Snap Mode, that gimmick on the Xbox One that let you "snap" something else onto the side of your screen while you were watching a movie or playing a game, is no more. Microsoft is ditching the feature to make the console's dash run faster.
We replaced Snap to improve multitasking, reduce memory use, improve overall speed, and free up resources going forward for bigger things.
— Mike Ybarra (@XboxQwik) January 24, 2017
The Xbox One's dash has always been a tad sluggish in parts, so few people are going to miss it if they're getting faster response times elsewhere.
Snap will be removed as part of an Xbox One update that will be rolling out in a few months' time, and which is also bringing stuff like an interesting new "copilot" mode.