On Yosemite, double-clicking a window's title bar zooms it.  (This is to
compensate for the fact that the zoom button has been replaced by a full-screen
button.)  Sometimes, double-clicking in the content area would count as double-
clicking in the title bar.

This is controlled, in part, by the -mouseDownCanMoveWindow method of the view
that was hit in the window.  The default implementation of that returns YES
for non-opaque views, as the views are in the Mac driver.  Overriding it to
return NO prevents the problem.

Thanks to Piotr Caban for the test.