feat: update Headroom implementation to support recent Hugo versions (#2950)

It has been reported that recent Hugo versions broke user's configuration of Headroom JS via the `header.on_scroll` option.

With this PR, Headroom JS configuration is working again.

Fixes #2935

modules/wowchemy/layouts/partials/site_js.html

@@ -126,12 +126,43 @@
 <script src="{{ $js_algolia_search.RelPermalink }}" type="module"></script>
 {{ end }}
 
-{{/* Page Data */}}
-{{ $default_headroom := not (.IsHome | or (eq .Type "book")) }}
-{{ $use_headroom := cond (isset $.Params "header.on_scroll") (eq $.Params.header.on_scroll "disappear") (default $default_headroom) }}
+{{/* Headroom */}}
+{{/*
+  Headroom JS enables hiding the header on scrolling down and showing it on scrolling up.
+  This is especially valuable on smaller displays such as mobile devices.
+  This option can be set in the following ways:
+    - globally, in the site's 'params.yaml':
+      ```
+      header:
+        on_scroll: "disappear" | "sticky"
+      ```
+    - per page, in the page front matter:
+      ```
+      header:
+        on_scroll: "disappear" | "sticky"
+      ```
+  When the header is set to disappear, the Headroom JS is loaded.
+
+  By default, the header is sticky in the homepage and in the book layout,
+  and it disappears when scrolling in all other pages.
+*/}}
+
+{{/* By default don't use Headroom in the homepage and book layout */}}
+{{ $use_headroom := not (.IsHome | or (eq .Type "book")) }}
+
+{{/* Check for the global site parameter `header.on_scroll` */}}
+{{ if (isset site.Params.header "on_scroll") }}
+  {{ $use_headroom = eq site.Params.header.on_scroll "disappear" }}
+{{ end }}
+
+{{/* Check for the page parameter `header.on_scroll` */}}
+{{ if (isset $.Params.header "on_scroll") }}
+  {{ $use_headroom = eq $.Params.header.on_scroll "disappear" }}
+{{ end }}
+
+{{/* Output `use_headroom` variable for Headroom initialization in Wowchemy JS */}}
 {{ printf "<script id=\"page-data\" type=\"application/json\">%s</script>" (dict "use_headroom" $use_headroom | jsonify) | safeHTML}}
 
-{{/* Headroom */}}
 {{ if $use_headroom }}
   {{ $headroom := resources.Get "js/wowchemy-headroom.js" | js.Build (dict "format" "esm" "minify" true) }}
   {{- if hugo.IsProduction -}}