The collection is not fixed when I animate the section in which this collection is located

Hey. I need animation of the collection, but there is a small nuance that does not allow it to be done. I need to click on the image and the bottom should appear div and was fixed (so that the user could not scroll the page).

Animation should be such: Section (all page) rises on 100vh upwards and on 100vh the fixed div rises.

But the problem is that if I animate the section (raise all the page) the div stops to be fixed. If I delete animation of the sections - the div, as it should be, is fixed on the page. Why and how to fix it?

See a short video that I recorded. And you will understand my problem better: