When I opened Minecraft, there was a white flickering bar at the top of the screen.

What I expected to happen:
When I loaded the game, there should have been no flickering on-screen.

What actually happened:
On the main menu, the top of the screen was flickering white. In the options menu it turned black, and in-game it went the colour of the sky. When the game was paused and I opened the Dash (in Ubuntu), it intermittently covered the entire screen, and when the inventory was open, it was much thinner than on the menu screen.

Steps to reproduce:
1. Open Minecraft snapshot 13w49a.
2. Look at the top of the screen.