Only make the check for sameness if there are listeners or if we plan to fire

notifications.  Trying to fix small Tp regression. Bug still 209634, r=caillon,
sr=jag

content/html/content/src/nsGenericHTMLElement.cpp

@@ -1968,12 +1968,29 @@ nsGenericHTMLElement::SetHTMLAttribute(nsIAtom* aAttribute,
 {
   nsresult  result = NS_OK;
 
-  // Do nothing if there is no change.  Note that this assumes that two
-  // attributes that have the same nsISupports value are in fact equal.
-  nsHTMLValue oldValue;
-  result = GetHTMLAttribute(aAttribute, oldValue);
-  if (result != NS_CONTENT_ATTR_NOT_THERE && oldValue == aValue) {
-    return NS_OK;
+  PRBool haveListeners =
+    mDocument &&
+    nsGenericElement::HasMutationListeners(this,
+                                           NS_EVENT_BITS_MUTATION_ATTRMODIFIED);
+  
+  // If we have no listeners and aNotify is false, we are almost certainly
+  // coming from the content sink and will almost certainly have no previous
+  // value.  Even if we do, setting the value is cheap when we have no
+  // listeners and don't plan to notify.  The check for aNotify here is an
+  // optimization; the check for haveListeners is a correctness issue.
+  if (haveListeners || aNotify) {
+    // Do nothing if there is no change.  Note that operator== on nsHTMLValue
+    // assumes that two nsHTMLValues are the same if they have the same unit
+    // and value.  For nsHTMLValues whose value is an nsISupports, this means
+    // that a new nsISupports pointer _must_ be created in order for attribute
+    // changes to take proper effect. Currently, this only applies to inline
+    // style, which satisfies this constraint because style rules are
+    // immutable.
+    nsHTMLValue oldValue;
+    result = GetHTMLAttribute(aAttribute, oldValue);
+    if (result != NS_CONTENT_ATTR_NOT_THERE && oldValue == aValue) {
+      return NS_OK;
+    }
   }
   
   nsChangeHint impact = NS_STYLE_HINT_NONE;
@@ -1981,9 +1998,6 @@ nsGenericHTMLElement::SetHTMLAttribute(nsIAtom* aAttribute,
                            impact);
   nsCOMPtr<nsIHTMLStyleSheet> sheet;
   if (mDocument) {
-    PRBool haveListeners =
-      nsGenericElement::HasMutationListeners(this,
-                                             NS_EVENT_BITS_MUTATION_ATTRMODIFIED);
     PRBool modification = PR_TRUE;
     nsAutoString oldValueStr;
     if (haveListeners) {