Hi there,
Thanks for writing in! Please add the following jQuery script in your Customizer via Appearance > Customize > Custom > Edit Global Javascript
function stop_jumps() {
jQuery('header.masthead.masthead-inline').css('height', jQuery('.x-topbar').height() + jQuery('.x-navbar').height());
}
jQuery(document).ready(function(){
stop_jumps();
});
jQuery(window).resize(function(){
stop_jumps();
});
Don’t forget to clear your browser’s cache after adding the code. Let us know how this goes!