Horizontal scrolling with shadow

Been banging my head against a wall all weekend trying to adapt this vertical implementation:
//codepen.io/haibnu/details/Fouvp

to horizontal. Here’s where I got to:
//codepen.io/danwick/pen/jJfyi

Narrowed it down to the linear-gradient (vertical vs horizontal?) and/or the background-size.

The background size (length|percentage|cover|contain) is throwing me off.

//www.w3schools.com/cssref/css3_pr_background-size.asp