Feature: Allow unlimited multi-level navigation (#1431)

* Allow unlimited multi-level navigation

This PR supersedes #462.

The only user-level difference from #462 is that disambiguation of parent pages has to use either `grand_parent` or `ancestor` titles: the somewhat unnatural `section_id` and `in_section` fields are not supported.

The implementation has been significantly simplified by the changes introduced in v0.7.0 of the theme.

* Detect cyclic parenthood

A page should not have a parent or ancestor with the same title. If it does, the location of the repeated link is marked by ∞, to facilitate debugging the navigation (and an unbounded loop leading to a build exception is avoided).

* Add nav_error_report warning in main navigation

When activated by `nav_error_report: true` in `_config.yml`, displays warnings about pages with the same title as their parent page or an ancestral page.

* Cache site-nav with links to all pages

The extra cached site-nav is used for determining breadcrumbs and children navigation, which may involve pages that are excluded from the main navigation.

* Replace code for determining children by inclusion of components/nav/children.html

* Update CHANGELOG.md

---------

Co-authored-by: Matt Wang <matt@matthewwang.me>

CHANGELOG.md

@@ -17,12 +17,16 @@ This website is built from the `HEAD` of the `main` branch of the theme reposito
 
 Code changes to `main` that are *not* in the latest release:
 
-- N/A
+### New Features
+
+- Added: Allow unlimited multi-level navigation by [@pdmosses] in [#1431]
 
 Docs changes made since the latest release:
 
 - N/A
 
+[#1431]: https://github.com/just-the-docs/just-the-docs/pull/1431
+
 ## Release v0.9.0
 
 Hi folks! This minor release adds a `nav_enabled` set of variables to enable/disable the navigation at a site, layout, and page level --- and uses that to add search and auxilary links to the `minimal` layout. In addition, it fixes `search-data.json` corruption with default layouts and some minor CSS/SCSS issues.

_config.yml

@@ -122,6 +122,9 @@ nav_external_links:
   - title: Just the Docs on GitHub
     url: https://github.com/just-the-docs/just-the-docs
 
+# Show navigation error report
+nav_error_report: true # default is false/nil.
+
 liquid:
    error_mode: strict
    strict_filters: true

_includes/components/breadcrumbs.html

@@ -1,40 +1,41 @@
 {%- comment -%}
   Include as: {%- include components/breadcrumbs.html -%}
   Depends on: page, site.
+  Includes: components/site_nav.html.
   Results in: HTML for the breadcrumbs component.
   Overwrites:
-    node, pages_list, parent_page, grandparent_page.
+    nav_list_link, site_nav, nav_list_simple, nav_list_link_class, nav_category,
+    nav_anchor_splits, nav_breadcrumbs, nav_split, nav_split_next, nav_split_test,
+    nav_breadcrumb_link, nav_list_end_less, nav_list_end_count, nav_end_index, nav_breadcrumb.
 {%- endcomment -%}
 
-{%- if page.url != "/" and page.parent -%}
+{%- if page.url != "/" and page.parent and page.title -%}
 
 {%- capture nav_list_link -%}
 <a href="{{ page.url | relative_url }}" class="nav-list-link">
 {%- endcapture -%}
 
 {%- capture site_nav -%}
-{%- include_cached components/site_nav.html -%}
+{%- include_cached components/site_nav.html all=true -%}
 {%- endcapture -%}
 
-{%- if site_nav contains nav_list_link -%}
+{%- capture nav_list_simple -%}
+<ul class="nav-list">
+{%- endcapture -%}
 
-  {%- capture nav_list_simple -%}
-  <ul class="nav-list">
-  {%- endcapture -%}
+{%- capture nav_list_link_class %} class="nav-list-link">
+{%- endcapture -%}
 
-  {%- capture nav_list_link_class %} class="nav-list-link">
-  {%- endcapture -%}
+{%- capture nav_category -%}
+<div class="nav-category">
+{%- endcapture -%}
 
-  {%- capture nav_category -%}
-  <div class="nav-category">
-  {%- endcapture -%}
-
-  {%- assign nav_anchor_splits =
+{%- assign nav_anchor_splits =
       site_nav | split: nav_list_link |
       first | split: nav_category |
       last | split: "</a>" -%}
 
-  {%- comment -%}
+{%- comment -%}
   The ordinary pages (if any) and the collections pages (if any) are separated by
   occurrences of nav_category.
   
