Windows 10 build 18946

Windows 10 build 18946 is a private 20H1 build of Windows 10 which was not intended to be released but accidentally flighted to all insiders in the Fast Ring on 2019-07-24. It's currently the first leaked build of the rs_xbox_dev branch.

Breaking the known insider releases routine, Microsoft managed to also release 18947.rs_prerelease as well as 18947.rs_xbox before pulling all three releases after publishing the AMD64 variant of this build within ~5 minutes.

They also broke the UX routine and showed up some new major changes.

New features
Both features can be enabled using mach2.
 * Fully redesigned Start Menu. It is likely intended to be used in the new Windows Lite operating system. Currently it works properly only with Light theme.
 * New Control Center and Notification Center design. Now it's more compact and convenient. By default it only shows buttons, but it expands as you receive notifications.