Cute game! I had to play it after seeing it was Mac only, since that never happens :)
When I start the game, it forces itself to be full screen and then just displays nothing but black; it's only after I force it back into windowed mode through Mission Control that it actually displays any graphics. I am using a bit of an older version of macOS (10.14.6), so it may just be my system because I imagine this doesn't happen to you
I might suggest listing the controls somewhere, since it wasn't obvious to me what to press for the third pickup and I had to try a bunch of keys to find it
Also I love the little fluttery sound effect for the double jump, it harmonizes so well with the background music that I didn't even initially recognize that's what it was