@@ -49,96 +50,47 @@
   
   The number of occurrences of a string in nav_split_next is computed by removing
   them all, then dividing the resulting size difference by the length of the string.
-  {%- endcomment %}
+{%- endcomment %}
 
-  {%- assign nav_breadcrumbs = "" | split: "" -%}
+{%- assign nav_breadcrumbs = "" | split: "" -%}
 
-  {%- for nav_split in nav_anchor_splits -%}
-  {%- unless forloop.last -%}
+{%- for nav_split in nav_anchor_splits -%}
+{%- unless forloop.last -%}
 
-  {%- assign nav_split_next = nav_anchor_splits[forloop.index] | strip -%}
+{%- assign nav_split_next = nav_anchor_splits[forloop.index] | strip -%}
 
-  {%- assign nav_split_test =
+{%- assign nav_split_test =
       nav_split_next | remove_first: nav_list_simple | prepend: nav_list_simple -%}
-  {%- if nav_split_test == nav_split_next -%}
+{%- if nav_split_test == nav_split_next -%}
   {%- assign nav_breadcrumb_link =
         nav_split | split: "<a " | last | prepend: "<a " |
         replace: nav_list_link_class, ">" | append: "</a>" -%}
   {%- assign nav_breadcrumbs = nav_breadcrumbs | push: nav_breadcrumb_link -%}
-  {%- endif -%}
+{%- endif -%}
 
-  {%- if nav_split_next contains "</ul>" -%}
+{%- if nav_split_next contains "</ul>" -%}
   {%- assign nav_list_end_less = nav_split_next | remove: "</ul>" -%}
   {%- assign nav_list_end_count =
         nav_split_next.size | minus: nav_list_end_less.size | divided_by: 5 -%}
   {% for nav_end_index in (1..nav_list_end_count) %}
     {%- assign nav_breadcrumbs = nav_breadcrumbs | pop -%}
   {%- endfor -%}
-  {%- endif -%}
-
-  {%- endunless -%}
-  {%- endfor -%}
-
-  {%- assign nav_parent_link = nav_breadcrumbs[-1] -%}
-  {%- assign nav_grandparent_link = nav_breadcrumbs[-2] -%}
-
-{%- else -%}
-
-  {%- comment -%}
-    Pages whose links are excluded from the main navigation may still have
-    breadcrumbs. Determining them appears to require inspecting the front matter
-    of all the pages in the same group. For sites with 100s of pages, this is too
-    inefficient in Jekyll 3 (also when the for-loop is replaced by where-filters).
-  {%- endcomment -%}
-
-  {%- assign pages_list = site[page.collection] | default: site.html_pages -%}
-  
-  {%- assign parent_page = nil -%}
-  {%- assign grandparent_page = nil -%}
-
-  {%- for node in pages_list -%}
-  
-    {%- if node.has_children and page.grand_parent -%}
-
-      {%- if node.title == page.parent and node.parent == page.grand_parent -%}
-        {%- assign parent_page = node -%}
-      {%- endif -%}
-      {%- if node.title ==  page.grand_parent -%}
-        {%- assign grandparent_page = node -%}
-      {%- endif -%}
-      {%- if parent_page and grandparent_page -%}
-        {%- break -%}
-      {%- endif -%}
-
-    {%- elsif node.has_children and node.title == page.parent and node.parent == nil -%}
-
-      {%- assign parent_page = node -%}
-      {%- break -%}
-
-    {%- endif -%}
-
-  {%- endfor -%}
-  
-  {%- capture nav_parent_link -%}
-    <a href="{{ parent_page.url | relative_url }}">{{ page.parent }}</a>
-  {%- endcapture -%}
-
-  {%- if page.grand_parent %}
-    {%- capture nav_grandparent_link -%}
-      <a href="{{ grandparent_page.url | relative_url }}">{{ page.grand_parent }}</a>
-    {%- endcapture -%}
-  {%- endif -%}
-
 {%- endif -%}
 
+{%- endunless -%}
+{%- endfor -%}
+
 <nav aria-label="Breadcrumb" class="breadcrumb-nav">
   <ol class="breadcrumb-nav-list">
-  {%- if nav_grandparent_link %}
-    <li class="breadcrumb-nav-list-item">{{ nav_grandparent_link }}</li>
-  {%- endif %}
-    <li class="breadcrumb-nav-list-item">{{ nav_parent_link }}</li>
+    {%- for nav_breadcrumb in nav_breadcrumbs %}
+    <li class="breadcrumb-nav-list-item">{{ nav_breadcrumb }}</li>
+    {%- endfor %}
     <li class="breadcrumb-nav-list-item"><span>{{ page.title }}</span></li>
   </ol>
 </nav>
 
+{% if site.nav_error_report %}
+{{ nav_error_report }}
+{% endif %}
+
 {%- endif -%}

