Theme: Expand at most one sidebar group for the current page

Among the sidebar groups that use block_expand=active, only expand
one group for the "current" page.

Example: On qunitjs.com, some pages cross-promoted in multiple groups
for improved discovery. Such as "QUnit.config.current" which is
primarily a config page, but also listed under "Extension interfaces".
Instead of expanding both "Configuration" and "Ext interfaces", and
thus pushing down the TOC even further and making navivation unclear,
expand only the first match.

When interested in exploring other pages in the same secondary group
related to a page, one can still click the sidebar heading to expand
it, and/or the category link atop the current page.

Author: Krinkle

_data/sidebar_api.yml

@@ -4,6 +4,8 @@
 
 - group: lorem
 
+- group: quux
+
 - group: deprecated
   expand: false
   title: Deprecated

_includes/sidebar.html

@@ -41,14 +41,19 @@
   Whether a block should be expanded.
   - true: Always expanded.
   - false: Never expanded.
-  - active: Expand when the current page is in the list.
+  - active: Expand when the current page is in the list,
+            unless another block has already been expanded for the current page.
+            (If a page is in multiple categories, only expand the first and
+            presumed most-important once. The others can be navigated by clicking
+            on the category atop the page and/or in the sidebar instead.)
   - initial: Expand on the home page, and when the current page is in the list.
 
 * initial: [Default: "/"]
   Override which address the "initial" expand mode will compare to.
 
 {%- endcomment -%}
 <aside class="sidebar" role="complementary">
+{%- assign block_expand_active_once = false -%}
 {%- for block in site.data[include.blocks] -%}
 	{%- assign block_type = block.type | default: "group" -%}
 	{%- assign block_title = block.title -%}
@@ -94,8 +99,9 @@
 
 	{%- if block_expand == "active" -%}
 		{%- assign self_in_contents = block_contents | where: 'url', page.url | size -%}
-		{%- if page.url == block_url or self_in_contents > 0 -%}
+		{%- if page.url == block_url or self_in_contents > 0 and block_expand_active_once == false -%}
 			{%- assign block_expand = true -%}
+			{%- assign block_expand_active_once = true -%}
 		{%- else -%}
 			{%- assign block_expand = false -%}
 		{%- endif -%}

api/foo/quux.md

@@ -4,8 +4,9 @@ title: .quuz()
 excerpt: Hello, where was I?
 groups:
 - foo
+- quux
 ---
 
-That's weird. So, where was I? Oh, that's right, Barcelona!
+New teeth. That's weird. So, where was I? Oh, that's right. Barcelona!
 
 This entry has no version information.

api/lorem/quux.md

@@ -0,0 +1,21 @@
+---
+layout: page-api
+title: Example.quux()
+excerpt: Thanks for coming.
+groups:
+- lorem
+- quux
+version_added: "2.0.0"
+---
+
+A quote from an funny scene:
+
+> Hammond: Thank you for coming, Thor.<br>
+> Thor: It is my pleasure, General Hammond.
+>
+> […]
+>
+> Kinsey: My name is - <br>
+> Thor: Senator Kinsey. O'Neill suggested I send you to a distant planet for your actions, but I am reasonably certain that his statement was in jest.<br>
+> Kinsey: I am sure it was, Commander - <br>
+> Thor: Supreme Commander.

api/quux/index.md

@@ -0,0 +1,8 @@
+---
+layout: group
+group: quux
+title: Special quux
+amethyst:
+  toc: true
+---
+