Plugin has issues in non-typical WordPress installs

Please take a look at your plugin. It fails to include its templates correctly if the wp-content directory is not in the same directory as site_url. It is a perfectly valid WordPress install if WordPress is in a sub-directory from document_root and wp-content is also a sub-directory in document root.

Your plugin also breaks if the name of your plugin folder is changed – take a look at the plugin_url function.

Similarly your templates will fail since they attempt to climb the directory structure to find wp-load.php. I suggest you look at writing a hidden div on wp-footer and using the standard WordPress constants to find your files and values.

Try making your plugin work in this kind of structure:

wordpress (directory with WordPress codebase)
index.php
wp-config.php
wp-content

https://wordpress.org/plugins/tab-slide/