_includes/components/children_nav.html

@@ -1,33 +1,89 @@
 {%- comment -%}
   Include as: {%- include components/children_nav.html -%}
-  Depends on: page, site.
+  Depends on: page, site, nav_breadcrumbs.
   Results in: HTML for the children-navigation component.
-  Includes:
-    sorted_pages.html
-    toc_heading_custom.html
+  Includes: components/nav/sorted.html, toc_heading_custom.html.
   Overwrites:
-    child_pages.
+    nav_ancestor_links, nav_top_node_titles, nav_child_candidates, nav_children,
+    nav_child, nav_child_ok, nav_child_ancestor, nav_sorted.
 {%- endcomment -%}
 
-{%- if page.has_children == true and page.has_toc != false -%}
-  {%- assign child_pages = site[page.collection]
-        | default: site.html_pages
-        | where: "parent", page.title
-        | where: "grand_parent", page.parent -%}
+{%- comment -%}
+  Whether a page has any children is checked efficiently by inspecting the cached
+  site_nav. If the page has no children, nav_children is set to an empty array;
+  otherwise nav_children is left unset.
+{%- endcomment -%}
 
-  {%- include sorted_pages.html pages = child_pages -%}
+{%- if page.has_children == false -%}
+  {%- assign nav_children = "" | split: "" -%}
+{%- else -%}
+
+  {%- assign nav_children = nil -%}
+
+  {%- capture nav_list_link -%}
+  <a href="{{ page.url | relative_url }}" class="nav-list-link">
+  {%- endcapture -%}
+
+  {%- capture site_nav -%}
+  {%- include_cached components/site_nav.html all=true -%}
+  {%- endcapture -%}
+
+  {%- capture nav_list_simple -%}
+  <ul class="nav-list">
+  {%- endcapture -%}
+
+  {%- assign nav_child_start = site_nav
+        | split: nav_list_link | last
+        | split: "</a>" | slice: 1 | first -%}
+
+  {%- assign nav_child_test = nav_child_start 
+        | remove_first: nav_list_simple | prepend: nav_list_simple -%}
+    
+  {%- if nav_child_start != nav_child_test -%}
+    {%- assign nav_children = "" | split: "" -%}
+  {%- endif -%}
+
+{%- endif -%}
+
+{%- unless nav_children -%}
+
+  {%- comment -%}
+    The layout is assumed to include components/breadcrumbs.html before this file,
+    otherwise it needs to be included here.
+  {%- endcomment -%}
+
+  {%- assign nav_ancestors = "" | split: "" -%}
+  {%- for nav_link in nav_breadcrumbs -%}
+    {%- assign nav_title = nav_link | split: ">" | slice: 1 | first | append: ">" | remove: "</a>" -%}
+    {%- assign nav_ancestors = nav_ancestors | push: nav_title -%}
+  {%- endfor -%}
+
+  {%- assign nav_parenthood = site[page.collection] | default: site.html_pages
+    | where_exp: "item", "item.title != nil" | group_by: "parent" -%}
+
+  {%- assign nav_top_nodes = nav_parenthood
+      | where_exp: "item", "item.name == ''" | map: "items" | first -%}
+
+  {% assign nav_top_node_titles = nav_top_nodes | map: "title" -%}
+
+  {%- include components/nav/children.html node=page ancestors=nav_ancestors all=true -%}
+
+{%- endunless -%}
+
+{%- if nav_children.size >= 1 -%}
 
   {%- if page.child_nav_order == 'desc' or page.child_nav_order == 'reversed' -%}
-    {%- assign sorted_pages = sorted_pages | reverse -%}
+    {%- assign nav_children = nav_children | reverse -%}
   {%- endif -%}
-{%- endif -%}
 
 <hr>
 {% include toc_heading_custom.html %}
 <ul>
-{% for child in sorted_pages %}
+  {% for nav_child in nav_children %}
   <li>
-    <a href="{{ child.url | relative_url }}">{{ child.title }}</a>{% if child.summary %} - {{ child.summary }}{% endif %}
+    <a href="{{ nav_child.url | relative_url }}">{{ nav_child.title }}</a>{% if nav_child.summary %} - {{ nav_child.summary }}{% endif %}
   </li>
-{% endfor %}
+  {% endfor %}
 </ul>
+
+{%- endif -%}

