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).
The old strategy of switching a transparent window to solid and back
doesn't work on 10.11; it appears that queued messages must be handled
for the window to become visible, but that's not allowed during a GC.
The strategy for 10.11 and up create an OpenGL canvas, which acts as a
direct-to-screen drawing area that a GC callback can affect without
Racket-level allocation.
