Bug 272552: XUL tree splitters visible post-merge. Recheckin toolkit part of bug 265165.

toolkit/content/xul.css

@@ -803,6 +803,8 @@ grippy {
 .tree-splitter {
   width: 0px;
   max-width: 0px;
+  min-width: 0% ! important;
+  min-height: 0% ! important;
   -moz-box-ordinal-group: 2147483646;
 }