_includes/components/nav.html

@@ -1,75 +0,0 @@
-{%- comment -%}
-  Include as: {%- include components/nav.html pages=pages -%}
-  Depends on: include.pages.
-  Results in: HTML for the navigation panel.
-  Includes:
-    sorted_pages.html
-  Overwrites:
-    nav_pages, first_level_pages, second_level_pages, third_level_pages,
-    node, children_list, child, grand_children_list, grand_child.
-{%- endcomment -%}
-
-{%- assign nav_pages = include.pages
-    | where_exp: "item", "item.title != nil"
-    | where_exp: "item", "item.nav_exclude != true" -%}
-
-{%- include sorted_pages.html pages = nav_pages -%}
-
-{%- comment -%}
-  It might be more efficient to sort the pages at each level separately.
-{%- endcomment -%}
-
-{%- assign first_level_pages = sorted_pages
-    | where_exp: "item", "item.parent == nil" -%}
-{%- assign second_level_pages = sorted_pages
-    | where_exp: "item", "item.parent != nil"
-    | where_exp: "item", "item.grand_parent == nil" -%}
-{%- assign third_level_pages = sorted_pages
-    | where_exp: "item", "item.grand_parent != nil" -%}
-
-<ul class="nav-list">
-{%- for node in first_level_pages -%}
-  <li class="nav-list-item">
-    {%- if node.has_children -%}
-    <button class="nav-list-expander btn-reset" aria-label="toggle items in {{ node.title }} category" aria-pressed="false">
-      <svg viewBox="0 0 24 24" aria-hidden="true"><use xlink:href="#svg-arrow-right"></use></svg>
-    </button>
-    {%- endif -%}
-    <a href="{{ node.url | relative_url }}" class="nav-list-link">{{ node.title }}</a>
-    {%- if node.has_children -%}
-      {%- assign children_list = second_level_pages
-            | where: "parent", node.title -%}
-      {%- if node.child_nav_order == 'desc' or node.child_nav_order == 'reversed' -%}
-        {%- assign children_list = children_list | reverse -%}
-      {%- endif -%}
-      <ul class="nav-list">
-      {%- for child in children_list -%}
-        <li class="nav-list-item">
-          {%- if child.has_children -%}
-          <button class="nav-list-expander btn-reset" aria-label="toggle items in {{ child.title }} category" aria-pressed="false">
-            <svg viewBox="0 0 24 24" aria-hidden="true"><use xlink:href="#svg-arrow-right"></use></svg>
-          </button>
-          {%- endif -%}
-          <a href="{{ child.url | relative_url }}" class="nav-list-link">{{ child.title }}</a>
-          {%- if child.has_children -%}
-            {%- assign grand_children_list = third_level_pages
-                  | where: "parent", child.title
-                  | where: "grand_parent", node.title -%}
-            {%- if child.child_nav_order == 'desc' or child.child_nav_order == 'reversed' -%}
-              {%- assign grand_children_list = grand_children_list | reverse -%}
-            {%- endif -%}
-            <ul class="nav-list">
-            {%- for grand_child in grand_children_list -%}
-              <li class="nav-list-item">
-                <a href="{{ grand_child.url | relative_url }}" class="nav-list-link">{{ grand_child.title }}</a>
-              </li>
-            {%- endfor -%}
-            </ul>
-          {%- endif -%}
-        </li>
-      {%- endfor -%}
-      </ul>
-    {%- endif -%}
-  </li>
-{%- endfor -%}
-</ul>

_includes/components/nav/children.html

@@ -0,0 +1,48 @@
+{%- comment -%}
+  Include as: {%- include components/nav/children.html node=node ancestors=title_array all=bool -%}
+  Depends on: include.node, include.ancestors, include.all, nav_parenthood, nav_top_node_titles.
+  Includes: components/nav/sorted.html.
+  Assigns to: nav_children.
+  Overwrites:
+    nav_candidates, nav_child, nav_child_ok.
+{%- endcomment -%}
+
+{%- assign nav_children = "" | split: "" -%}
+
+{%- if include.all == true or include.node.has_children != false -%}
+
+  {%- assign nav_candidates = nav_parenthood
+        | where: "name", include.node.title | map: "items" | first -%}
+
+  {%- for nav_child in nav_candidates -%}
+    {%- assign nav_child_ok = true -%}
+
+    {%- if nav_child.grand_parent and nav_child.grand_parent != include.node.parent -%}
+      {%- assign nav_child_ok = false -%}
+    {%- endif -%}
+
+    {%- if nav_child.ancestor and nav_child.ancestor != include.node.title -%}
+      {%- unless include.ancestors contains nav_child.ancestor -%}
+        {%- assign nav_child_ok = false -%}
+      {%- endunless -%}
+    {%- endif -%}
+
+    {%- comment -%}
+      The following check rejects nav_child as 3rd-level when include.node is 2nd-level
+      and nav_child can also be 2nd-level. This is for backwards compatibility with
+      existing 3-level sites.
+    {%- endcomment -%}
+    {%- if nav_child.grand_parent == nil and nav_child.ancestor == nil and 
+          nav_top_node_titles contains nav_child.parent and include.ancestors.size >= 1 -%}
+      {%- assign nav_child_ok = false -%}
+    {%- endif -%}
+
+    {%- if nav_child_ok -%}
+      {%- assign nav_children = nav_children | push: nav_child -%}
+    {%- endif -%}
+  {%- endfor -%}
+
+{%- endif -%}
+
+{%- include components/nav/sorted.html pages=nav_children -%}
+{%- assign nav_children = nav_sorted -%}

