Sergey.S.Betke

When your blog is loaded, all links of this class .more-link (standard wordpress css class for “Read more…” 
links) are modified to no longer send the user to that post’s 
single page display when clicked. Instead, the WordPress database is queried for that specific post, and all content 
after the <!–more–> tag (i.e. only what you need) is returned. 
The new content is then immediately displayed to the user with light animation (You can disable animation in plugin options), 
inline with the opening content.
<ul>
<li>AJAX-read-more uses the jQuery framework already included with WordPress installations.</li>
<li>This plugin used GET ajax action (not POST, like other plugins), wordpress “template_redirect” action.</li>
<li>It’s compatible with cache plugins (like Hyper Cache Extended).</li>
<li>AJAX-read-more: design of progress indicator and other behaviors – by CSS (example – in css/all.css)</li>
<li>cache-safe script and css url</li>
</ul>
Check plugin options on options page.
Thanks for debugging to den@10-13.ru.
Theme requirements:
<ul>
<li>Your theme must generate correct DOM structure: 
(div id=”#content”) 
… 
(div class=”post”) 
 … 
(/div) 
(/div)</li>
<li>Theme must support footer (wp_footer). If not, you can change corresponding option on plugin option page.</li>
</ul>
For more information, please visit the <a href="http://sergey-s-betke.blogs.novgaro.ru/category/web/wordpress/read-more" rel="nofollow ugc">Sergey S. Betke blog</a>.
<h3>ToDo</h3>
The next version or later: 
* wordpress core integration for cache headers processing??? 
* optional http cache-control max-age header value and must-revalidate 
* just private cache headers, when post has limited access 
* check cache plugins support (Hyper Cache Extended): clear cached responces when post modified 
* cache-control and Last-Modification headers – to separate plugins 
* if-modified http request support (separate plugins) 
* the_excerpt optional support 
* AJAX-read-more send correct http cache-control headers, and fully support client-side public cache. 
* WP 3.3: http://wpdevel.wordpress.com/tag/3-3-dev-notes/ 
* WP 3.3: replace wp_localize_script() with wp_add_script_before(). wp_localize_script doesn’t support non-string params in WP 3.3+ 
* WP 3.3: wp_enqueue_script() now works mid-page – insert scripts just into pages when it’t needed

AJAX Read More

Automatically transform your  links into links that immediately display the rest of your entry (AJAX).

Miguel Useche

This plugin provides a widget and a filter to display a collapsible list of categories in your sidebar or posts by using vanilla Javascript.
<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 [JsCategoriesList]</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.

AJAX Read More vs JS Categories List Widget