Help me understand your intent.
You want a preloader or fullscreen section to be visible and un-scrollable? Then once all elements are loaded, the preloader section stays but it’s now possible to scroll down. Is that right?
If so you need to section elements. The first on should be set to 100vw width and 100vh height. Add a background image or color if you want and place all desired elements of the preloader inside this section.
The second section element should contain the remaining content for the page. On this element add a Load triggered interaction. Initial state should be to display none. The first step, after assets load, is to set the display to block. You can add a step at the beginning with wait time value if the load is faster than desired.