diff --git a/lib/codemirror.css b/lib/codemirror.css
index 23eaf74d449bcd5cdb4e952e5b2846428e8ebb71..3df9948513e51056ea0159e10c8b8ce8dcca9a99 100644
--- a/lib/codemirror.css
+++ b/lib/codemirror.css
@@ -36,6 +36,8 @@
   min-width: 20px;
   text-align: right;
   color: #999;
+  -moz-box-sizing: content-box;
+  box-sizing: content-box;
 }
 
 /* CURSOR */