diff --git a/accessible/generic/Accessible.h b/accessible/generic/Accessible.h
index 2385c776c853..7b49672cdaf6 100644
--- a/accessible/generic/Accessible.h
+++ b/accessible/generic/Accessible.h
@@ -14,6 +14,7 @@
 #include "mozilla/UniquePtr.h"
 
 #include "nsIContent.h"
+#include "nsIContentInlines.h"
 #include "nsString.h"
 #include "nsTArray.h"
 #include "nsRefPtrHashtable.h"
diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index 646891d8eea9..7df6a7cdac19 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -151,10 +151,15 @@ nsIContent::FindFirstNonChromeOnlyAccessContent() const
   return nullptr;
 }
 
-nsIContent*
-nsIContent::GetFlattenedTreeParent() const
+nsINode*
+nsIContent::GetFlattenedTreeParentNodeInternal() const
 {
-  nsIContent* parent = GetParent();
+  nsINode* parentNode = GetParentNode();
+  if (!parentNode || !parentNode->IsContent()) {
+    MOZ_ASSERT(!parentNode || parentNode == OwnerDoc());
+    return parentNode;
+  }
+  nsIContent* parent = parentNode->AsContent();
 
   if (parent && nsContentUtils::HasDistributedChildren(parent) &&
       nsContentUtils::IsInSameAnonymousTree(parent, this)) {
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 4a29532292e4..0878a74bdb7b 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -107,6 +107,7 @@
 #include "nsIChromeRegistry.h"
 #include "nsIConsoleService.h"
 #include "nsIContent.h"
+#include "nsIContentInlines.h"
 #include "nsIContentSecurityPolicy.h"
 #include "nsIContentSink.h"
 #include "nsIContentViewer.h"
diff --git a/dom/base/nsIContent.h b/dom/base/nsIContent.h
index 6a03e6f06537..c52396258ec7 100644
--- a/dom/base/nsIContent.h
+++ b/dom/base/nsIContent.h
@@ -719,13 +719,15 @@ public:
   virtual void SetXBLInsertionParent(nsIContent* aContent) = 0;
 
   /**
-   * Returns the content node that is the parent of this node in the flattened
-   * tree. For nodes that are not filtered into an insertion point, this
-   * simply returns their DOM parent in the original DOM tree.
-   *
-   * @return the flattened tree parent
+   * Same as GetFlattenedTreeParentNode, but returns null if the parent is
+   * non-nsIContent.
    */
-  nsIContent *GetFlattenedTreeParent() const;
+  inline nsIContent *GetFlattenedTreeParent() const;
+
+  /**
+   * Helper method, which we leave public so that it's accessible from nsINode.
+   */
+  nsINode *GetFlattenedTreeParentNodeInternal() const;
 
   /**
    * Gets the custom element data used by web components custom element.
diff --git a/dom/base/nsIContentInlines.h b/dom/base/nsIContentInlines.h
index a9fd95bfeda8..16fde9e56e26 100644
--- a/dom/base/nsIContentInlines.h
+++ b/dom/base/nsIContentInlines.h
@@ -33,5 +33,39 @@ inline mozilla::dom::ShadowRoot* nsIContent::GetShadowRoot() const
   return AsElement()->FastGetShadowRoot();
 }
 
+inline nsINode* nsINode::GetFlattenedTreeParentNode() const
+{
+  nsINode* parent = GetParentNode();
+
+  // Try to short-circuit past the complicated and not-exactly-fast logic for
+  // computing the flattened parent.
+  //
+  // There are three cases where we need might something other than parentNode:
+  //   (1) The node is an explicit child of an XBL-bound element, re-bound
+  //       to an XBL insertion point.
+  //   (2) The node is a top-level element in a shadow tree, whose flattened
+  //       parent is the host element (as opposed to the actual parent which
+  //       is the shadow root).
+  //   (3) The node is an explicit child of an element with a shadow root,
+  //       re-bound to an insertion point.
+  bool needSlowCall = HasFlag(NODE_MAY_BE_IN_BINDING_MNGR) ||
+                      IsInShadowTree() ||
+                      (parent && parent->IsContent() &&
+                       parent->AsContent()->GetShadowRoot());
+  if (MOZ_UNLIKELY(needSlowCall)) {
+    MOZ_ASSERT(IsContent());
+    return AsContent()->GetFlattenedTreeParentNodeInternal();
+  }
+
+  return parent;
+}
+
+inline nsIContent*
+nsIContent::GetFlattenedTreeParent() const
+{
+  nsINode* parent = GetFlattenedTreeParentNode();
+  return (parent && parent->IsContent()) ? parent->AsContent() : nullptr;
+}
+
 
 #endif // nsIContentInlines_h
diff --git a/dom/base/nsINode.h b/dom/base/nsINode.h
index 215dce51d6ee..893f014f7e01 100644
--- a/dom/base/nsINode.h
+++ b/dom/base/nsINode.h
@@ -928,6 +928,16 @@ public:
     return mParent;
   }
 
