No, this is a new feature for the display manager. Previously in Ubuntu (with unity) the window would be the whole window area would get covered by an orange transparent film, and it was the orange film that would be resized. The actual window would only resize on release.
Yes, not exactly a revolutionary new feature, but newly enabled in unity by default.
> I thought this feature was present in OS GUIs since about 1997?
It was, until all the fancy compositing WMs like Compiz came around. Those usually had it as an option, but not as the default because it lagged way too much.
If someone has a better link, please share!