Windows 10 build 9821

Windows 10 build 9821 is a pre-Technical Preview 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 Engagement Access Program (EEAP), as indicated by the  branch.

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

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. Title bar of them also recieved 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.

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.

Window thumbnail background color
The background color of window thumbnails on the taskbar retains the default color even if the user changes the theme color. Restarting will fix this bug.

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

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.