You could disable the stickiness by creating a theme and adding this to your CSS:
.d-header {
position: relative;
}
This may impact the layout in other ways and cause issues with the dropdown menus, so you’d need to write some additional CSS to get around those problems. Getting started with themes: Structure of themes and theme components.
There are also a lot of additional topics that might help you with general Discourse questions in the #howto category.