Forcing z-index is not needed anymore.

services/web/public/stylesheets/app/editor.less

@@ -305,7 +305,6 @@
 }
 
 .ui-layout-resizer when (@is-overleaf = true) {
-	z-index: 5 !important;
 	width: @ui-resizer-size !important;
 	background-color: @editor-resizer-bg-color;
 	&.ui-layout-resizer-closed {