Smooth transition of site divider

On the https://gif.com.ai/ splash page there is a site divider on the left half of the page that when scrolled abruptly disappears and reveals a full-page navigation. To me and others this feels clunky and it is surprising that the page all of a sudden goes full width and reveals everything. Is there any way to address this UI feature and ease the user into the experience better? Perhaps changing the opacity gradually on scroll or shifting the screen slowly to the left to reveal the 100% width view. Or a combination of ideas. Any examples of and related links would be very helpful.