Build 9879 was compiled around November 2nd, and includes a number of small improvements over the last public Technical Preview build being 9860. According to the screenshots published by IThome, Microsoft is fine tuning certain elements of the UI, as well as adding more options for the user.
One option which users have requested is the ability to hide the Search and Task View buttons on the taskbar. In build 9879, you now have that ability by right clicking the taskbar. The Home area in File Explorer has also seen some additional changes, now with the ability to pin things to the Home area.
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