I'm not sure if this problem is Launcher related or game related, but to my knowledge, it only does it when loading Minecraft version 1.8.3

What happened:
I was beginning to play Minecraft, but when I noticed the Maximize button on the window was grayed out I thought I had found a bug.

Reproducing the issue myself:
I pressed "Play" on the Launcher, and while loading the game, I dragged a separate window across the screen. Sure enough, when exiting "Full Screen" by pressing F11, the Maximize button was grayed out.

Here is how to reproduce the issue yourself:
1. Load up Minecraft in version 1.8.3
2. Make sure it is set to "Full Screen"
3. Exit Minecraft
4. Have another window ready (i.e. web browser) and press "Play"
5. While Minecraft is loading, grab the other window and move it around
6. When it loads completely, click somewhere on the "Main Menu"
7. Exit "Full Screen"