docs: Add more information to the HTML sidebar

Add a new sidebar template that creates a more RTD-like "fisheye" view of
the current place in the document hierarchy.  It is far from ideal, but
some readers may find it better for navigating through the documentation as
a whole.

Add some CSS trickery as well to make the table of contents less intrusive
when viewing the pages on a small screen.
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Reviewed-by: David Gow <davidgow@google.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/conf.py

@@ -153,7 +153,7 @@ else:
     math_renderer = 'mathjax'
 
 # Add any paths that contain templates here, relative to this directory.
-templates_path = ['_templates']
+templates_path = ['sphinx/templates']
 
 # The suffix(es) of source filenames.
 # You can specify multiple suffix as a list of string:
@@ -328,6 +328,7 @@ if  html_theme == 'alabaster':
         'description': get_cline_version(),
         'page_width': '65em',
         'sidebar_width': '15em',
+        'fixed_sidebar': 'true',
         'font_size': 'inherit',
         'font_family': 'serif',
     }
@@ -345,7 +346,7 @@ html_use_smartypants = False
 
 # Custom sidebar templates, maps document names to template names.
 # Note that the RTD theme ignores this
-html_sidebars = { '**': ['searchbox.html', 'localtoc.html', 'sourcelink.html']}
+html_sidebars = { '**': ['searchbox.html', 'kernel-toc.html', 'sourcelink.html']}
 
 # about.html is available for alabaster theme. Add it at the front.
 if html_theme == 'alabaster':

Documentation/sphinx-static/custom.css

@@ -11,7 +11,9 @@ div.body h3 { font-size: 130%; }
 /* Tighten up the layout slightly */
 div.body { padding: 0 15px 0 10px; }
 div.sphinxsidebarwrapper { padding: 1em 0.4em; }
-div.sphinxsidebar { font-size: inherit; }
+div.sphinxsidebar { font-size: inherit;
+		    max-height: 100%;
+		    overflow-y: auto; }
 /* Tweak document margins and don't force width */
 div.document {
     margin: 20px 10px 0 10px; 
@@ -27,3 +29,47 @@ dl.function, dl.struct, dl.enum { margin-top: 2em; background-color: #ecf0f3; }
 dl.function dt { margin-left: 10em; text-indent: -10em; }
 dt.sig-object { font-size: larger; }
 div.kernelindent { margin-left: 2em; margin-right: 4em; }
+
+/*
+ * Tweaks for our local TOC
+ */
+div.kerneltoc li.toctree-l1 { font-size: smaller;
+		text-indent: -1em;
+		margin-left: 1em; }
+div.kerneltoc li.current > a {font-weight: bold; }
+div.kerneltoc li.toctree-l2,li.toctree-l3 { font-size: small;
+		text-indent: -1em;
+		margin-left: 1em;
+		list-style-type: none;
+	      }
+div.kerneltoc li.current ul { margin-left: 0; }
+div.kerneltoc { background-color: #eeeeee; }
+div.kerneltoc li.current ul { background-color: white; }
+
+/*
+ * The CSS magic to toggle the contents on small screens.
+ */
+label.kernel-toc-title { display: none; }
+label.kernel-toc-title:after {
+    content: "[Hide]";
+}
+input[type=checkbox]:checked ~ label.kernel-toc-title:after {
+    content: "[Show]";
+}
+/* Hide the toggle on large screens */
+input.kernel-toc-toggle { display: none; }
+
+/*
+ * Show and implement the toggle on small screens.
+ * The 875px width seems to be wired into alabaster.
+ */
+@media screen and (max-width: 875px) {
+    label.kernel-toc-title { display: inline;
+			     font-weight: bold;
+			     font-size: larger; }
+    input[type=checkbox]:checked ~ div.kerneltoc {
+	display: none;
+    }
+    h3.kernel-toc-contents { display: inline; }
+    div.kerneltoc a { color: black; }
+}

Documentation/sphinx/templates/kernel-toc.html

+{# SPDX-License-Identifier: GPL-2.0 #}
+{# Create a local TOC the kernel way #}
+<p>
+<h3 class="kernel-toc-contents">Contents</h3>
+<input type="checkbox" class="kernel-toc-toggle" id = "kernel-toc-toggle" checked>
+<label class="kernel-toc-title" for="kernel-toc-toggle"></label>
+
+<div class="kerneltoc" id="kerneltoc">
+{{ toctree(maxdepth=3) }}
+</div>
+{# hacky script to try to position the left column #}
+<script type="text/javascript"> <!--
+  var sbar = document.getElementsByClassName("sphinxsidebar")[0];
+  let currents = document.getElementsByClassName("current")
+  sbar.scrollTop = currents[currents.length - 1].offsetTop;
+  --> </script>