Apply z-index rule to container, not to the button.

services/web/public/stylesheets/app/editor/review-panel.less

@@ -889,6 +889,7 @@
 	top: 0;
 	right: 0;
 	font-size: 11px;
+	z-index: 2;
 	
 	.rp-size-mini & {
 		right: @review-off-width;
@@ -901,7 +902,6 @@
 		color: @rp-type-blue;
 		text-align: center;
 		border-bottom-left-radius: 3px;
-		z-index: 2;
 		white-space: nowrap;
 
 		&.rp-track-changes-indicator-on-dark {