TinyMCE fullscreen mode (DFW) not working properly in meta boxes

[rilwis]

Describe the bug

As meta box is supported, users might create an extra TinyMCE editor in a meta box, which has the fullscreen (DFW - distraction free writing) mode. When clicking on the fullscreen button of the TinyMCE, the UI of the Gutenberg overlaps the UI of the TinyMCE editor, which makes the TinyMCE editor useless.

To reproduce Steps to reproduce the behavior:

1. Install & activate the Meta Box plugin from wordpress.org: https://wordpress.org/plugins/meta-box/
2. Create a simple meta box with only one editor field by adding the following code in the functions.php file of the active theme:

add_filter( 'rwmb_meta_boxes', function( $meta_boxes ) {
    $meta_boxes[] = [
        'title' => 'Test',
        'fields' => [
            [
                'id' => 'myeditor',
                'type' => 'wysiwyg',
            ],
        ],
    ];
} ); 

3. Go to Posts > Add New and create a new post
4. Click on fullscreen button in the editor and see the error

Expected behavior

When the fullscreen mode is active, the editor of the TinyMCE should be above the Gutenberg editor and accessible to users.

Desktop (please complete the following information):

• OS: Windows 10
• Browser: Chrome, Firefox

Additional context

• Gutenberg version: I'm using the version in WordPress 5.2.2.

[draganescu]

This looks more like a classic editor z-index issue? cc @azaozz