182099 - lock up on stylesheets tab double-clicking the twisty

r=varga, sr=bzbarsky, a=sspitzer

extensions/inspector/resources/content/viewers/stylesheets/stylesheets.js

@@ -238,7 +238,7 @@ function(aRow)
     if (this.mLevels[i] < baseLevel)
       return i;
   }
-  return 0;
+  return -1;
 }
 
 StyleSheetsView.prototype.hasNextSibling = 

layout/xul/base/src/tree/public/nsITreeView.idl

@@ -116,7 +116,8 @@ interface nsITreeView : nsISupports
   
   /** 
    * Methods used by the tree to draw vertical lines in the tree.
-   * GetParentIndex is used to obtain the index of a parent row.
+   * getParentIndex is used to obtain the index of a parent row.
+   * If there is no parent row, getParentIndex returns -1.
    */
   long getParentIndex(in long rowIndex);