Regarding BlackThief07's working code, there is one huge flaw in it. When you cycle left or right per game with a single press, the videosnap changes properly with the selected game. However, if you hold the left or right button and it spins over multiple games, then the moment you let go and it lands on the selected game, the videosnap is playing the one previous game from the selected one.
Here is a video demo of it happening:
https://www.youtube.com/watch?v=H3KhbTAw2m8&feature=youtu.beMegaMan6 and Metal Gear show up properly with single clicks. But when you long scroll to Ninja Gaiden, the videosnap is of Punchout playing. You can see in the video the same thing happens to River City Ransom, where Prince of Persia is instead playing after a long single scroll. This is with Horizontal Scrolling enabled in the Display Options. When you turn it off, then this problem disappears but then you can't scroll through the games with "left" or "right" anymore and instead must use the "up" and "down" keys.
I don’t know. I have no issues at all using LEFT and RIGHT for game navigation, so maybe I don’t understand.
Are you saying you can use the left and right keys in the theme and then hit tab and use the same left and right properly in the config menus? Because I can't. Unless there's a specific code implemented (like the Blackthief07's code) into the theme, every time I try to re-bind the left and right keys to prev_game and next_game, the default bindings for left and right gets overwritten/erased (like the picture I posted previously) so that the left and right do not work in the config menus.
What horizontal theme are you using? Mind posting a snap of your button config file that works with that theme?