Hey YouTube users. I've got something for you to try. Start a video and tap the little overflow button at the top right, it'll appear for a short time if you tap the player. For the last few years, you would have seen three or four grayish buttons overlay onto the video with the labels: Captions, Quality, Report, and Cardboard. Here's the thing, you might not see that anymore. Instead, a little white card might slide up from the bottom of the screen with the same options. Take a peek at the screenshots to see what I'm talking about.
Left: old style with a video overlay. Right: new style with pop-up menu.
If you're still stuck with the old style, don't fret, you're not alone. There's no specific APK to download or any tricks to go through to get this new and massively improved interface, it's controlled by a server-side switch.
YouTube is running what's called an A/B test. It's a practice in marketing and product development that involves creating two (or more) similar versions of a product with (usually) minor differences and asking groups of random people to try them out. This sort of thing is usually done with focus groups, but some companies run these experiments in the wild.
We first started receiving reports about the menu about four or five months ago. Over the next couple of months, the same menu appeared on some of my own devices and those of other readers. Oddly, it only turned up on about half of my gadgets, but never the rest. The even weirder thing is that the new menu has since disappeared entirely from every device I own; meanwhile we've received dozens of tips from readers to say that they just started seeing it. – Many thanks to everybody who has sent in tips, we definitely appreciate it... but we're probably good with this one. ;-)
Hopefully this new menu rolls out officially to everybody in the near future, because I can say I much prefer it to the old style.