Minecraft launches to black screen.

What I expected to happen was...:
After clicking play from the launcher, I expected to see the title screen in the window.

What actually happened was...:
The Mojang logo appeared as usual, then the entire screen went black with the arrow cursor visible.

I noted that pointing the arrow at the bottom of the screen to show the dock resulted in a partial drawing of the expected menu screen roughly the same size and shape of the dock.

Steps to Reproduce:
1. Start Launcher 1.2.5
2. Click Play button