Steps to reproduce the issue

Add a 3rd part menu module to site.
Such as MenuCk
maximenuck-pro_9.1.11.zip
mobilemenuck-pro_1.3.3.zip

Expected result

When viewed on PC as window width is reduced except the mobile menu burger bar image to kick in.

Actual result

Something else kicks in and messed up what MenuCK is trying to do.
Turns out it its the bootstrap collapse code in the cassiopeia temlate (index.php and error.php).

System information (as much as possible)

Joomla 4 Beta 7
PHP 7.4

Additional comments

I suggest a 'collpsiblenavbar' option be added to cassiopeia.

if ($this->params->get('collapsiblenavbar', 1)) :
    HTMLHelper::_('bootstrap.collapse', '.navbar-toggler');
    ?>
    <button class="navbar-toggler navbar-toggler-right" type="button" data-bs-toggle="collapse" data-bs-target="#navbar" aria-controls="navbar" aria-expanded="false" aria-label="<?php echo Text::_('TPL_BFUKCASSIOPEIA_TOGGLE'); ?>">
        <span class="icon-menu" aria-hidden="true"></span>
    </button>
    <div class="collapse navbar-collapse" id="navbar">
        <jdoc:include type="modules" name="menu" style="none" />
    </div>
    <?php
else :
    ?>
    <div id="navbar">
        <jdoc:include type="modules" name="menu" style="none" />
    </div>
    <?php
endif;