After playing for a while (Patch 1.3.2, fullscreen, OS X El Capitan 10.11.2, 15" Retina Macbook Pro 2012, GT650M GPU) the default OS X mouse cursor appears on top of in-game cursor.
Don't recall this happening in the past but that was older OS X versions and older game versions.
Cursor just appears after some time playing. Have yet to identify a cause or repeatable trigger. Did not invoke Steam Overlay or anything, might have to try toggling that on/off and see what happens.
Customer support service by UserEcho