The interface toolkit was drawing all live updates (while using menus/buttons) in the frontbuffer. This isn't well supported cross-platform, involves quite some hackish code too, so was time to be killed once.
Live button updates or menus now are drawn in backbuffer only, and copied to front when needed. It only uses the glReadPixels and glCopyPixels for frontbuffer access now. These two calls are available since OpenGL 1.0, and known to be supported cross platform a lot better.
Another benefit of the recode is that drawing updates in frontbuffer now is a centralized call, allowing experiments by developers for optimal solutions for specific platforms. For example, for Windows systems the KTX OpengGL extensions could be used here, this has being enforced by Discreet for 3DStudio and being used by many games.
Results so far are very encouraging, especially for ATI cards, which had a lot of troubles with frontbuffer drawing. It still needs a thorough review though. On SGI/SUN workstations it should work smooth (the current method was in Blender 1.0 too); but I prefer feedback on that. Some (older) Linux systems might get a degraded performance as well.
You can switch the "copy from back to frontbuffer" to use another method by setting the "Rt" button in "Display Buttons" (F10) at value '2'. It then doesn't use glCopyPixels, but a glReadPixels and glDrawPixels combo.
One of the funny benefits of a cleaner menu drawing system also is that it allows more advanced drawing of buttons and menus. In the theme editor you now can define alpha values for menu backdrop and items, to have them showing transparent in the UI.
You can download a transparent UI .B.blend here. Load the file and if you like it, save it over as startup file with CTRL + U.