The bug

The Fullscreen function (as enabled by F11 or by accessing the Graphics menu button) fills only part of an external monitor at the top left.

It appears (though unconfirmed) to be filling a section equal in pixel size to the integrated screen of the laptop (which in my case is 1366x768), which is closed when the bug occurs, but still running off of the external monitor only.

When I open my laptop, the issue resolves itself.

How to reproduce

1. Change settings on a laptop running Windows 10 control panel to "Do nothing" in "Choose what happens when I close the lid" section
2. Connect laptop to external monitor of a larger size than the integrated screen
3. Start Minecraft 1.11 with the lid closed and an external keyboard, mouse, and monitor connected
4. Change to Fullscreen mode (using F11 or Graphics menu) if not already enabled.