_roots_of_absolute_flow_tree()
+    }
+
+    fn is_absolute_containing_block(&self) -> bool {
+        self.block_flow.is_absolute_containing_block()
+    }
+
     fn generated_containing_block_size(&self, flow: OpaqueFlow) -> LogicalSize<Au> {
         self.block_flow.generated_containing_block_size(flow)
     }
@@ -283,6 +291,14 @@ impl Flow for MulticolColumnFlow {
         self.block_flow.compute_overflow()
     }
 
+    fn contains_roots_of_absolute_flow_tree(&self) -> bool {
+        self.block_flow.contains_roots_of_absolute_flow_tree()
+    }
+
+    fn is_absolute_containing_block(&self) -> bool {
+        self.block_flow.is_absolute_containing_block()
+    }
+
     fn generated_containing_block_size(&self, flow: OpaqueFlow) -> LogicalSize<Au> {
         self.block_flow.generated_containing_block_size(flow)
     }

components/layout/table_caption.rs

@@ -91,6 +91,14 @@ impl Flow for TableCaptionFlow {
         self.block_flow.compute_overflow()
     }
 
+    fn contains_roots_of_absolute_flow_tree(&self) -> bool {
+        self.block_flow.contains_roots_of_absolute_flow_tree()
+    }
+
+    fn is_absolute_containing_block(&self) -> bool {
+        self.block_flow.is_absolute_containing_block()
+    }
+
     fn generated_containing_block_size(&self, flow: OpaqueFlow) -> LogicalSize<Au> {
         self.block_flow.generated_containing_block_size(flow)
     }

components/layout/table_cell.rs

@@ -271,6 +271,14 @@ impl Flow for TableCellFlow {
         self.block_flow.compute_overflow()
     }
 
+    fn contains_roots_of_absolute_flow_tree(&self) -> bool {
+        self.block_flow.contains_roots_of_absolute_flow_tree()
+    }
+
+    fn is_absolute_containing_block(&self) -> bool {
+        self.block_flow.is_absolute_containing_block()
+    }
+
     fn generated_containing_block_size(&self, flow: OpaqueFlow) -> LogicalSize<Au> {
         self.block_flow.generated_containing_block_size(flow)
     }

components/layout/table_row.rs

@@ -489,6 +489,14 @@ impl Flow for TableRowFlow {
         self.block_flow.compute_overflow()
     }
 
+    fn contains_roots_of_absolute_flow_tree(&self) -> bool {
+        self.block_flow.contains_roots_of_absolute_flow_tree()
+    }
+
+    fn is_absolute_containing_block(&self) -> bool {
+        self.block_flow.is_absolute_containing_block()
+    }
+
     fn generated_containing_block_size(&self, flow: OpaqueFlow) -> LogicalSize<Au> {
         self.block_flow.generated_containing_block_size(flow)
     }

components/layout/table_rowgroup.rs

@@ -194,6 +194,14 @@ impl Flow for TableRowGroupFlow {
         self.block_flow.compute_overflow()
     }
 
+    fn contains_roots_of_absolute_flow_tree(&self) -> bool {
+        self.block_flow.contains_roots_of_absolute_flow_tree()
+    }
+
+    fn is_absolute_containing_block(&self) -> bool {
+        self.block_flow.is_absolute_containing_block()
+    }
+
     fn generated_containing_block_size(&self, flow: OpaqueFlow) -> LogicalSize<Au> {
         self.block_flow.generated_containing_block_size(flow)
     }

tests/wpt/mozilla/meta/MANIFEST.json

    "reftest"
   ],
   "css/translate_clip_nested.html": [
-   "319619a3ae8d8d1f935fedfecf41f895cb205033",
+   "11adf8b4fa7c3176b002b32f8309c12bfb40043d",
    "reftest"
   ],
   "css/translate_clip_nested_ref.html": [
-   "2f0dc9fe89d71550e1acc76865e950bbb9f2db79",
+   "7481c266d32dc59034490a89321e45a86b47bb1b",
    "support"
   ],
   "css/translate_clip_ref.html": [

tests/wpt/mozilla/tests/css/translate_clip_nested.html

@@ -10,7 +10,14 @@
 <div style="position: relative; overflow: hidden; background: red;">
   <div style="transform: translateX(-100px);">
     <div style="position: absolute; right: -100px; background: green;">
-      1 2 3 4 5 6 7 8 9 10 11 12 13
+    </div>
+  </div>
+</div>
+
+<!-- Repeat the same test, but this time make the outer div a stacking context. -->
+<div style="position: relative; z-index: 1; overflow: hidden; background: red;">
+  <div style="transform: translateX(-100px);">
+    <div style="position: absolute; right: -100px; background: green;">
     </div>
   </div>
 </div>

tests/wpt/mozilla/tests/css/translate_clip_nested_ref.html

@@ -6,8 +6,5 @@
     width: 200px;
   }
 </style>
-<div style="position: relative; overflow: hidden; background: red;">
-  <div style="position: absolute; right: 0; background: green;">
-    1 2 3 4 5 6 7 8 9 10 11 12 13
-  </div>
-</div>
+<div style="position: relative; overflow: hidden; background: green;"> </div>
+<div style="position: relative; overflow: hidden; background: green;"> </div>