Invincibility after first hit when pressing ALT

If you press ALT when not using fullscreen you will get invincibility after a single hit. When you get hit, you will not get knocked back and no players can proceed to hit you. Once you press ALT again, you will finally get knocked back. I've tested this out with the vanilla Minecraft server and CraftBukkit 1.4.6-R0.3.

I've taken a video of this bug:
http://youtu.be/6bS6kZeL71U