WordPress keeps getting better and more interesting after each major update. But not everyone likes every new feature. I am personally not a fan of the new editor window. Before WordPress 4.0, I could resize it any way I wanted. After WordPress 4.0, you don’t have that option as the editor expands to match the window height automatically. The good news is you can make WordPress go back to the way it was before the latest update. All you need to do is click on the “Screen Options” tab and disable “Expand the editor to match the window height.”
Many folks happen to like this feature. I happen to be more productive when this feature is turned off. The good news is WordPress 4.0 lets you choose how you want to edit your posts.