[IMP] website: add a z-index on the header for shadow option

We need a z-index on the header to make the shadow option work properly
We also needed to increase the z-index of the popup snippet so that it
goes above the header when it is displayed.

Part of https://github.com/odoo/odoo/pull/49142
task-2229551

closes odoo/odoo#49142

Related: odoo/design-themes#238
Related: odoo/upgrade#1432
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website/static/src/scss/website.scss

@@ -637,7 +637,6 @@ $-transition-duration: 200ms;
     display: block;
     @include o-position-absolute(0, 0, auto, 0);
     position: fixed;
-    z-index: $zindex-modal - 20;
     background: $light;
 
     &:not(.o_header_no_transition) {
@@ -703,6 +702,10 @@ $-transition-duration: 200ms;
 
 // Header templates
 header {
+    &#top {
+        // We need this z-index for the shadow option of the header
+        z-index: $zindex-modal - 20;
+    }
     &:not(.o_header_no_transition) {
         .navbar-brand {
             transition: font-size $-transition-duration, opacity $-transition-duration ease-out;

addons/website/static/src/snippets/s_popup/000.scss

@@ -68,7 +68,7 @@
             }
         }
         .s_popup_content {
-            z-index: 1;
+            z-index: $zindex-modal;
             position: fixed;
             right: 20px;
         }