Sets—a new Windows interface feature that was first previewed in November 2017 and will make every window into a tabbed window—has been removed from the latest Insider Preview build of Windows 10. Moreover, the Verge is reporting that the feature won't be coming back in this year's next major update, due in October.

This marks the second time that Sets have been included in a preview release only to be removed at a later stage prior to the release of an update. When first announcing Sets, Microsoft was careful to note that it wasn't promising Sets for any particular release—or possibly even ever, given the complexities of application compatibility and uncertainty about how people will actually use the feature.

The promise of Sets is certainly high. Putting tabs in every window is a way of solving certain long-standing requests, such as the demand for tabs in Explorer. But Sets went far beyond this, allowing collections of different applications to be grouped together with tabs to switch them. As such, Sets became a way of managing one's workspace, allowing you to combine, say, a Word window of a school paper with the online resources that you're using to write the paper.

Complicating this, however, is the issue that many Windows applications do all manner of weird and occasionally wonderful things with their title bars. With a simple application like Notepad, which does nothing at all non-standard, it's straightforward for the operating system to take the window's titlebar and add tabs to it. That's because the operating system is entirely responsible for drawing the titlebar in the first place.

But in an application such as Office or Visual Studio, where the window titlebars are customized by the application, the task becomes much harder: the operating system has to ensure that the tabs don't break the application. We'll likely see an API somewhere down the line that enables the Sets system to coordinate the tabbing with the applications. But that's still very much a work in progress.

The next version of Windows, likely to be version 1809 with the codename "Redstone 5," is also going to be a special release. Version 1809 is going to be blessed as a long-term support channel (LTSC) release, meaning that instead of 18 months of patches and support, it'll receive the traditional 10 years of support and security fixes. If it weren't an LTSC release, one might expect Microsoft to release a first iteration of Sets—for example, a Sets that only worked with modern applications built using the UWP API along with Win32 applications that used standard titlebars. Then you might use the next semi-annual update to add support for Win32 applications with custom titlebars. The long lifetime for version 1809 makes this kind of partial, phased release much less appealing, as those people using it will forever be stuck with the partial feature, at least until the next LTSC release is made in 2021.

Microsoft says that it's still working on improving the Sets experience and that the feature isn't gone for good. But Sets looks like it needs more time in development than is available.

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