Redo

- simplify the logic for the editor.css
- add the code from joomla#35669 for the template snippets to support files placed in the media folder (aka publically accessible)

plugins/editors/tinymce/forms/setoptions.xml

@@ -72,13 +72,9 @@
 
 		<field
 			name="content_template_path"
-			type="folderlist"
+			type="templateslist"
 			label="PLG_TINY_FIELD_CUSTOM_CONTENT_TEMPLATE_PATH_LABEL"
 			description="PLG_TINY_FIELD_CUSTOM_CONTENT_TEMPLATE_PATH_DESC"
-			directory="/templates"
-			hide_none="true"
-			recursive="true"
-			exclude="system"
 			default=""
 			folderFilter="^tinymce"
 			validate="options"

plugins/editors/tinymce/src/Field/TemplateslistField.php

@@ -0,0 +1,100 @@
+<?php
+
+/**
+ * @package     Joomla.Plugin
+ * @subpackage  Editors.tinymce
+ *
+ * @copyright   (C) 2021 Open Source Matters, Inc. <https://www.joomla.org>
+ * @license     GNU General Public License version 2 or later; see LICENSE.txt
+ */
+
+namespace Joomla\Plugin\Editors\TinyMCE\Field;
+
+\defined('_JEXEC') or die;
+
+use Joomla\CMS\Form\Field\FolderlistField;
+use Joomla\CMS\HTML\HTMLHelper;
+use Joomla\CMS\Language\Text;
+
+/**
+ * Generates the list of directories  available for drag and drop upload.
+ *
+ * @package     Joomla.Plugin
+ * @subpackage  Editors.tinymce
+ * @since       __DEPLOY_VERSION__
+ */
+class TemplatesListField extends FolderlistField
+{
+  protected $type = 'templatesList';
+
+  /**
+   * Method to attach a JForm object to the field.
+   *
+   * @param   \SimpleXMLElement  $element  The SimpleXMLElement object representing the `<field>` tag for the form field object.
+   * @param   mixed              $value    The form field value to validate.
+   * @param   string             $group    The field name group control value. This acts as an array container for the field.
+   *                                       For example if the field has name="foo" and the group value is set to "bar" then the
+   *                                       full field name would end up being "bar[foo]".
+   *
+   * @return  boolean  True on success.
+   *
+   * @see     \Joomla\CMS\Form\FormField::setup()
+   * @since   __DEPLOY_VERSION__
+   */
+  public function setup(\SimpleXMLElement $element, $value, $group = null)
+  {
+    $return = parent::setup($element, $value, $group);
+
+    // Set some defaults.
+    $this->recursive   = true;
+    $this->hideDefault = true;
+    $this->exclude     = 'system';
+    $this->hideNone    = true;
+
+    return $return;
+  }
+
+  /**
+   * Method to get the directories options.
+   *
+   * @return  array  The dirs option objects.
+   *
+   * @since   __DEPLOY_VERSION__
+   */
+  public function getOptions()
+  {
+    $def         = new \stdClass;
+    $def->value  = '';
+    $def->text   = Text::_('JOPTION_DO_NOT_USE');
+    $options     = [0 => $def];
+    $directories = [JPATH_ROOT . '/templates', JPATH_ROOT . '/media/templates/site'];
+
+    foreach ($directories as $directory) {
+      $this->directory = $directory;
+      $options         = array_merge($options, parent::getOptions());
+    }
+
+    return $options;
+  }
+
+  /**
+   * Method to get the field input markup for the list of directories.
+   *
+   * @return  string  The field input markup.
+   *
+   * @since   __DEPLOY_VERSION__
+   */
+  protected function getInput()
+  {
+    return HTMLHelper::_(
+      'select.genericlist',
+      (array) $this->getOptions(),
+      $this->name,
+      'class="form-select"',
+      'value',
+      'text',
+      $this->value,
+      $this->id
+    );
+  }
+}