Bug 1431000 - Introduce nsINode::InsertChildBefore, r=catalinb

dom/html/HTMLFieldSetElement.cpp

@@ -139,6 +139,39 @@ HTMLFieldSetElement::SubmitNamesValues(HTMLFormSubmission* aFormSubmission)
   return NS_OK;
 }
 
+nsresult
+HTMLFieldSetElement::InsertChildBefore(nsIContent* aChild,
+                                       nsIContent* aBeforeThis,
+                                       bool aNotify)
+{
+  bool firstLegendHasChanged = false;
+
+  if (aChild->IsHTMLElement(nsGkAtoms::legend)) {
+    if (!mFirstLegend) {
+      mFirstLegend = aChild;
+      // We do not want to notify the first time mFirstElement is set.
+    } else {
+      // If mFirstLegend is before aIndex, we do not change it.
+      // Otherwise, mFirstLegend is now aChild.
+      int32_t index = aBeforeThis ? ComputeIndexOf(aBeforeThis) : GetChildCount();
+      if (index <= ComputeIndexOf(mFirstLegend)) {
+        mFirstLegend = aChild;
+        firstLegendHasChanged = true;
+      }
+    }
+  }
+
+  nsresult rv =
+    nsGenericHTMLFormElement::InsertChildBefore(aChild, aBeforeThis, aNotify);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (firstLegendHasChanged) {
+    NotifyElementsForFirstLegendChange(aNotify);
+  }
+
+  return rv;
+}
+
 nsresult
 HTMLFieldSetElement::InsertChildAt_Deprecated(nsIContent* aChild,
                                               uint32_t aIndex,