Apple is now releasing test builds via Software Update to developers with access to the latest test builds of Mac OS X 10.6. The latest build, 10A402, includes a number of fixes and optimizations, as well as two new visual UI changes.

So far, some of the changes that have been uncovered in the 1.3GB update include a fix for Expos? issues when used with multiple monitors as well as activation of third-party preference panes. Reports also indicate that overall system performance has significantly improved, with notable increases in speed and responsiveness as well as decreases in startup and shutdown times. It also appears that battery life indicators are showing much longer estimated running times, though as of yet no tests can confirm that the times are accurate.

A couple interesting UI changes accompanied the new build as well. The most obvious visual change is that contextual menus in the Dock now use a semi-transperent black background with white text, much like that of the current Stacks view. This is in contrast to the current contextual menus, or folders set to list view, which show a light gray background with black text. The Quantum Byte speculates that it is part of the rumored "Marble" interface theme that is expected to be featured in Snow Leopard.

From left to right: 10.5 sliders, iLife '09 sliders, 10.6 sliders

Adding some credence to that idea is the new appearance of sliders in the newest test build. "Marble" is said to be very similar to the overall UI look of iLife '09, introduced this past January. You'll notice that the appearance of the new slider controls is the same as those in iTunes 8 and iPhoto '09.

Apple still has just under three months left to ship Snow Leopard by its promised September release. The tweaks and optimizations are a good sign that things are progressing on schedule, but the UI changes may be a sign that Apple is saving a few small things to wow us with when it finally hits store shelves.