To drop a stack on a mac requires pressing Cmd + Q. Unfortunately this by default is also a shortcut to quit Minecraft.

How I know Cmd + Q drops a stack:
Disabling the Cmd + Q = Quit shortcut by going into system preferences, keyboard, shortcuts, and binding "Quit Minecraft" to something else, like Cmd + Option + Q, enables me to drop a stack by pressing Cmd + Option + Q

My suggestion:
Rebind it to Ctrl + Q