For two periods last year, those using preview builds of Windows 10 could access a feature called Sets: a tabbed interface that was eventually to allow tabs to be put in the titlebar of just about any window. These tabs would allow both multiple copies of the same application to be combined—a tabbed Explorer or Command Prompt, say—and multiple disparate windows to be grouped—combining, say, a browser window containing research with the Word window. However, both times the feature was enabled only for a few weeks, so Microsoft could gather data before disabling it. Sets aren't in the Windows 10 May 2019 update.

The Shell-provided tab experience is no more, but adding tabs is high on our to do list. — Rich Turner (@richturn_ms) April 20, 2019

It seems now that Sets are unlikely to ever materialize. Rich Turner, who oversees Microsoft's revamping of the Windows command-line infrastructure and the Windows Subsystem for Linux tweeted that the interface "is no more." Having everything tabbed everywhere isn't going to happen. Adding tabs specifically for command-line windows is, however, "high on [Microsoft's] to do list."

There was initially some confusion that the tweet might have meant that some other system-wide approach to tabs was going to be used. But Turner clarified today that the command-line tabs will be purpose-built for command-line windows, not a general feature for the entire operating system.

Sets was an extremely complicated feature. Initially, Microsoft planned only to use it with new programs written using the Universal Windows Platform (UWP). That's because UWP programs are generally well-behaved. Programs built using the old Win32 API can do all sorts of strange things, such as drawing menus into their titlebars (as Visual Studio does), having ribbon controls merged into the title bars (as Office does), or even putting tabs into their titlebars (as Chrome does). UWP applications, by contrast, do have some control over their titlebars, but it's much more controlled and consistent.

However, since most Windows applications use Win32, a UWP-only feature wouldn't be too useful. Besides, that would have meant excluding Explorer and the command-line windows, arguably the two areas where tabs are most in demand. So Microsoft expanded the reach of Sets, allowing it to be used with Win32 applications. This created an enormously complex interoperability issue that Microsoft never quite cracked.

Compounding that, Sets used the tabs designed for the Edge browser. With the new Chromium-based Edge, Microsoft is now using Chromium's tab implementation. This forfeits Edge's nice features such as Fluent design and tab thumbnails. It also means that the company has much less interest in maintaining that source code.

Taken together, the result? No more sets.