How to disable widget content nesting in CKEditor?

I have an editable custom widget that can be placed in CKEditor 4 by

• clicking a button in the toolbar - works fine = does not allow nesting
• drag & drop from outside of the editor - allows nesting

I do not want to let user to have nested content of the widget. On the other hand I do want users to able to edit the content of the widget.

NOTE

1. click the button to insert widget's content. Click inserted text and the button becomes not available for clicking. Click some other text and you would be able to insert again. This is desired behavior.
2. the widget button will not be present in the final version of the CKEditor application / widget. Only drag&drop way of inserting text will be available
3. Insert two widgets in the editor and try to drag&drop one inside the other one. It will not work. This is desired behavior.

Now try to insert a widget and then insert another one by drag&drop of the text "master or editable" into existing widget. It will be possible.

Could someone help me to set CKEditor a way so nesting is NOT possible?

Working jsfiddle.

 init: function( editor ) {
    editor.widgets.add( 'simplebox', {

        button: 'Create a simple box',
        template:
            '<div class="simplebox">' +
                '<h2 class="simplebox-title">Title</h2>' +
                '<div class="simplebox-content"><p>Content<br>.<br>.<br>.</p></div>' +
            '</div>',

        editables: {
            title: {
                selector: '.simplebox-title',
                allowedContent: 'br strong'
            },
            content: {
                selector: '.simplebox-content',
                allowedContent: 'p br ul ol li strong em'
            }
        },

        allowedContent:
            'div(!simplebox); div(!simplebox-content); h2(!simplebox-title)',

        requiredContent: 'div(simplebox)',

        upcast: function( element ) {
            return element.name == 'div' && element.hasClass( 'simplebox' );
        }
    } );
}
} );