m popup is opened by pressing a button from the editor's toolbar.
        // Get the button's object in order to place the popup relative to it.
        var $btn = editor.$tb.find('.fr-command[data-cmd="myButton"]')

        // Compute the popup's position.
        var left = $btn.offset().left
        var top = $btn.offset().top + (editor.opts.toolbarBottom ? 10 : $btn.outerHeight() - 10)

        // Show the custom popup.
        // The button's outerHeight is required in case the popup needs to be displa