Checkout: Fix Payment Box Radio (#37177)

* Register z-indexes

* Include z-index

Author: Aurorum

client/assets/stylesheets/shared/functions/_z-index.scss

@@ -35,6 +35,7 @@ $z-layers: (
 		'.NuxWelcome:before': -1,
 		'.is-group-editor::before': -1,
 		'.is-group-gutenberg::before': -1,
+		'.payment-box__pm-wrapper .section-nav-tab::after': -1,
 		'.site-icon.is-blank .gridicon': 0,
 		'.chart__bar-section.is-spacer': 0,
 		'.reader__featured-post': 0,
@@ -64,6 +65,7 @@ $z-layers: (
 		'.plugin-item__label': 1,
 		'.wp-editor-tools': 1,
 		'.plan .gridicons-checkmark-circle': 1,
+		'.payment-box__pm-wrapper .section-nav-tab': 1,
 		'.plan-discount-message': 1,
 		'.post-editor__switch-mode': 1,
 		'.auth__form .form-fieldset input': 1,

client/my-sites/checkout/checkout/style.scss

@@ -1213,6 +1213,7 @@
 		border: 0;
 		margin: 8px 0 0;
 		position: relative;
+		z-index: z-index( 'root', '.payment-box__pm-wrapper .section-nav-tab' );
 
 		&:first-child {
 			margin-top: 0 !important;
@@ -1230,6 +1231,7 @@
 			border: 1px solid var( --color-neutral-10 );
 			border-radius: 50%;
 			box-sizing: border-box;
+			z-index: z-index( 'root', '.payment-box__pm-wrapper .section-nav-tab::after' );
 		}
 
 		&::before {