The bug

The death screen disables the "Respawn" and "Title screen" buttons to avoid the player accidentally pressing them after dying, but it does not grab the mouse while the buttons are disabled.

Steps to Reproduce:

1. Die

   /kill @p
   

Observed Results:

The player will die, the "You Died!" screen will show, and the mouse will immediately be free to move around.

Expected Results:

The mouse would be disabled for the brief moment that the "return to title screen" and "respawn" options are grayed out.

Notes:

When you are playing in windowed mode you can unintentionally close Minecraft because you were rotating your camera before you died, and your mouse ends up on the close window button.