+  /**
+   * Returns the node that is the parent of this node in the flattened
+   * tree. This differs from the normal parent if the node is filtered
+   * into an insertion point, or if the node is a direct child of a
+   * shadow root.
+   *
+   * @return the flattened tree parent
+   */
+  inline nsINode* GetFlattenedTreeParentNode() const;
+
   /**
    * Get the parent nsINode for this node if it is an Element.
    * @return the parent node
diff --git a/dom/events/EventStateManager.cpp b/dom/events/EventStateManager.cpp
index fc0734c9dc2f..f9a5e5075969 100644
--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -29,6 +29,7 @@
 #include "nsCOMPtr.h"
 #include "nsFocusManager.h"
 #include "nsIContent.h"
+#include "nsIContentInlines.h"
 #include "nsIDocument.h"
 #include "nsIFrame.h"
 #include "nsIWidget.h"
diff --git a/dom/html/HTMLLinkElement.cpp b/dom/html/HTMLLinkElement.cpp
index 4743570d088a..1e166b172d67 100644
--- a/dom/html/HTMLLinkElement.cpp
+++ b/dom/html/HTMLLinkElement.cpp
@@ -17,6 +17,7 @@
 #include "nsGenericHTMLElement.h"
 #include "nsGkAtoms.h"
 #include "nsDOMTokenList.h"
+#include "nsIContentInlines.h"
 #include "nsIDocument.h"
 #include "nsIDOMEvent.h"
 #include "nsIDOMStyleSheet.h"
diff --git a/dom/svg/SVGAnimationElement.cpp b/dom/svg/SVGAnimationElement.cpp
index c9d1191daf4b..838db997abcc 100644
--- a/dom/svg/SVGAnimationElement.cpp
+++ b/dom/svg/SVGAnimationElement.cpp
@@ -10,6 +10,7 @@
 #include "nsSMILAnimationController.h"
 #include "nsSMILAnimationFunction.h"
 #include "nsContentUtils.h"
+#include "nsIContentInlines.h"
 #include "nsIURI.h"
 #include "prtime.h"
 
diff --git a/dom/svg/SVGSVGElement.h b/dom/svg/SVGSVGElement.h
index 56738380fe36..f0a056e69fc1 100644
--- a/dom/svg/SVGSVGElement.h
+++ b/dom/svg/SVGSVGElement.h
@@ -9,6 +9,7 @@
 
 #include "mozilla/dom/FromParser.h"
 #include "nsAutoPtr.h"
+#include "nsIContentInlines.h"
 #include "nsISVGPoint.h"
 #include "nsSVGEnum.h"
 #include "nsSVGLength2.h"
diff --git a/layout/base/RestyleTracker.h b/layout/base/RestyleTracker.h
index 5247ed423c46..95ec9064d80f 100644
--- a/layout/base/RestyleTracker.h
+++ b/layout/base/RestyleTracker.h
@@ -16,6 +16,7 @@
 #include "nsAutoPtr.h"
 #include "nsClassHashtable.h"
 #include "nsContainerFrame.h"
+#include "nsIContentInlines.h"
 #include "mozilla/SplayTree.h"
 #include "mozilla/RestyleLogging.h"
 #include "GeckoProfiler.h"
diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index 9f80bd953890..98cc84ac7284 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -23,6 +23,7 @@
 #include "nsFrameList.h"
 #include "nsPlaceholderFrame.h"
 #include "nsIContent.h"
+#include "nsIContentInlines.h"
 #include "nsContentUtils.h"
 #include "nsCSSPseudoElements.h"
 #include "nsIAtom.h"
diff --git a/layout/generic/nsPlaceholderFrame.cpp b/layout/generic/nsPlaceholderFrame.cpp
index 5c083ba1850b..2b6799f48539 100644
--- a/layout/generic/nsPlaceholderFrame.cpp
+++ b/layout/generic/nsPlaceholderFrame.cpp
@@ -18,6 +18,7 @@
 #include "nsPresContext.h"
 #include "nsRenderingContext.h"
 #include "nsIFrameInlines.h"
+#include "nsIContentInlines.h"
 
 using namespace mozilla;
 using namespace mozilla::gfx;
diff --git a/layout/inspector/inDOMUtils.cpp b/layout/inspector/inDOMUtils.cpp
index 8a9fed8a13be..c979cb1ab4c1 100644
--- a/layout/inspector/inDOMUtils.cpp
+++ b/layout/inspector/inDOMUtils.cpp
@@ -14,6 +14,7 @@
 #include "nsISupportsArray.h"
 #include "nsString.h"
 #include "nsIStyleSheetLinkingElement.h"
+#include "nsIContentInlines.h"
 #include "nsIDOMElement.h"
 #include "nsIDocument.h"
 #include "nsIPresShell.h"