1 files changed, 24 insertions, 0 deletions
diff --git a/components/layout/display_list/mod.rs b/components/layout/display_list/mod.rs
index aa6d3d2f500..ccd7763144f 100644
--- a/components/layout/display_list/mod.rs
+++ b/components/layout/display_list/mod.rs
@@ -2,6 +2,9 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use malloc_size_of_derive::MallocSizeOf;
+use serde::{Deserialize, Serialize};
+
 pub use self::builder::{
     BorderPaintingMode, DisplayListBuildState, IndexableText, StackingContextCollectionFlags,
     StackingContextCollectionState,
@@ -15,3 +18,24 @@ pub(crate) mod conversions;
 mod gradient;
 pub mod items;
 mod webrender_helpers;
+
+/// A unique ID for every stacking context.
+#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]
+pub struct StackingContextId(
+    /// The identifier for this StackingContext, derived from the Flow's memory address
+    /// and fragment type.  As a space optimization, these are combined into a single word.
+    pub u64,
+);
+
+impl StackingContextId {
+    /// Returns the stacking context ID for the outer document/layout root.
+    #[inline]
+    pub fn root() -> StackingContextId {
+        StackingContextId(0)
+    }
+
+    pub fn next(&self) -> StackingContextId {
+        let StackingContextId(id) = *self;
+        StackingContextId(id + 1)
+    }
+}