When you are dead and the text "You died!" is visible, toggling fullscreen (default F11) will make the "Title screen" and "Respawn" buttons darker and unpressable, making it impossible to do anything else than to close minecraft.
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Thank you for your report.
However, your ticket has been resolved as a duplicate.
This issue has been linked to the original bugreport: MC-2835. You can find or add further information on that report.
Please search before reporting any bugs, as it's likely that one exists already.