fix invalidating of thread pane when deleting last message in group sr=mscott, a=asa 268969

mailnews/base/src/nsMsgGroupView.cpp

@@ -508,7 +508,10 @@ NS_IMETHODIMP nsMsgGroupView::OnHdrDeleted(nsIMsgDBHdr *aHdrDeleted, nsMsgKey aP
     {
       thread->RemoveChildAt(0); // get rid of dummy
       nsMsgDBView::RemoveByIndex(viewIndexOfThread - 1);
-      NoteChange(viewIndexOfThread - 1, -1, nsMsgViewNotificationCode::insertOrDelete); // an example where view is not the listener - D&D messages
+      if (m_deletingRows)
+        mIndicesToNoteChange.Add(viewIndexOfThread - 1);
+      else
+        NoteChange(viewIndexOfThread - 1, -1, nsMsgViewNotificationCode::insertOrDelete); // an example where view is not the listener - D&D messages
     }
     else if (rootDeleted)
     {
@@ -520,7 +523,7 @@ NS_IMETHODIMP nsMsgGroupView::OnHdrDeleted(nsIMsgDBHdr *aHdrDeleted, nsMsgKey aP
   {
     nsHashKey *hashKey = AllocHashKeyForHdr(aHdrDeleted);
     if (hashKey)
-    m_groupsTable.Remove(hashKey);
+      m_groupsTable.Remove(hashKey);
     delete hashKey;
   }
   return rv;