Hi Tim,
I see the issue now.
I can see that you have a #page-container
element around all your content, when scrolling, an inline style of margin-top: -121px;
is added to this element which is causing it to shoot up off the page.
If you can track down where in your theme/plugins this is happening then you can try and remove it as that is the cause of the issue. I don’t know why it works when my plugin is disabled, potentially the JavaScript is using my plugins HTML to calculate something and getting confused.
Can I also suggest using the plugin shortcode and placing that inside the #page-container
element to see if that fixes it. It could be that placing the menu HTML inside the container will stop the JavaScript from getting confused.
Please let me know if either of the above work.
Peter