Minecraft window resizes when doing focusing on another window and focusing back on minecraft window.

When i play minecraft, I half the window on my screen so one is minecraft and one is google chrome. When i focus on the google chrome window and focus back on the minecraft window, it resizes to a smaller window. It is annoying because i have to resize it.

Here is a link to a youtube video that i made demonstrating the bug.
http://www.youtube.com/watch?v=n58KsIg4Mqg