prevent jittery ajaxify in ACP when switching from a short page to a long one

public/less/admin/admin.less

@@ -30,6 +30,9 @@
 @import "./modules/snackbar";
 @import "./modules/nprogress";
 
+body {
+	overflow-y: scroll;
+}
 
 .admin {
 	background: #fff;