I have an external display as the main display connected via USB-C to HDMI adapter to my 2017 Macbook Pro running OS X 12.1 with the built-in retina as an extended display. Launching grid navigation with Scoot 0.8 overlays both display grids on the external display and none on the built-in retina display. Relatedly, launching element navigation only shows targets when the focused window is on the external display.