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Windows 10 Technical Preview Build 9879 brought small but useful interface changes, OneDrive selective sync, expanded MKV support, and several touchpad gestures. It reached the Windows Insider Fast ring on November 12, 2014; Microsoft announced Slow-ring availability and ISO images on November 24. This is a historical preview release, not a current Windows installation recommendation.
What Build 9879 changed at a glance
Microsoft delivered Build 9879 as a full in-place upgrade. Its most practical changes addressed taskbar clutter, app-window controls, OneDrive offline access, media compatibility, and feedback tools. The release also had significant known problems, so its new features should be considered alongside its limitations. Microsoft’s November 12, 2014 announcement describes the release.
How to hide Search or Task View buttons
To remove either button from the taskbar, right-click an empty area of the taskbar and clear the checkbox for Show Search or Show Task View. You could hide one or both, depending on which controls you wanted visible.
Other taskbar, window, and snapping changes
App-window controls and animations
Microsoft revised the minimize and restore animations in response to feedback. It also replaced the three-dot control on modern app windows with a three-line, hamburger-style icon, intended to make app commands and full-screen controls easier to find.
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Snap Assist across monitors
Snap Assist could now help when arranging apps across multiple monitors, making it easier to choose another window after snapping one into place.
Precision touchpad gestures
On a supported precision touchpad, Build 9879 added these three-finger actions:
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- 15.6" diagonal, HD (1366 x 768), micro-edge, BrightView, 220 nits, 45% NTSC.
- Swipe up to open Task View.
- Swipe down to show the desktop.
- Flick left or right to switch to the previous app.
- Move left or right to bring up Alt+Tab for app selection.
- Tap to open Search.
Microsoft’s release notes specify a precision touchpad; they do not establish that these gestures work with every external touchpad.
OneDrive selective sync: choose what stays local
Build 9879 changed OneDrive to selective sync. In Windows 8.1, users could see online-only placeholders in File Explorer and mistake them for files already available offline. The new approach let users choose which folders were synced to the PC, clarifying what was stored locally.
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- 10th Generation Intel Core i5-1035G1 processor
- 12GB system memory for full-power multitasking
- 256GB Solid State Drive
- 15.6" Micro-edge touchscreen display
File Explorer became the single route to OneDrive files. For settings such as automatic saving and which folders were available offline, right-click the OneDrive icon in the notification area and open Settings. Microsoft’s Build 9879 announcement explains the change.
MKV playback, H.265, and Internet Explorer’s engine experiment
More complete MKV support
Build 9879 expanded MKV support in several ways: Windows Media Player and other apps could play MKV files, File Explorer could display their thumbnails and metadata, and MKV was supported in DLNA and Play To scenarios. Microsoft also announced platform support for H.265, also known as HEVC. These were capabilities documented for the Technical Preview build, not a claim about every app or later Windows configuration.
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- Latitude 7480 Laptop 14"
- Intel Core i7 6th Gen i7-6600U -Core Processor 2.6GHz (3.4GHz With Turbo Boost)
- 256 GB SSD Hard Drive & 16GB Memory
- 1920x1080 FHD resolution Non-Touch with Webcam and an integrated graphics chip
- Wireless Wifi & Bluetooth
Edge rendering engine enabled for a subset of Insiders
The Internet Explorer team enabled its interoperability-focused Edge rendering engine for 10% of Windows Insiders in this build. This was an experiment for that subset, not a universal default for all Build 9879 installations. Microsoft’s release notes describe the scope.
File Explorer, dialogs, and Insider tools
- Pin folders: Users could pin favorite folders to File Explorer Home.
- Windowed dialogs: Modern dialogs could appear in windows rather than only in a full-screen interface.
- Insider Hub: The build included the Hub for program news and quests.
- Feedback app: Users could sort feedback by trending topics, “Me Too” votes, and recency. Microsoft also improved the return behavior after submitting feedback, fixed a search/filter issue, and made the screenshot control easier to find.
ISO images were published with the Slow-ring release announced on November 24, rather than the initial Fast-ring delivery on November 12. Microsoft’s announcement covers the release timing.
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Build 9879 known problems
Microsoft’s November 12 release announcement listed these issues and said it expected Windows Update to address the first two; the remaining listed issues would not be fixed for Build 9879. The list was specific to this historical preview build:
- Black screen at sign-in or unlock: Some users could encounter an intermittent black screen and might need a hard reboot.
- Distributed File System connections: Users could be unable to connect to DFS network locations.
- Unexpected disk growth: Duplicate driver installation caused disk use to grow by 20GB or more on some systems. The resulting loss of free space could prevent setup from completing and lead to rollback.
- Skype and Music interruptions: Skype calls could disconnect and Music could stop when minimized.
- Lync screen sharing: Microsoft listed several screen-sharing issues.
The release notes do not say that buying extra storage or using third-party repair software fixes the duplicate-driver issue. These are historical Build 9879 caveats, not a general troubleshooting guide for current Windows installations.
What Microsoft later said was fixed—and what KB3025380 did
In a December 17, 2014 update, Gabe Aul reported almost 1,300 bugs that participants had reported or upvoted as fixed across the Insider effort. That aggregate was not 1,300 fixes exclusive to Build 9879. The update separately said that the 0xAB bluescreen and Explorer.exe crash had been patched for Build 9879. The December 17 Insider update also reported more than 1.5 million registered Insiders and about 450,000 highly active participants.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsKB3025380, published December 18, 2014, served a different purpose: Microsoft said it allowed Windows Insider members to continue receiving future Technical Preview builds. Its support article says there were no prerequisites and that a restart might be required. It does not describe the update as a general repair for the black-screen, DFS, disk-growth, Skype, Music, or Lync problems. Microsoft Support: KB3025380.
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