Fixes #13066: Add z-index to the form control feedback icons so input groups don't render on top of them

less/forms.less

@@ -292,6 +292,7 @@ input[type="checkbox"],
   position: absolute;
   top: (@line-height-computed + 5); // Height of the `label` and its margin
   right: 0;
+  z-index: 2; // Ensure icon is above input groups
   display: block;
   width: @input-height-base;
   height: @input-height-base;