Microsoft has released Insider Preview Build 15031 for PC to insiders in the fast ring. This release introduces two new features called Dynamic Lock and Compact Overlay Windows. Dynamic Lock automatically locks your Windows 10 PC when you re not around based the proximity of a Bluetooth-paired phone. Compact overlay windows allow developers to place their applications into a small window that overlays all other applications on the screen. This allows you to continue watching a video or monitor an application, while performing another task in another application.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-insider-build-15031-for-pc-adds-dynamic-lock-and-compact-overlay-windows/