chore(sfc-playground): fix header z-index issue (#14479)

close #14478

packages-private/sfc-playground/src/Header.vue

@@ -170,7 +170,6 @@ nav {
   background-color: var(--bg);
   box-shadow: 0 0 4px rgba(0, 0, 0, 0.33);
   position: relative;
-  z-index: 999;
   display: flex;
   justify-content: space-between;
 }

packages-private/sfc-playground/src/VersionSelect.vue

@@ -106,6 +106,7 @@ onMounted(() => {
 
 <style>
 .version {
+  z-index: 1;
   margin-right: 12px;
   position: relative;
 }