_includes/components/nav/links.html

@@ -0,0 +1,53 @@
+{%- comment -%}
+  Include as: {%- include components/nav/links.html pages=page_array ancestors=title_array all=bool -%}
+  Depends on: include.pages, include.ancestors, include.all.
+  Results in: HTML for the main navigation when all is nil or false;
+    includes links to pages excluded from the main navigation when all is true.
+  Includes: components/nav/sorted.html, components/nav/children.html, components/nav/links.html.
+  Overwrites:
+    node, nav_children, nav_ancestors.
+{%- endcomment -%}
+
+<ul class="nav-list">
+  {%- for node in include.pages -%}
+  {%- if include.all == true or node.nav_exclude != true -%}
+
+  {%- if include.ancestors contains node.title -%}
+  
+    <li class="nav-list-item">
+      <a href="{{ node.url | relative_url }}" class="nav-list-link"> ∞ </a>
+    </li>
+    {%- capture nav_error_report -%}
+      <blockquote class="warning">
+        A page has the same title as its parent page or one of its ancestral pages!<br>
+        This causes an incorrect link in the main navigation panel.<br>
+        Page title: <code>{{ node.title }}</code>, location: <code>{{ node.path }}</code>.
+      </blockquote>
+    {%- endcapture -%}
+
+  {%- else -%}
+  
+    {%- include components/nav/children.html node=node ancestors=include.ancestors all=include.all -%}
+
+    <li class="nav-list-item">
+      {%- if nav_children.size >= 1 -%}
+      <button class="nav-list-expander btn-reset" aria-label="toggle items in {{ node.title }} category" aria-pressed="false">
+        <svg viewBox="0 0 24 24" aria-hidden="true"><use xlink:href="#svg-arrow-right"></use></svg>
+      </button>
+      {%- endif -%}
+      <a href="{{ node.url | relative_url }}" class="nav-list-link">{{ node.title }}</a>
+      {%- if nav_children.size >= 1 -%}
+        {%- if node.child_nav_order == 'desc' or node.child_nav_order == 'reversed' -%}
+          {%- assign nav_children = nav_children | reverse -%}
+        {%- endif -%}
+        {%- assign nav_ancestors = include.ancestors | push: node.title -%}
+        {%- include components/nav/links.html pages=nav_children ancestors=nav_ancestors all=include.all -%}
+      {%- endif -%}
+    </li>
+  
+  {%- endif -%}
+  
+  {%- endif -%}
+  {%- endfor -%}
+</ul>
+{%- comment -%}{%- endcomment -%}

_includes/components/nav/pages.html

@@ -0,0 +1,23 @@
+{%- comment -%}
+  Include as: {%- include components/nav/pages.html pages=page_array all=bool -%}
+  Depends on: include.pages.
+  Results in: HTML for the main navigation when all is nil or false;
+    adds links to pages excluded from the main navigation when all is true.
+  Includes: components/nav/links.html
+  Assigns to:
+    nav_parenthood, nav_top_nodes, nav_top_node_titles, nav_ancestors.
+{%- endcomment -%}
+
+{%- assign nav_parenthood = include.pages
+      | where_exp: "item", "item.title != nil" | group_by: "parent" -%}
+
+{%- assign nav_top_nodes = nav_parenthood
+      | where_exp: "item", "item.name == ''" | map: "items" | first -%}
+
+{%- include components/nav/sorted.html pages=nav_top_nodes -%}
+
+{% assign nav_top_node_titles = nav_top_nodes | map: "title" -%}
+
+{%- assign nav_ancestors = "" | split: "" -%}
+
+{%- include components/nav/links.html pages=nav_sorted ancestors=nav_ancestors all=include.all -%}

_includes/components/nav/sorted.html

@@ -1,7 +1,7 @@
 {%- comment -%}
-  Include as: {%- include sorted_pages.html pages=array_of_pages -%}
+  Include as: {%- include components/nav/sorted.html pages=page_array -%}
   Depends on: include.pages.
-  Assigns to: sorted_pages.
+  Assigns to: nav_sorted.
   Overwrites: 
     nav_order_pages, title_order_pages, double_quote, empty_array,
     nav_number_pages, nav_string_pages, nav_order_groups, group,
@@ -103,7 +103,7 @@
   {%- endif -%}
 {%- endunless -%}
 
