change mask background to sticky position

Author: dabeng

dist/css/jquery.orgchart.css

@@ -51,7 +51,9 @@
 }
 
 .orgchart ~ .mask {
-  position: absolute;
+  position: sticky;
+  width: 100%;
+  height: 100%;
   top: 0;
   right: 0;
   bottom: 0;