YouTube 12.40 brought one small but neat change: a pinch-to-zoom gesture that lets you expand the video to fit an entire 18:9 display. Since most videos are made for the 16:9 format, black bars would sit on each side of the display when you're viewing YouTube videos in fullscreen mode, which sort of detracted from the illusion of having that big shiny nearly full-front display.
With 12.40, only the Pixel 2 supported the gesture, leaving other 18:9 smartphones behind. Now we've verified that v12.44 of the YouTube app makes the gesture available to the Galaxy S8+, LG V30, and possibly more devices. You can see it in action in the gif below:
Artem verified that the gesture wasn't working for him in v12.43 on the S8+ but is on v12.44.52. You can grab that on from the Play Store or from APK Mirror if it isn't live for you yet.