Fixed widget covers scrollbar when "stack all pages vertically" is turned off

Hi!

I found a very problematic issue. I made an horizontal scrolling gallery, following this tutorial:

The problem is that I only need it in a page of my website, and my website doesn’t have the option “stack all pages vertically” because, actually, I don’t need that.

My problem is that WITHOUT that option turned on, my fixed widgets appear on top of the scrollbar, giving it an confusing apperance.

Please, hope somebody could help me with these. Thank you in advance!