diff --git a/content/base/public/nsContentUtils.h b/content/base/public/nsContentUtils.h
index 86860aee6dfc..f53c8bf323c6 100644
--- a/content/base/public/nsContentUtils.h
+++ b/content/base/public/nsContentUtils.h
@@ -187,6 +187,7 @@ public:
 
   /**
    * Returns the parent node of aChild crossing document boundaries.
+   * Uses the parent node in the composed document.
    */
   static nsINode* GetCrossDocParentNode(nsINode* aChild);
 
@@ -217,7 +218,8 @@ public:
 
   /**
    * Similar to ContentIsDescendantOf except it crosses document boundaries,
-   * also crosses ShadowRoot boundaries from ShadowRoot to its host.
+   * this function uses ancestor/descendant relations in the composed document
+   * (see shadow DOM spec).
    */
   static bool ContentIsCrossDocDescendantOf(nsINode* aPossibleDescendant,
                                               nsINode* aPossibleAncestor);
diff --git a/content/base/src/nsContentUtils.cpp b/content/base/src/nsContentUtils.cpp
index c4adf16fdc07..015f26156e24 100644
--- a/content/base/src/nsContentUtils.cpp
+++ b/content/base/src/nsContentUtils.cpp
@@ -2025,6 +2025,10 @@ nsContentUtils::GetCrossDocParentNode(nsINode* aChild)
   NS_PRECONDITION(aChild, "The child is null!");
 
   nsINode* parent = aChild->GetParentNode();
+  if (parent && parent->IsContent() && aChild->IsContent()) {
+    parent = aChild->AsContent()->GetFlattenedTreeParent();
+  }
+
   if (parent || !aChild->IsNodeOfType(nsINode::eDOCUMENT))
     return parent;
 
@@ -2083,12 +2087,6 @@ nsContentUtils::ContentIsCrossDocDescendantOf(nsINode* aPossibleDescendant,
     if (aPossibleDescendant == aPossibleAncestor)
       return true;
 
-    // Step over shadow root to the host node.
-    ShadowRoot* shadowRoot = ShadowRoot::FromNode(aPossibleDescendant);
-    if (shadowRoot) {
-      aPossibleDescendant = shadowRoot->GetHost();
-    }
-
     aPossibleDescendant = GetCrossDocParentNode(aPossibleDescendant);
   } while (aPossibleDescendant);