-{%- assign sorted_pages = nav_number_pages
+{%- assign nav_sorted = nav_number_pages
       | concat: nav_string_pages
       | concat: title_number_pages
       | concat: title_string_pages -%}

_includes/components/site_nav.html

@@ -1,9 +1,10 @@
 {%- comment -%}
-  Include as: {%- include_cached components/site_nav.html -%}
+  Include as: {%- include_cached components/site_nav.html all=bool -%}
   Depends on: site.
-  Results in: HTML for the site-nav.
+  Results in: cached HTML for the main navigation when `all` is nil or false;
+    includes links to pages excluded from the main navigation when `all` is true.
   Includes:
-    components/nav.html
+    components/nav/pages.html
   Overwrites:
     pages_top_size, collections_size, collection_entry,
     collection_key, collection_value, collection.
@@ -16,7 +17,7 @@
         | where_exp:"item", "item.nav_exclude != true"
         | size %}
   {% if pages_top_size > 0 %}
-    {% include components/nav.html pages=site.html_pages %}
+    {% include components/nav/pages.html pages=site.html_pages all=include.all %}
   {% endif %}
   {%- if site.nav_external_links -%}
     <ul class="nav-list">
@@ -51,17 +52,21 @@
                 </button>
                 {%- endif -%}
                 <div class="nav-category">{{ collection_value.name }}</div>
-                {% include components/nav.html pages=collection %}
+                {% include components/nav/pages.html pages=collection all=include.all %}
               </li>
             </ul>
           {% else %}
             <div class="nav-category">{{ collection_value.name }}</div>
-            {% include components/nav.html pages=collection %}
+            {% include components/nav/pages.html pages=collection all=include.all %}
           {% endif %}
         {% else %}
-          {% include components/nav.html pages=collection %}
+          {% include components/nav/pages.html pages=collection all=include.all %}
         {% endif %}
       {% endif %}
     {% endfor %}
   {% endif %}
 </nav>
+
+{% if site.nav_error_report %}
+{{ nav_error_report }}
+{%- endif %}

_includes/css/activation.scss.liquid

@@ -21,18 +21,18 @@
 {%- endcomment -%}
 
 {%- capture activation_no_nav_link %}
-.site-nav ul li a {
+    .site-nav ul li a {
       background-image: none;
-}
+    }
 
-{%- if site.just_the_docs.collections %}
-.site-nav > ul.nav-category-list > li > button svg {
+    {%- if site.just_the_docs.collections %}
+    .site-nav > ul.nav-category-list > li > button svg {
       transform: rotate(-90deg);
-}
-.site-nav > ul.nav-category-list > li.nav-list-item > ul.nav-list {
+    }
+    .site-nav > ul.nav-category-list > li.nav-list-item > ul.nav-list {
       display: block;
-}
-{%- endif %}
+    }
+    {%- endif %}
 {% endcapture -%}
 
 {%- capture nav_list_link -%}
@@ -45,29 +45,29 @@
 
 {%- if site_nav contains nav_list_link -%}
 
-{%- capture nav_list -%}
-<ul class="nav-list
-{%- endcapture -%}
+  {%- capture nav_list -%}
+  <ul class="nav-list
+  {%- endcapture -%}
 
-{%- assign nav_list_end = "</ul>" -%}
+  {%- assign nav_list_end = "</ul>" -%}
 
-{%- capture nav_list_item -%}
-<li class="nav-list-item
-{%- endcapture -%}
+  {%- capture nav_list_item -%}
+  <li class="nav-list-item
+  {%- endcapture -%}
 
-{%- capture nav_category_list -%}
-<ul class="nav-list nav-category-list">
-{%- endcapture -%}
+  {%- capture nav_category_list -%}
+  <ul class="nav-list nav-category-list">
+  {%- endcapture -%}
 
-{%- assign nav_list_link_prefix =
+  {%- assign nav_list_link_prefix =
         site_nav | split: nav_list_link | first | append: nav_list_link -%}
 
-{%- assign nav_splits =
+  {%- assign nav_splits =
         nav_list_link_prefix | split: nav_list_item | pop -%}
 
-{%- assign nav_levels = "" | split: "" | push: 1 -%}
+  {%- assign nav_levels = "" | split: "" | push: 1 -%}
 
-{%- comment -%}
+  {%- comment -%}
     The pattern of occurrences of list and list-item tags in the site_nav string
     is included in the language defined by the following context-free grammar:
     
@@ -125,9 +125,9 @@
       followed by the just the start of a collection has two occurrences of nav_list,
       and the increment needs to be one less; similarly when the first nav_split
       starts with an empty non-foldable collection.
-{%- endcomment %}
+  {%- endcomment %}
 
