Fix duplicate stacking context creation for anonymous Flows

Anonymous nodes were previously creating duplicate stacking contexts,
one for each node in the anonymous node chain. This change eliminates
that for tables.

Additionally the use of stacking context ids based on node addresses is
no longer necessary since stacking contexts no longer control scrolling.
This is the first step in eliminating the dependency between node
addresses and ClipScrollNodes which causes issues like #16425.

1 files changed, 48 insertions, 13 deletions

diff --git a/components/layout/display_list_builder.rs b/components/layout/display_list_builder.rs
index f29a1f377ae..af08a690ca0 100644
--- a/components/layout/display_list_builder.rs
+++ b/components/layout/display_list_builder.rs
@@ -181,6 +181,9 @@ pub struct StackingContextCollectionState {
     /// The current stacking real context id, which doesn't include pseudo-stacking contexts.
     pub current_real_stacking_context_id: StackingContextId,
 
+    /// The next stacking context id that we will assign to a stacking context.
+    pub next_stacking_context_id: StackingContextId,
+
     /// The current clip and scroll info, used to keep track of state when
     /// recursively building and processing the display list.
     pub current_clip_and_scroll_info: ClipAndScrollInfo,
@@ -212,6 +215,7 @@ impl StackingContextCollectionState {
             clip_scroll_node_parents: FnvHashMap::default(),
             current_stacking_context_id: StackingContextId::root(),
             current_real_stacking_context_id: StackingContextId::root(),
+            next_stacking_context_id: StackingContextId::root().next(),
             current_clip_and_scroll_info: root_clip_info,
             containing_block_clip_and_scroll_info: root_clip_info,
             clip_stack: Vec::new(),
@@ -220,6 +224,11 @@ impl StackingContextCollectionState {
         }
     }
 
+    fn generate_stacking_context_id(&mut self) -> StackingContextId {
+        let next_stacking_context_id = self.next_stacking_context_id.next();
+        mem::replace(&mut self.next_stacking_context_id, next_stacking_context_id)
+    }
+
     fn add_stacking_context(&mut self,
                             parent_id: StackingContextId,
                             stacking_context: StackingContext) {
@@ -611,10 +620,6 @@ pub trait FragmentDisplayListBuilding {
                                parent_clip_and_scroll_info: ClipAndScrollInfo)
                                -> StackingContext;
 
-
-    /// The id of stacking context this fragment would create.
-    fn stacking_context_id(&self) -> StackingContextId;
-
     fn unique_id(&self, id_type: IdType) -> u64;
 
     fn fragment_type(&self) -> FragmentType;
@@ -2169,10 +2174,6 @@ impl FragmentDisplayListBuilding for Fragment {
         }
     }
 
-    fn stacking_context_id(&self) -> StackingContextId {
-        StackingContextId::new(self.unique_id(IdType::StackingContext))
-    }
-
     fn create_stacking_context(&self,
                                id: StackingContextId,
                                base_flow: &BaseFlow,
@@ -2394,9 +2395,11 @@ impl FragmentDisplayListBuilding for Fragment {
 bitflags! {
     pub flags StackingContextCollectionFlags: u8 {
         /// This flow never establishes a containing block.
-        const NEVER_CREATES_CONTAINING_BLOCK = 0x01,
+        const NEVER_CREATES_CONTAINING_BLOCK = 0b001,
         /// This flow never creates a ClipScrollNode.
-        const NEVER_CREATES_CLIP_SCROLL_NODE = 0x02,
+        const NEVER_CREATES_CLIP_SCROLL_NODE = 0b010,
+        /// This flow never creates a stacking context.
+        const NEVER_CREATES_STACKING_CONTEXT = 0b100,
     }
 }
 
@@ -2435,6 +2438,11 @@ pub trait BlockFlowDisplayListBuilding {
     fn build_display_list_for_block(&mut self,
                                     state: &mut DisplayListBuildState,
                                     border_painting_mode: BorderPaintingMode);
+
+    fn block_stacking_context_type(
+        &self,
+        flags: StackingContextCollectionFlags,
+    ) -> BlockStackingContextType;
 }
 
 /// This structure manages ensuring that modification to StackingContextCollectionState is
@@ -2572,11 +2580,11 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
                                            flags: StackingContextCollectionFlags) {
         let mut preserved_state = SavedStackingContextCollectionState::new(state);
 
-        let block_stacking_context_type = self.block_stacking_context_type();
+        let block_stacking_context_type = self.block_stacking_context_type(flags);
         self.base.stacking_context_id = match block_stacking_context_type {
             BlockStackingContextType::NonstackingContext => state.current_stacking_context_id,
             BlockStackingContextType::PseudoStackingContext |
-            BlockStackingContextType::StackingContext => self.fragment.stacking_context_id(),
+            BlockStackingContextType::StackingContext => state.generate_stacking_context_id(),
         };
         state.current_stacking_context_id = self.base.stacking_context_id;
 
@@ -2963,6 +2971,33 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
         state.processing_scrolling_overflow_element = false;
     }
 
+    #[inline]
+    fn block_stacking_context_type(
+        &self,
+        flags: StackingContextCollectionFlags,
+    ) -> BlockStackingContextType {
+        if flags.contains(NEVER_CREATES_STACKING_CONTEXT) {
+            return BlockStackingContextType::NonstackingContext;
+        }
+
+        if self.fragment.establishes_stacking_context() {
+            return BlockStackingContextType::StackingContext
+        }
+
+        if self.base.flags.contains(IS_ABSOLUTELY_POSITIONED) {
+            return BlockStackingContextType::PseudoStackingContext
+        }
+
+        if self.fragment.style.get_box().position != position::T::static_ {
+            return BlockStackingContextType::PseudoStackingContext
+        }
+
+        if self.base.flags.is_float() {
+            return BlockStackingContextType::PseudoStackingContext
+        }
+
+        BlockStackingContextType::NonstackingContext
+    }
 }
 
 pub trait InlineFlowDisplayListBuilding {
@@ -2988,7 +3023,7 @@ impl InlineFlowDisplayListBuilding for InlineFlow {
 
             if !fragment.collect_stacking_contexts_for_blocklike_fragment(state) {
                 if fragment.establishes_stacking_context() {
-                    fragment.stacking_context_id = fragment.stacking_context_id();
+                    fragment.stacking_context_id = state.generate_stacking_context_id();
 
                     let current_stacking_context_id = state.current_stacking_context_id;
                     let stacking_context =