Ctrl + Q would usually drop all items you are hovering over. On my Mac though, it doesn't work. I tried Cmd + Q since that is what sometimes replaces Ctrl on a Mac, but by trying that I learned that Cmd + Q closes the running application.

You would also be able to hold shift, middle click, then drag the items across your inventory to create multiple stacks (In creative mode of course). On my Mac though, I use an Apple Magic Mouse, which doesn't have a middle click button. So I changed the key bind in Minecraft's controls to 'R'. For some reason, holding shift, R, then dragging doesn't do what you would expect it to do when it was the middle click button.