add invisible splitter

www/code/code.less

@@ -59,6 +59,7 @@ body {
     box-sizing: border-box;
     font-family: Calibri,Ubuntu,sans-serif;
     word-wrap: break-word;
+    position: relative;
     media-tag {
         * {
             max-width:100%;
@@ -77,6 +78,17 @@ body {
     .markdown_gfm-table(black);
 }
 
+.cp-splitter {
+    position: absolute;
+    height: 100%;
+    width: 8px;
+    top: 0;
+    left: 0;
+    z-index: 9999;
+
+    cursor: col-resize;
+}
+
 @media (max-width: @media-medium-screen) {
     .CodeMirror {
         flex: 1;