Windows 10 build 9821

Windows 10 build 9821 is a build of Windows 10, which was shared on the BetaWiki Discord server on 11 May 2020. Prior to being shared, Polish website WinClub.pl had released one screenshot of it. This build was originally released to Microsoft partners participating in the Ecosystem Engineering Access Program (EEAP), as indicated by the  branch.

It is the first available build to identify itself as "Windows Technical Preview" in many areas and the first available build to update the kernel version to 6.4.

The copyright year in winver was updated from 2013 to 2014. This persisted up to build 9883.

Aero changes
DWM-based window animations (particularly when opening and closing windows) have been changed to simpler transitions. In addition, the Aero theme's window borders have been made thinner, though the Aero Lite theme still has the thicker border style.

Branding
This build uses the "Windows Technical Preview" branding on the desktop and "Windows" branding in winver instead of "Windows 8.1" and "Windows 8" in those areas respectively.

Windowed Metro
Native support for windowed Metro apps is now enabled by default. The title bar of them also received its own style.

New icons
Some icons (including Explorer's) have been changed to a flatter style.

Task View
An multi-desktop mode has been introduced.

Taskbar

 * Action Center, a Windows Phone-like notification area, has been added.
 * Search and Task View icons have been added to the taskbar.

Bug with the Metro
Dragging a window in full-screen mode in Metro apps will result in Metro transitions from Windows 8.1.

Explorer.exe error during shutdown
When the system shuts down, a memory access error related to  may occur.

Timebomb
Like other Windows 10 builds found in the  branch, this build contains a non-functional timebomb and as a result can be installed on the current date.

Taskbar Language Bar
The language bar is not displayed on the taskbar, but however the old language bar is working properly.

Window thumbnail border color error
If window auto-colorization is enabled, the window thumbnail borders on the taskbar do not change color even after the wallpaper changes. This issue can be remedied by manually changing the window color through the Control Panel or restarting Windows.

Incorrect file operation progress window size
The file operation progress window (which appears while copying, moving, and deleting files) may not extend automatically when more than one operation is in progress.

Closing notifications in the Notification Center
When swiping away all notifications from a system tray application, particularly the Action Center,  will crash.