thumbnailLevelUp option doesn't create a clickable link in iOS

[ChristopherWerby]

thumbnailLevelUp option doesn't create a clickable link in iOS when trying to use the thumbnailLevelUp: true option.

The 'chevron-up' thumbnail within an album can't be clicked on iOS, even though it works fine on a desktop machine. I forked one of your pens and modified the options -- it displays the problem when loaded on an iOS device: https://codepen.io/Pipsqueaks/pen/wmqRgZ

I forked it again and stripped down the options for a minimum set, and I still have the problem that the navigation "thumbnail" isn't clickable in iOS. https://codepen.io/Pipsqueaks/pen/VXzgRL