With more smartphones coming out with 18:9 displays, developers have to work around the new display aspect to get the most out of their apps. Google is helping out Pixel 2 XL users with a new YouTube mode just for them. The streaming app now has a zoom-to-fill mode. By default, the video will fit inside the screen but you can do the zoom in gesture to fill the entire display. To revert to the default size, all you need to do is make the zoom out gesture. This feature comes to the Pixel 2 XL via version 12.40 of the YouTube app.
Source: Android Police