updated 10:00 pm EDT, Thu March 13, 2014
First beta bumped 4K support to 60Hz from 10.9.2's 24Hz
Apple has begun seeding the second beta of OS X Mavericks 10.9.3, again asking developers to focus testing on graphics drivers and audio, the same areas mentioned in the first beta release. The new build, 13D17, continues to include the enhanced 4K monitor support found in the first beta, which raised the refresh rate of 4K monitors considerably, offered a pixel-doubling scaling option for external monitors, and expanded the range of supported monitors.
The discovery of a "Retina" pixel-doubling option has raised speculation that Apple plans to finally upgrade its own Apple Cinema Display series to 4K, also known as "UltraHD," which is starting to catch on in North American markets both for home entertainment and creative-professional workstations, despite still-high entry costs. Support for 4K has been a major selling point for the latest Mac Pro.
The first beta of 10.9.3 included new options to scale resolutions to allow 60hz refresh rates at up to 4096x2160 for Retina MacBook Pros, allowing them to retain "Retina" level output over HDMI. Currently, 10.9.2 limits resolution to 3840x2160, with refresh rates at a maximum of 24Hz, or 30Hz maximum for a supported (Asus or Sharp) external display -- which caused flickering and choppy, under-performing graphics.
The new 10.9.3 beta 2 is available for registered developers through Apple's developer website.