Turning off fullscreen makes it so that the taskbar covers up the bottom of minecraft. Minecraft cannot be reshaped into a bigger or smaller screen like it used to and the close/minimize/fullscreen icons on the top right are not showing. The game is kind of in a fullscreen state, but at the same time it is not. I like being out of fullscreen but still having the game stretched out to cover the entire screen except the taskbar so that the game doesn't create a lagspike when minimizing every time I click somewhere else. If this is an intended change, please, remove it. So anoying.
Please check if that's still an issue for you in the latest 1.14 development snapshot.