Windows 10 build 21277

Windows 10 build 21277 is the first Insider Preview build of Cobalt, the 21H2 development semester of Windows as a service. This build was released on 10 December 2020 to all Insiders on the Dev Channel along with Iron build 20277.

Features such as redesigned touch keyboard, emoji picker, voice typing, and theme-aware splash screens that were removed in Iron build 20246 have been reintroduced in this build. However, those that were only available to a small group of Insiders haven't been completely rolled out yet. In addition, the Meet Now integration on the Windows taskbar introduced in Iron build 20221 is now available for the Enterprise SKUs and Azure Active Directory (AAD) user accounts, where it was previously disabled.

x64 emulation has been added as a preview on ARM64 devices. Unlike WOW64 and the previous x86 emulation, the new emulator does not use a separate System directory with x64-compatible binaries. Instead, existing ARM64 binaries in  have been extended with x64-compatible code, such executables are then referred to as ARM64X executables. The implementation in this build requires the user to download a preview version of the C++ runtime, a requirement that would be removed in build 21318.

Changes

 * Support for Unicode Emoji versions 12.1 and 13.0 have been added. This includes over 200 new glyphs, a new gender-neutral design, and some existing emojis being redesigned.
 * The animations when opening and closing a window have been updated.
 * Snip & Sketch can now be used to create a screen snip and paste it directly into a File Explorer folder to save the screenshot there. This change was initially added to the Beta Channel in Feature Experience Pack build 120.2212.1070.0.
 * Snipping Tool is now an optional feature, and it can be uninstalled if preferred from Settings → Apps and features → Optional features.
 * The scaling logic of the graphs in Task Manager’s Performance tab has been adjusted.
 * Registry Editor dialog windows have been updated to support the Ctrl key stroke to remove words.
 * The Start menu has been updated to display items at their parent folder's level if they are the only item in that folder.
 * Xbox Console Companion is now completely replaced by the new Xbox app.
 * Mixed Reality Portal's setup screens now feature a video showing a headset and some experiences.
 * Microsoft Wallet is now considered unsupported, blocking users from installing it.

Bugs

 * The update process to this build has a chance of hanging for extended periods of time.
 * Some users might see a popup that says “Your PC ran into a problem and will restart in 1 min” after logging in to Windows with either an Azure Active Directory or Active Directory account. If encountered, save any data immediately and let the PC restart.
 * might hang or crash in this build.
 * Live preview doesn't work for pinned site tabs on some machines, a gray window is seen instead of the intended content.
 * The modern Disk Management page located at Settings → System → Storage → Manage Disks and Volumes in the Settings app is completely blank, and as a result it will not display any drives present on the system. To workaround this issue, use the legacy Disk Management tool to manage disk drives.
 * Aero Shake is disabled in this build. To enable it again, merge the following to the registry and restart :

Availability
Starting with this build's release, the Dev Channel went under a temporary splitting of released builds. While the channel still received mandatory updates from the Iron semester, builds from this version would be released at the same time as optional updates, giving Insiders the ability to choose to keep receiving Iron builds or moving forward. For this case, the accompanying Iron build was 20277.