-{%- for nav_split in nav_splits -%}
+  {%- for nav_split in nav_splits -%}
 
     {%- assign nav_list_less = nav_split | remove: nav_list -%}
     {%- assign nav_list_count =
@@ -174,30 +174,24 @@
 
     {%- endif -%}
 
-{%- endfor -%}
+  {%- endfor -%}
 
-{%- assign nav_levels = nav_levels | where_exp: "item", "item >= 1" -%}
+  {%- assign nav_levels = nav_levels | where_exp: "item", "item >= 1" -%}
 
-{%- endif -%}
-
-{%- comment -%}
+  {%- comment -%}
     The generated CSS depends on the position of the current page in each level in
     the navigation.
-{%- endcomment -%}
 
-{%- if nav_levels[1] == nil -%}
+    nav_page_level is the depth of the navigation hierarchy (ignoring collections).
+    In existing 3-level sites, nav_page_level <= 3.
+    In multi-level sites, nav_page_level is unbounded.
+  {%- endcomment -%}
 
-{{ activation_no_nav_link }}
+  {%- assign nav_page_level = nav_levels.size | minus: 1 -%}
+  {%- assign nav_page_parent_level = nav_page_level | minus: 1 -%}
+  {%- assign nav_page_grandparent_level = nav_page_level | minus: 2 -%}
 
-{%- else -%}
-
-{%- if nav_levels[2] == nil and nav_levels[3] -%}
-
-{{ activation_no_nav_link }}
-
-{%- else -%}
-
-{%- comment -%}
+  {%- comment -%}
     The site-nav is:
     - an optional ul.nav-list with li.nav-list-items for non-collection top-level pages
     - an optional ul.nav-list with li.nav-list-item.externals
@@ -216,9 +210,9 @@
     The generated CSS uses:
     - activation_collection_prefix, to select the site-nav > ul.nav-list for the page
     - activation_other_collection_prefix, to select all the other site-nav > ul.nav-lists
-{%- endcomment -%}
+  {%- endcomment -%}
 
-{%- if page.collection == nil -%}
+  {%- if page.collection == nil -%}
 
     {%- capture activation_collection_prefix -%}
     .site-nav > ul.nav-list:first-child
@@ -228,7 +222,7 @@
     .site-nav > ul.nav-list:not(:first-child)
     {%- endcapture -%}
 
-{%- else -%}
+  {%- else -%}
 
     {%- capture activation_collection_prefix -%}
     .site-nav > ul:nth-of-type({{ nav_levels[0] }})
@@ -240,64 +234,52 @@
     .site-nav > ul:not(:nth-of-type({{ nav_levels[0] }}))
     {%- endcapture -%}
 
-{%- endif -%}
+  {%- endif -%}
 
-{%- comment -%}
+  {%- comment -%}
     The required background image of the link to the current page may involve SCSS.
     To avoid page-dependent SCSS, all nav links initially have that background image.
     The following rule removes the image from the links to all parents, siblings,
     and children of the current page.
-{%- endcomment %}
+  {%- endcomment %}
 
-{% if nav_levels[3] -%}
-
-{{ activation_collection_prefix }} > li > a,
-{{ activation_collection_prefix }} > li > ul > li > a,
-{{ activation_collection_prefix }} > li > ul > li > ul > li:not(:nth-child({{ nav_levels[3] }})) > a {
+    {% if nav_page_level >= 2 -%}
+    {%- for i in (1..nav_page_parent_level) -%}
+    {{ activation_collection_prefix }} >
+    {%- for j in (2..i) %} li > ul >
+    {%- endfor %} li > a,
+    {% endfor -%}
+    {%- endif %}
+    {{ activation_collection_prefix }} >
+    {%- for i in (1..nav_page_parent_level) %} li > ul >
+    {%- endfor %} li:not(:nth-child({{ nav_levels[nav_page_level] }})) > a,
+    {{ activation_collection_prefix }} >
+    {%- for i in (1..nav_page_level) %} li > ul >
+    {%- endfor %} li a {
       background-image: none;
-}
+    }
 
-{%- elsif nav_levels[2] -%}
-
-{{ activation_collection_prefix }} > li > a,
-{{ activation_collection_prefix }} > li > ul > li:not(:nth-child({{ nav_levels[2] }})) > a,
-{{ activation_collection_prefix }} > li > ul > li > ul > li > a {
-  background-image: none;
-}
-
-{%- else -%}
-
-{{ activation_collection_prefix }} > li:not(:nth-child({{ nav_levels[1] }})) > a,
-{{ activation_collection_prefix }} > li > ul > li > a,
-{{ activation_collection_prefix }} > li > ul > li > ul > li > a {
-  background-image: none;
-}
-
-{%- endif %}
-
-{%- comment -%}
+  {%- comment -%}
     The following rule removes the image from the links to pages in other collections.
-{%- endcomment %}
+  {%- endcomment %}
 
