I posted the following on the Card Kingdom development blog.
Another new feature for the engine this go-around is an enhanced UI. Our previous UI was built using objects and custom components for each type of widget. Buttons, text labels, sliders, and other such controls could be placed in the game interface with a few custom animations, such as sliding in and out of the frame and rollover/highlight states. These controls functioned well and were pretty simple to use and extend, but it took a lot of initial work developing the widgets that could have been used in other areas of the engine.