Упорядочил описание групп полей, селектов, кнопок и пр.

src/blocks/fields-group/fields-group.less

@@ -65,10 +65,10 @@
   .field-select__select,
   .btn {
     position: relative;
-    z-index: 1;
+    z-index: 0;
 
-    &:not(:focus) {
-      z-index: 0;
+    &:hover, &:focus {
+      z-index: 1;
     }
   }
 }

src/less/mixins/btn-mixins.less

@@ -10,6 +10,7 @@
 .btn() {
   display: inline-flex;
   align-items: center;
+  justify-content: center;
   position: relative;
   padding: @field-padding-top @field-padding-horizontal @field-padding-bottom;
   text-align: center;