OS X 10.11.6 Titan X 12GB + Radeon XT 2600
Worked: Always had this problem (years), sorry for not reporting before...
Typically I run my mac (classic mac pro) with 2 video cards and run my secondary monitor off the basic Radeon card, for boot menus etc.
If I drag or duplicate the Blender window across to 2nd display I get garbled UI text. Bring it back to primary display, I can read it again. Works normally if both monitors connected to primary card.