When launching Retroarch in "retina" mode (the default), all clicks are offset by a factor of 2 horizontally and vertically (i.e. a tap in the center of the screen is scaled to the bottom right, and taps outside the upper left quadrant are ignored).
Everything works correctly when launching RetroArch in "low resolution" mode (by clicking "get info" in Finder on the .app).
Mu version 1.9.9 (v1.3.0 7ead066)