Snap Mode allows you to dedicate a portion of the screen to a single app, taking up a large column typically for viewing media apps like OneGuide's TV side-by-side with a game.
It's a throwback to Windows 8 apps, which could be snapped side-by-side in a similar way. Microsoft killed the feature for Xbox app developers last summer when it launched UWP for Xbox One. Xbox owners were still able to access legacy apps that made use of Snap Mode, but no longer, with the builds on the latest Xbox Insider Program update wave.
We reported last November that Microsoft was working on picture-in-picture functionality as a replacement for Snap Mode, and today, we've gotten a glimpse at what that could look like.
In Windows 10 Fast ring build 15031, Microsoft has introduced a new Compact Overlay mode for UWP app developers. This enables a picture-in-picture experience that we heard would be coming to Xbox One. In fact, one of the first apps that will utilize this feature is Movies & TV, and it stands to reason that the Xbox One version of the app will also pick up the same update.
The Xbox One is currently using overlays for Cortana and the Achievement Tracker, and we've seen that Beam will also be using overlays too for displaying chat, and more.
While I have no hard confirmation (yet) that Compact Overlay mode specifically is the replacement for Snap Mode, it matches up with everything we've heard previously. I'm willing to bet money that this is the replacement for Snap Mode.
We've reached out to Microsoft for confirmation, but we'll probably find out as soon as the Movies & TV app for Xbox One is updated in the Alpha or Beta ring very shortly.