Back to top (scroll up) displays incorrectly on mobile

When viewing my home page on a mobile, the “back to top button” is treated like a column running the full length of the page down the right-hand side, squashing all other content across to the left. I can’t see anything in theme options to amend this. It looks ugly, as there’s nothing in the yellow column, apart from the back to top button right at the bottom. So there’s a “phantom” yellow box all the way down the right-hand side for no apparent reason. Also, it’s taking up valuable screen space, which makes everything else on this page smaller.

All other pages seem fine, and the back to top button is overlaid as expected, it’s just the home page. You can see it here: http://www.inkypixel.co.uk