Add a z-index to CodeMirror-scroll to optimize drawing

Issue codemirror#6859

Author: marijnh

lib/codemirror.css

@@ -164,6 +164,7 @@ div.CodeMirror span.CodeMirror-nonmatchingbracket {color: #a22;}
  height: 100%;
  outline: none; /* Prevent dragging from highlighting the element */
  position: relative;
+ z-index: 0;
 }
 .CodeMirror-sizer {
  position: relative;