I remember one of the features I admired on the Macintosh in the late 80s was support for multiple displays. First time I saw this was on an SE/30, using the internal display along with a big external CRT. It had all the expected behavior of one giant desktop, allowing the user to drag the mouse and windows between the displays. Pretty cool for 1988, or so I thought. This may have used some 3rd party product (Radius?), but I am pretty sure the Macintosh II had the functionality out-of-the-box at this time, just by dropping in additional NuBus display card.
I can't recall when Windows started to support multiple displays like this. Which version of Windows first offered this functionality and with what sorts of display cards? Were there any popular 3rd party solutions for Windows before this, that allowed the Desktop to span multiple displays?
NOTE: Specifically asking about the Desktop/OS recognizing both displays - not just "mirroring" a display to an external monitor or projector as any laptop with external video port could do.