Bug 1272409 - Part 1: Add GetNodeDepth() to nsContentUtils. r=smaug

`GetNodeDepth()` will be used in the next patch.

Depends on D27614

Differential Revision: https://phabricator.services.mozilla.com/D27615

--HG--
extra : moz-landing-system : lando

dom/base/nsContentUtils.cpp

@@ -10621,6 +10621,21 @@ bool nsContentUtils::
          nsThreadManager::MainThreadHasPendingHighPriorityEvents();
 }
 
+/* static */
+uint32_t nsContentUtils::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;
+}
+
 /* static */
 bool nsContentUtils::IsURIInPrefList(nsIURI* aURI, const char* aPrefName) {
   MOZ_ASSERT(aPrefName);

dom/base/nsContentUtils.h

@@ -3360,6 +3360,24 @@ class nsContentUtils {
   static bool HighPriorityEventPendingForTopLevelDocumentBeforeContentfulPaint(
       Document* aDocument);
 
+  /**
+   * 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.
+   *
+   * However, these is a spec issue about how to handle shadow DOM case. We
+   * may need to update this function later:
+   *   https://github.com/w3c/csswg-drafts/issues/3840
+   *
+   * https://drafts.csswg.org/resize-observer/#calculate-depth-for-node-h
+   */
+  static uint32_t GetNodeDepth(nsINode* aNode);
+
  private:
   static bool InitializeEventTable();