Better integration of Read the Docs' embed

This makes it feel more integrated with the rest of the theme.

src/furo/assets/styles/_scaffold.sass

@@ -117,6 +117,7 @@ article
 
 .sidebar-scroll,
 .toc-scroll
+  flex-grow: 1
   flex-shrink: 1
 
   overflow: auto

src/furo/assets/styles/extensions/_readthedocs.sass

@@ -17,3 +17,43 @@
 
   .ethical-callout a
     color: var(--color-foreground-secondary) !important
+
+#furo-readthedocs-versions
+  position: static
+  width: 100%
+  background: transparent
+
+  .rst-current-version
+    background: var(--color-sidebar-item-background)
+    &:hover
+      background: var(--color-sidebar-item-background--hover)
+    .fa-book
+      color: var(--color-foreground-primary)
+
+  > .rst-other-versions
+    padding: 0
+
+  .injected
+    .rst-versions
+      position: unset
+
+  &:hover,
+  &:focus-within
+    .rst-current-version
+      font-size: inherit
+      background: #272525
+      &:hover
+        background: #272525
+      .fa-book
+        color: white
+
+    .fa-caret-down
+      display: none
+
+    .rst-current-version,
+    .rst-other-versions,
+    .injected
+      display: block
+
+    > .rst-current-version
+      display: none

src/furo/theme/furo/sidebar/variant-selector.html

@@ -0,0 +1,33 @@
+{# Add variant selector for Read the Docs embedding #}
+{% if READTHEDOCS %}
+<div id="furo-readthedocs-versions" class="rst-versions" data-toggle="rst-versions" role="note" aria-label="{{ _('Versions') }}" tabindex="0">
+  <span class="rst-current-version" data-toggle="rst-current-version">
+    <span class="fa fa-book">&nbsp;</span>
+    {{ current_version }}
+  </span>
+  <div class="rst-other-versions">
+    <dl>
+      <dt>{{ _('Versions') }}</dt>
+      {% for slug, url in versions %}
+        <dd><a href="{{ url }}">{{ slug }}</a></dd>
+      {% endfor %}
+    </dl>
+    <dl>
+      <dt>{{ _('Downloads') }}</dt>
+      {% for type, url in downloads %}
+        <dd><a href="{{ url }}">{{ type }}</a></dd>
+      {% endfor %}
+    </dl>
+    <dl>
+      {# Translators: The phrase "Read the Docs" is not translated #}
+      <dt>{{ _('On Read the Docs') }}</dt>
+        <dd>
+          <a href="//{{ PRODUCTION_DOMAIN }}/projects/{{ slug }}/?fromdocs={{ slug }}">{{ _('Project Home') }}</a>
+        </dd>
+        <dd>
+          <a href="//{{ PRODUCTION_DOMAIN }}/builds/{{ slug }}/?fromdocs={{ slug }}">{{ _('Builds') }}</a>
+        </dd>
+    </dl>
+  </div>
+</div>
+{% endif %}

src/furo/theme/furo/theme.conf

@@ -9,7 +9,8 @@ sidebars =
   sidebar/scroll-start.html,
   sidebar/navigation.html,
   sidebar/ethical-ads.html,
-  sidebar/scroll-end.html
+  sidebar/scroll-end.html,
+  sidebar/variant-selector.html
 # sidebar-end
 
 [options]