MDL-76447 editor_tiny: Nest the dropdown menu in the parent DOM.

The TinyMCE menu has a significant issue with the Overflow style,
and the Boost theme heavily uses Overflow for drawer navigation.
Nest the dropdown menu container into the parent editor container makes it work correctly.

Co-authored-by: davewoloszyn <[email protected]>
Co-authored-by: xr0master <[email protected]>

lib/editor/tiny/classes/editor.php

@@ -194,6 +194,9 @@ class editor extends \texteditor {
 
             // Plugin configuration.
             'plugins' => $this->manager->get_plugin_configuration($context, $options, $fpoptions, $this),
+
+            // Nest menu inside parent DOM.
+            'nestedmenu' => true,
         ];
 
         if (defined('BEHAT_SITE_RUNNING') && BEHAT_SITE_RUNNING) {