Bug 1685146 - Add ActiveScrolledRoot::IsProperAncestor(). r=tnikkel

Depends on D123700

Differential Revision: https://phabricator.services.mozilla.com/D123701

layout/painting/nsDisplayList.cpp

@@ -207,6 +207,13 @@ bool ActiveScrolledRoot::IsAncestor(const ActiveScrolledRoot* aAncestor,
   return false;
 }
 
+/* static */
+bool ActiveScrolledRoot::IsProperAncestor(
+    const ActiveScrolledRoot* aAncestor,
+    const ActiveScrolledRoot* aDescendant) {
+  return aAncestor != aDescendant && IsAncestor(aAncestor, aDescendant);
+}
+
 /* static */
 nsCString ActiveScrolledRoot::ToString(
     const ActiveScrolledRoot* aActiveScrolledRoot) {

layout/painting/nsDisplayList.h

@@ -278,6 +278,8 @@ struct ActiveScrolledRoot {
 
   static bool IsAncestor(const ActiveScrolledRoot* aAncestor,
                          const ActiveScrolledRoot* aDescendant);
+  static bool IsProperAncestor(const ActiveScrolledRoot* aAncestor,
+                               const ActiveScrolledRoot* aDescendant);
 
   static nsCString ToString(const ActiveScrolledRoot* aActiveScrolledRoot);