Issue #2401 - Part 2: Add GetCommonFlattenedTreeAncestor.

Passing GetFlattenedTreeParent() as the "get parent" function.

This way we can reduce the complexity in our EventStateManager by simply
calling out to the templatized function from Part 1.

dom/base/nsContentUtils.cpp

@@ -2442,6 +2442,16 @@ nsContentUtils::GetCommonAncestor(nsINode* aNode1, nsINode* aNode2)
   });
 }
 
+/* static */
+nsIContent*
+nsContentUtils::GetCommonFlattenedTreeAncestor(nsIContent* aContent1,
+                                               nsIContent* aContent2)
+{
+  return GetCommonAncestorInternal(aContent1, aContent2, [](nsIContent* aContent) {
+    return aContent->GetFlattenedTreeParent();
+  });
+}
+
 // static
 nsINode*
 nsContentUtils::GetCommonAncestorUnderInteractiveContent(nsINode* aNode1,

dom/base/nsContentUtils.h

@@ -338,6 +338,13 @@ public:
    */
   static nsINode* GetCommonAncestor(nsINode* aNode1, nsINode* aNode2);
 
+  /**
+   * Returns the common flattened tree ancestor, if any, for two given content
+   * nodes.
+   */
+  static nsIContent* GetCommonFlattenedTreeAncestor(nsIContent* aContent1,
+                                                    nsIContent* aContent2);
+ 
   /**
    * Returns the common ancestor under interactive content, if any.
    * If neither one has interactive content as ancestor, common ancestor will be

dom/events/EventStateManager.cpp

@@ -4849,48 +4849,13 @@ GetLabelTarget(nsIContent* aPossibleLabel)
   return label->GetLabeledElement();
 }
 
-static nsIContent* FindCommonAncestor(nsIContent *aNode1, nsIContent *aNode2)
+static nsIContent*
+FindCommonAncestor(nsIContent *aNode1, nsIContent *aNode2)
 {
-  // Find closest common ancestor
-  if (aNode1 && aNode2) {
-    // Find the nearest common ancestor by counting the distance to the
-    // root and then walking up again, in pairs.
-    int32_t offset = 0;
-    nsIContent *anc1 = aNode1;
-    for (;;) {
-      ++offset;
-      nsIContent* parent = anc1->GetFlattenedTreeParent();
-      if (!parent)
-        break;
-      anc1 = parent;
-    }
-    nsIContent *anc2 = aNode2;
-    for (;;) {
-      --offset;
-      nsIContent* parent = anc2->GetFlattenedTreeParent();
-      if (!parent)
-        break;
-      anc2 = parent;
-    }
-    if (anc1 == anc2) {
-      anc1 = aNode1;
-      anc2 = aNode2;
-      while (offset > 0) {
-        anc1 = anc1->GetFlattenedTreeParent();
-        --offset;
-      }
-      while (offset < 0) {
-        anc2 = anc2->GetFlattenedTreeParent();
-        ++offset;
-      }
-      while (anc1 != anc2) {
-        anc1 = anc1->GetFlattenedTreeParent();
-        anc2 = anc2->GetFlattenedTreeParent();
-      }
-      return anc1;
-    }
+  if (!aNode1 || !aNode2) {
+    return nullptr;
   }
-  return nullptr;
+  return nsContentUtils::GetCommonFlattenedTreeAncestor(aNode1, aNode2);
 }
 
 /* static */