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While Apple says that its latest MacBook Air will officially support external monitors with resolutions up to 2,560-pixels-by-1,600-pixels, tests have shown that the slim portable can actually drive an external 4K display at a full 60Hz.
Testing the early 2015 MacBook Air with a Dell P2415Q 24-inch 4K display yielded positive results, according to Ars Technica. The Air correctly identified the attached monitor at 4K resolution, switching the user interface into HiDPI mode.
Subsequent refresh rate tests confirmed that it was running at 60Hz, and the publication said that interface components were "more than smooth enough for desktop use." Problems cropped up when performing animation-intensive actions, like entering full-screen mode, with "clearly visible" frame lag, showing that the display pushes the Intel HD 6000 graphics chip near its limit.
That limit is exceeded when using OS X's built-in resolution scaling functions, with the integrated GPU turning animations into "flip books" when asked to render higher resolutions. Even scrolling performance is said to suffer in that situation.
While the MacBook Air's Tech Specs page does not list 4K compatibility, the early 2015 Air is listed on Apple's 4K display support document. There, however, it is only shown with the capability to push 30Hz or 24Hz, rather than the 60Hz Ars discovered was possible.
This is not out of line with Apple's specification history, as the company often lowers the "official" capabilities of its products below what they are theoretically capable of. Many of the early unibody MacBook Pro generations were listed with maximum RAM capacities of just 8 gigabytes, for instance, despite easily supporting 16 gigabytes.