Run the minecraft launcher. It launches as expected. Close the window. It quits. Run it again. Still launches as expected.

Now, quit by hitting command Q instead of closing the window.

Run the launcher again. Now, it launches, and a window comes up, but it never draws anything and you just get the pinwheel of death forever. Restarting it doesn't help. What does help is removing ~/"Library/Application Support/Minecraft Launcher/" and launching it again.

(At least on my machine, there appears to be nothing in that directory, so I'm not sure what deleting it is fixing. But this reproduces on 2 machines: a MacBookPro10,1 and an iMac14,2.)