It's not exactly the same, but I did something similar in my Arcadeflow theme: when you go down from the grid view you "enable" an horizontal scrollbar to scroll faster through games. It's not difficult to code, just requires a bit more complicated input management routine.
In my theme I have a "on signal" section which already tracks arrow buttons, and usually left and right are used to scroll through games. When the theme detects a "down" press from the last row it enables a switch variable (something like scrollbar_enabled = true) and this same variable is checked by the "left" and "right" case statement.
So if the variable is false "left" and "right" move the games, otherwise there's a routine to update the scrollbar.
I don't know if it helps