Windows 11 build 21380 is the earliest available build of Windows 11, which was first shown running on an Acer Aspire Vero prototype, which was first demonstrated during the 2021 Hong Kong Computer and Communications Festival. The build was obtained by BetaWorld members and uploaded to the aforementioned website on 8 June 2023 and was shared publicly two days later via the BetaWorld Blog.
This build is one out of many known builds to be compiled from a branch with the
fs prefix. The prefix stands for Firesteel, the codename for an internal Microsoft self-hosting effort related to Windows 11 development. It is currently the earliest available build to introduce a sizeable number of user interface changes that would be later included in the final release, which include but are not limited to reworked versions of the Start menu and taskbar, the new Widgets feature, and functional improvements to window snapping. A leak source detection implementation is also present in the build, and can be controlled through the use of a Velocity staging key.