Failing to re-size the window correctly during the 'loading world' screen, when selecting to maximize the window from the default size.
When maximize has been selected, the window will maximize, however Minecraft will only appear in the bottom left corner, and not adjusting to fit the whole screen.
A temporary fix involves resizing the screen.
Duplicate of MC-13661, please use the search function to see if your bug has already been submitted. Currently over 52% of tickets are being closed as duplicate.