Only unset :hover in NotifyMouseOut if we're not mousing over

anything. Otherwise, let NotifyMouseOver handle changing :hover; it's faster
that way.  Bug 292472, r+sr=roc, a=dbaron

content/events/src/nsEventStateManager.cpp

@@ -2570,9 +2570,15 @@ nsEventStateManager::NotifyMouseOut(nsGUIEvent* aEvent, nsIContent* aMovingInto)
   // Store the first mouseOut event we fire and don't refire mouseOut
   // to that element while the first mouseOut is still ongoing.
   mFirstMouseOutEventElement = mLastMouseOverElement;
-  
-  // Unset :hover
-  SetContentState(nsnull, NS_EVENT_STATE_HOVER);
+
+  // Don't touch hover state if aMovingInto is non-null.  Caller will update
+  // hover state itself, and we have optimizations for hover switching between
+  // two nearby elements both deep in the DOM tree that would be defeated by
+  // switching the hover state to null here.
+  if (!aMovingInto) {
+    // Unset :hover
+    SetContentState(nsnull, NS_EVENT_STATE_HOVER);
+  }
   
   // Fire mouseout
   DispatchMouseEvent(aEvent, NS_MOUSE_EXIT_SYNTH,

content/events/src/nsEventStateManager.h

@@ -180,6 +180,11 @@ protected:
   /**
    * Tell this ESM and ESMs in affected child documents that the mouse
    * has exited this document's currently hovered content.
+   * @param aEvent the event that triggered the mouseout
+   * @param aMovingInto the content node we've moved into.  This is used to set
+   *        the relatedTarget for mouseout events.  Also, if it's non-null
+   *        NotifyMouseOut will NOT change the current hover content to null;
+   *        in that case the caller is responsible for updating hover state.
    */
   void NotifyMouseOut(nsGUIEvent* aEvent, nsIContent* aMovingInto);
   void GenerateDragDropEnterExit(nsPresContext* aPresContext, nsGUIEvent* aEvent);