Issue #1783 - Part 1: Move GetNodeDepth to ResizeObserver.cpp

GetNodeDepth() is a specialized version for ResizeObserver to get the depth of
a node across Shadow DOM. It's better to have this in ResizeObserver.cpp
instead of the generic ContentUtils.

Also updated the function to bypass the shadow root itself and not count it
because ResizeObserver doesn't observe the shadow root and only needs the
relative depths among all observed targets. IOW we use the flattened tree here.

dom/base/ResizeObserver.cpp

@@ -6,13 +6,43 @@
 #include "mozilla/dom/ResizeObserver.h"
 
 #include "mozilla/dom/DOMRect.h"
-#include "nsContentUtils.h"
+#include "nsIContentInlines.h"
 #include "nsIFrame.h"
 #include "nsSVGUtils.h"
 
 namespace mozilla {
 namespace dom {
 
+/**
+ * Returns the length of the parent-traversal path (in terms of the number of
+ * nodes) to an unparented/root node from aNode. An unparented/root node is
+ * considered to have a depth of 1, its children have a depth of 2, etc.
+ * aNode is expected to be non-null.
+ * Note: The shadow root is not part of the calculation because the caller,
+ * ResizeObserver, doesn't observe the shadow root, and only needs relative
+ * depths among all the observed targets. In other words, we calculate the
+ * depth of the flattened tree.
+ *
+ * Note: these is a spec issue about how to handle shadow DOM case. We
+ * may need to update this function later.
+ *
+ *
+ * https://drafts.csswg.org/resize-observer/#calculate-depth-for-node-h
+ */
+static uint32_t GetNodeDepth(nsINode* aNode) {
+  uint32_t depth = 1;
+
+  MOZ_ASSERT(aNode, "Node shouldn't be null");
+
+  // Use GetFlattenedTreeParentNode to bypass the shadow root and cross the
+  // shadow boundary to calculate the node depth without the shadow root.
+  while ((aNode = aNode->GetFlattenedTreeParentNode())) {
+    ++depth;
+  }
+
+  return depth;
+}
+
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ResizeObserver)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
   NS_INTERFACE_MAP_ENTRY(nsISupports)
@@ -128,8 +158,7 @@ ResizeObserver::GatherActiveObservations(uint32_t aDepth)
 
   for (auto observation : mObservationList) {
     if (observation->IsActive()) {
-      uint32_t targetDepth =
-        nsContentUtils::GetNodeDepth(observation->Target());
+      uint32_t targetDepth = GetNodeDepth(observation->Target());
 
       if (targetDepth > aDepth) {
         mActiveTargets.AppendElement(observation);
@@ -176,8 +205,7 @@ ResizeObserver::BroadcastActiveObservations()
       // will be based on the updated size from last delivered observations.
       observation->UpdateBroadcastSize(rect);
 
-      uint32_t targetDepth =
-        nsContentUtils::GetNodeDepth(observation->Target());
+      uint32_t targetDepth = GetNodeDepth(observation->Target());
 
       if (targetDepth < shallowestTargetDepth) {
         shallowestTargetDepth = targetDepth;

dom/base/nsContentUtils.cpp

@@ -9781,17 +9781,3 @@ nsContentUtils::GetClosestNonNativeAnonymousAncestor(Element* aElement)
   }
   return e;
 }
-
-/* static */ uint32_t
-nsContentUtils::GetNodeDepth(nsINode* aNode)
-{
-  uint32_t depth = 1;
-
-  MOZ_ASSERT(aNode, "Node shouldn't be null");
-
-  while ((aNode = aNode->GetParentNode())) {
-    ++depth;
-  }
-
-  return depth;
-}

dom/base/nsContentUtils.h

@@ -2756,14 +2756,6 @@ public:
   static bool
   IsCustomElementsEnabled() { return sIsCustomElementsEnabled; }
 
-  /**
-   * Returns the length of the parent-traversal path (in terms of the number of
-   * nodes) to an unparented/root node from aNode. An unparented/root node is
-   * considered to have a depth of 1, its children have a depth of 2, etc.
-   * aNode is expected to be non-null.
-   */
-  static uint32_t GetNodeDepth(nsINode* aNode);
-
 private:
   static bool InitializeEventTable();