File Manager uses the multiple document interface to display multiple folders inside one window. It was changed in later versions of Windows but was the Windows standard for a decade.
Rather than being happy about the Volish news, some Open Sourcers have been seeing the glass as half empty. One Hacker News reader noted that: “Most of the MSFT open source stuff is either trash or completely unmaintained. Only a couple of high profile projects are maintained, and they jam opt-out telemetry in if you like it or not (despite hundreds of comments requesting them to go away). Even Scott Hanselman getting involved in one of our tickets got it nowhere. Same strong-arming and disregard for customers.”
So, in other words, Open Sourcers will only be happy once Microsoft open sources Windows 10 or Office. But given a few years ago Vole was calling Open Source a "cancer", Microsoft seems to have come a long way.