When generating a new single player map, attempting to maximize the window may prevented the game from rendering the game to the full frame size, while continuing to update the screen in the scaled down portion. The rest of the screen appeared to be a sky-blue-color. Restoring and re-maximizing the window fixes this issue.
Duplicate of MC-13661, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being closed as duplicate.