I've yet to see any platform where this is the case.
Windows 8 Metro, supposedly. They designed it to be either vector graphics or for bitmaps you have to supply multiple sizes. The largest size is chosen so that higher resolutions than that won't be neccesary because the human eye can't perceive it, so it will be future proof (at least that's what their marketing says).
FWIW, that's exactly the approach Apple has taken with both iOS and OS X. Some elements are rendered as vector graphics, most are rendered as bitmaps and the app provides a 1x and 2x bitmap.
Both Qt and Gtk are really good at that.
OSX UIs are generally bitmap - they actually don't scale well at all to high pixel density.
They are scaling them, is just they are scaling everything rather than allowing user control. I like scaling of stuff, as long as any window that I choose to push to native resolution goes to native resolution, especially on a machine that expensive. And I don't expect to have to go out and rebuy lots of software to enable that.