updated 09:57 am EST, Fri March 7, 2014
Appears intended to support 4K displays
The OS X 10.9.3 beta supports new graphics scaling options which appear aimed at supporting 4K displays, users say. While a Mac Pro or Retina MacBook Pro can connect to a 4K monitor, they normally do so at resolutions up to 4096x2160 at 24Hz, which results in OS X's interface being spread out and shrunk, and choppiness in the case of things like the mouse cursor. The MacBook Pro can alternately use 3840x2160 at 30Hz, but this isn't an improvement.
Apple is unlikely to build 4K support into any of its own computers or monitors for the foreseeable future. Native 4K is still expensive, although costs should come down by the end of 2014 as more manufacturers begin to ship products.