Optional
Microsoft VP and Technical Fellow, Laura Butler, weighed in by tweeting, "I don't wanna rain on the parade" and then did exactly that by pointing out that good old calc.exe had the topmost functionality back in 1994.
And I don't want to rain on Butler's or Microsoft's parade (I want to piss on it), but they should go back a little further and look at the calculator on RISC OS - or indeed any application on RISC OS. (To be fair, it has many problems due to the numbers involved - developers, users, and the money developers can make because of the number of users - but the active window not being forced to the top of the stack is one of its very good points).