-{{ activation_other_collection_prefix }} a,
-.site-nav li.external a {
+    {{ activation_other_collection_prefix }} a,
+    .site-nav li.external a {
       background-image: none;
-}
+    }
 
-{%- comment -%}
+  {%- comment -%}
     The following rule styles the link to the current page.
-{%- endcomment %}
+  {%- endcomment %}
 
-{{ activation_collection_prefix }} > li:nth-child({{ nav_levels[1] }})
-{%- if nav_levels[2] %} > ul > li:nth-child({{ nav_levels[2] }})
-{%- if nav_levels[3] %} > ul > li:nth-child({{ nav_levels[3] }})
-{%- endif -%}
-{%- endif %} > a {
+    {{ activation_collection_prefix }} > li:nth-child({{ nav_levels[1] }})
+    {%- for i in (2..nav_page_level) %} > ul > li:nth-child({{ nav_levels[i] }})
+    {%- endfor %} > a {
       font-weight: 600;
       text-decoration: none;
-}
+    }
 
-{%- comment -%}
+  {%- comment -%}
     The following rules unfold all collections, and display the links to any children
     of the current page.
     
@@ -306,27 +288,38 @@
     when nav-list-expander-right is true. This results in a minor visual difference
     between the appearance of active expander icons when JS is enabled/disabled and
     nav-list-expander-right is false, which seems unavoidable.
-{%- endcomment %}
+  {%- endcomment %}
 
-{%- if site.just_the_docs.collections %}
-.site-nav > ul.nav-category-list > li > button svg,
-{% endif -%}
-{{ activation_collection_prefix }} > li:nth-child({{ nav_levels[1] }}) > button svg
-{%- if nav_levels[2] -%},
-{{ activation_collection_prefix }} > li:nth-child({{ nav_levels[1] }}) > ul > li:nth-child({{ nav_levels[2] }}) > button svg
-{%- endif %} {
+    {%- if site.just_the_docs.collections %}
+    .site-nav > ul.nav-category-list > li > button svg,
+    {% endif -%}
+    {{ activation_collection_prefix }} > li:nth-child({{ nav_levels[1] }}) > button svg
+    {%- for i in (2..nav_page_level) %},
+    {{ activation_collection_prefix }} > li:nth-child({{ nav_levels[1] }}) > 
+    {%- for j in (2..i) %} ul > li:nth-child({{ nav_levels[j] }}) >
+    {%- endfor %} button svg
+    {%- endfor %} {
       transform: rotate(-90deg);
-}
+    }
 
-{%- if site.just_the_docs.collections %}
-.site-nav > ul.nav-category-list > li.nav-list-item > ul.nav-list,
-{% endif -%}
-{{ activation_collection_prefix }} > li.nav-list-item:nth-child({{ nav_levels[1] }}) > ul.nav-list
-{%- if nav_levels[2] %},
-{{ activation_collection_prefix }} > li.nav-list-item:nth-child({{ nav_levels[1] }}) > ul.nav-list > li.nav-list-item:nth-child({{ nav_levels[2] }}) > ul.nav-list
-{%- endif %} {
+    {%- if site.just_the_docs.collections %}
+    .site-nav > ul.nav-category-list > li.nav-list-item > ul.nav-list,
+    {% endif -%}
+    {{ activation_collection_prefix }} > li.nav-list-item:nth-child({{ nav_levels[1] }}) > ul.nav-list
+    {%- for i in (2..nav_page_level) %},
+    {{ activation_collection_prefix }} > li.nav-list-item:nth-child({{ nav_levels[1] }}) >
+    {%- for j in (2..i) %} ul.nav-list > li.nav-list-item:nth-child({{ nav_levels[j] }}) >
+    {%- endfor %} ul.nav-list
+    {%- endfor %} {
       display: block;
-}
+    }
+
+{%- else -%}
+
+  {%- comment -%}
+    not site_nav contains nav_list_link
+  {%- endcomment -%}
+
+    {{ activation_no_nav_link }}
 
 {%- endif -%}
-{%- endif -%}

_layouts/default.html

@@ -28,7 +28,7 @@ layout: table_wrappers
             {{ content }}
           {% endif %}
 
-          {% if page.has_children == true and page.has_toc != false %}
+          {% if page.has_toc != false %}
             {% include components/children_nav.html %}
           {% endif %}
         </main>