With the release of WordPress 4.0, they have made quite a lot of chances in the wp-admin user interface, and precisely in the editor if you see, you will notice that when you are writing a length paragraph or scrolling down at the bottom of the page, the height of the post/page editor will automatically match the height of your screen, which also removes the scroll bars there inside the editor.
It can be really annoying specially if you want to scroll down to panels below the editor such as Custom Fields, Meta Boxes of Excerpt Box. Thankfully there is a quick solution as well. You can uncheck the option in the Screen Options panel at the top of the screen and it will eliminate this feature in the UI.
Take a look at the screenshot above, to see how you can turn off the full-height editor.