Windows 10 build 17666 (rs prerelease)

Windows 10 build 17666 is the eleventh Insider Preview build of Windows 10 October 2018 Update. This build was released to Windows Insiders in the Fast Ring on 9 May 2018.

Sets

 * Sets now have an acrylic title bar. The window border color has also been changed to be gray.
 * All recent Microsoft Edge tabs are now included in the Alt+Tab switcher by default. You can go to the Multitasking settings and then choose the "Pressing Alt + Tab shows the recently used…" option to turn it off.
 * The Sets section of the Multitasking settings now contains ways to customize your windowing preference. If "Window" is chosen, new windows will not open into new tabs unless the user explicitly chooses to open a new tab using the "+" button or by dragging them into a window. If "Tab" is selected, new windows invoked from running apps will automatically open into tabs. You can also exclude apps from said section.
 * If a tab is playing audio, a volume icon now appears. Select it to mute the audio.
 * Tabs now restore with better performance, they now open in the background and don't use any system resources until you go into the tab.
 * Task Manager has now been added by default to the list of apps to not enable Sets.

General

 * Introduced a new clipboard copying experience. Highlights include the ability to paste from the clipboard history, the ability to pin the most frequently copied items. To enable it, navigate to Settings, then to the System page, and the go to the Clipboard section. Once there, select the option under "Save multiple items", and then press Win+V to use it.
 * Officially introduced the dark theme for File Explorer, the context menu for said app, and the common file dialog (the Open and Save dialogs). It was present in earlier builds, but required mach2 to enable it.
 * Notepad now supports Unix/Linux line endings (LF) and Macintosh line endings (CR).
 * Added the ability to search Bing within Notepad. It can be executed by way of three methods, these are right-clicking on the selected text and then choosing the "Search with Bing..." option, via the Edit menu and selecting the "Search with Bing..." option, or by pressing Ctrl+B on the keyboard.
 * Expanded the search previews to support apps, documents and other supported items.
 * Added the ability to rename tile folders on the Start menu. The new name will be visible if the folder's size is medium, wide, or large.
 * Added a question section in the Settings app to show common questions that are asked by Windows users.

General

 * Sometimes the update process appears to get stuck at percentages between 80% to 100% for a prolonged period of time.
 * After completing the Windows Mixed Reality First Run experience, the OOBE (in Mixed Reality) is completely black. Motion controllers are also not recognized in exclusive apps.
 * You may get to see the desktop before you get to the lock screen upon resuming from stand-by.
 * The Movies & TV app can crash if the user denies access to their video library and then navigates to the Personal tab.
 * Narrator can read extra text when Alt+Tab is invoked.
 * If you finish the setup for a Windows Mixed Reality headset on this build, the headset will remain black until you unplug and reconnect it to the PC.
 * If room boundary is enabled in your headset, some controllers may glitch out while playing a SteamVR game, make sure the blue bar is present at the top of the desktop. If it isn't, press Win+Y to make it appear. This makes the input go to Mixed Reality instead of another location.

Sets & Office

 * The top of some Win32 desktop apps may appear slightly underneath the tab bar when they're opened maximized. To fix this issue, restore and re-maximize the affected window.
 * Sometimes closing one tab can minimize the entire set.
 * Tiling and cascading windows do not work for inactive tabs.
 * Opening an Office document while the same app has an existing document open may cause an unintended switch to the last active document. It can also happen if you close a sheet in Excel while other sheets are open.
 * Locally stored files and cloud files that are on non-Microsoft cloud services are not automatically restored.