Android P's new Recents UI scrolls horizontally according to 9To5Google, who has obtained information about the look and feel of the new interface and other navigation bar tweaks that Google is making to the upcoming version of the mobile operating system. In addition to the information there are also screenshots which depict what users will see once they have Android P installed, though it's worth mentioning that the screenshots are not actual screenshots of the design but rather mock-ups based on the information. As noted the Recents UI will scroll horizontally instead of vertically like it does now, and the button that opens this UI, which is a square-shaped button in the current Android version and prior, is going to be removed in favor of a gesture-based interaction with the new pill-shaped home button that Android P will feature as well.
To access the recently opened apps from the Android home screen with Android P users will need to swipe up from the home button, which brings up large card-style panels of all apps that are currently open and running. In addition to the new scrolling orientation the panels will also have more rounded corners, whereas they're more squared off at the corners currently. Not much has changed in the actual functionality of how things work.
Instead of swiping up and down to scroll through your open apps, you swipe left or right, and instead of swiping left or right to close an app, you swipe up for the same action. Essentially Google has just flip-flopped things here. The back button is still in place with the new nav UI though it is said to disappear if the home screen is visible and sitting idle with no interaction long enough. Though the screenshots are just renders and not official screenshots of the software design changes, it's likely that things will be styled in a very similar way.