PSDCovers

<p>Developed internally for our Adjustly theme, this plugin allows authors to link 2 html elements together as trigger and target.  When the trigger is clicked the target will immediately expand to reveal its content.</p>
<h4>Try the Demo</h4>
<p>If you’re interested in seeing what a default installation of Adjustly Collapse has to offer, have a look at some samples on our site.</p>
<p><a href="http://www.psdcovers.com/adjustly-collapse/" rel="nofollow ugc">http://www.psdcovers.com/adjustly-collapse/</a></p>
<h4>Usage</h4>
<p>The basic structure without any CSS bells and whistles looks like this:</p>
<pre><code><a class="aj-collapse" rel="myslidingcontent">trigger</a>
<div class="aj-hidden" id="myslidingcontent">target: this content is hidden by default</div>
</code></pre>
<p>In the example above, the trigger is an href element and the target is a div element.  Note that the target can be any element you choose.</p>
<h4>rel=”[id]”</h4>
<p>Each trigger and target pair must contain a common label so that the trigger knows which target to expand or collapse. The trigger rel tag must be the same name as the target’s id tag.  If you plan on having multiple collapsible elements in a page, post or theme then you will need to ensure that the rel and id tags are always unique for each trigger/target combo.</p>
<h4>Class Names</h4>
<ol>
<li>The trigger must always have the following class name: class=”aj-collapse”</li>
<li>The target can have either of 2 classes: class=”aj-hidden” will make the target collapsed by default while class=”aj-visible” will display the content normally but allow the viewer to collapse it.</li>
</ol>
<h4>Notes</h4>
<p>The trigger and target do not have to be next to each other.  The target can be at the opposite end of the article or you can place it within the trigger. Theme developers can use it to make widgets collapsible.</p>


Adjustly Collapse

Developed internally for our Adjustly theme, this plugin allows authors to link 2 html elements together as trigger and target.

Miguel Useche

<p>This plugin provides a widget and a filter to display a collapsible list of categories in your sidebar or posts by using vanilla Javascript.</p>
<h4>Features</h4>
<ol>
<li>Support for Gutenberg blocks. Add it to any FSE theme or Gutenberg compatible theme.</li>
<li>Display a collapsed list of your archives to reduce space.</li>
<li>Uses vanilla JS to add effects and to be compatible with all browsers.</li>
<li>Select the symbol for expanding/collapsing categories</li>
<li>Select categories to exclude, so it shows only the categories you want.</li>
<li>Autoexpand selected category (including it parent or child).</li>
<li>Support for multiple instances.</li>
<li>Simple layout configuration</li>
<li>Shortcode support  <em>[JsCategoriesList]</em></li>
<li>Translated to Spanish, Italian Russian, Slovak, Czech.</li>
<li>Compatible with most JS cache and minify plugins.</li>
<li>And more to come…</li>
</ol>
<h3>Configuration</h3>
<ul>
<li>Title: title of the widget.</li>
<li>Trigger Symbol:  characters to be displayed as a bullet.</li>
<li>Symbol position: where to put the expand/collapse symbol.</li>
<li>Effect: JS’s effect to use.</li>
<li>Order by: display order of categories.</li>
<li>Expand: if list should be expanded or callapsed by default.</li>
<li>Show number of posts: display how many post are published in the category.</li>
<li>Show empty categories: display categories with no posts.</li>
<li>Open links in a new page: render category links with target=”_blank”.</li>
<li>Categories to include: categories to be included from the list.</li>
<li>Categories to exclude: categories to be excluded from the list.</li>
</ul>


JS Categories List Widget

Haven't seen any problems running this with WordPress 4.9.8, even though it's marked as untested for a long time.  It must have been well written.

Works well with 4.9.8

A simple Gutenberg block and JS widget (can be called from posts) for displaying categories in a list with some effects.

Adjustly Collapse vs JS Categories List Widget