On 10.11 in 64-bit mode, showing a frame with a PSMTabBarControl
instance somehow causes the event loop to become occupied on
mouse movements, so that moving the mouse in a `2htdp/universe`
program is slow when more than one tab is open in DrRacket. The
modern MMTabBarView widget doesn't have that problem. Also, the
MMTabBarView widget has a Yosemite look (to replace Aqua).