Sometimes we really need to send a message. Message so powerful and important that we need to nave nothing in between it and the viewers – even the page navigation, nevertheless it's no doubt one of the most important and needed parts of any page.
This is easily achievable in Mobirise Builder – we just need to use a full screen Header block and two little scripts you can get from here and just copy / paste in the page's head and end of body sections (Code Editor Extension needed)
So finally the page will load with no navigation visible like this:

And when user scrolls down the navbar gracefully slides down so the navigation gets accessible

It's really easy and actually – fun and takes just three steps:

<style>
.navbar-fixed-top  {
top:-70px;
opacity:0;
}
</style>
<script>
 /*---------------------------------------*/
 /*	NAVIGATION AND NAVIGATION VISIBLE ON SCROLL
 /*---------------------------------------*/
$( window ).on( "load", function() {
    mainNav();
    $(window).scroll(function() {
        mainNav();
    });
    function mainNav() {
        var top = (document.documentElement && document.documentElement.scrollTop) || document.body.scrollTop;
        if (top > 40) $('.navbar-fixed-top').stop().animate({
            "opacity": '1',
            "top": '0'
        });
        else $('.navbar-fixed-top').stop().animate({
            "top": '-70',
            "opacity": '0'
        });
	}
     });
</script>
That's basically it – you're done!
You won't see any changes in Mobirise Buidler visual editing interface since it's easier to handle and edit the navigation block this way – not chasing it around – but the effect will gracefully trigger as long as you Preview or Publish the page.
A tip: It's a good idea switching the scroll-down arrow of the first intro block on – it gets easier for the visitors to get once they should move down to experience the whole page.