Merge branch 'fix-header-z-index' into 'master'

Replaced duplicate z-index with index of 400

Closes #32609

See merge request !11547

Conflicts:
	app/assets/stylesheets/framework/header.scss

app/assets/stylesheets/framework/header.scss

@@ -24,12 +24,19 @@ header {
   &.navbar-gitlab {
     padding: 0 16px;
-    z-index: 100;
+    z-index: 400;
     margin-bottom: 0;
     min-height: $header-height;
     background-color: $gray-light;
     border: none;
     border-bottom: 1px solid $border-color;
+<<<<<<< HEAD
+=======
+    position: fixed;
+    top: 0;
+    left: 0;
+    right: 0;
+>>>>>>> 3c45906... Merge branch 'fix-header-z-index' into 'master'
     @media (max-width: $screen-xs-min) {
       padding: 0 16px;