WordPress 4.0 added a subtle UX change in the editor. If you are scrolling or writing a lengthy paragraph, the height of the editor will match the height of your screen, eliminating the scroll-bars inside the editor.
To reach some panels like the Excerpt and Custom Fields that reside below the editor, you need to scroll all the way down through the end of the paragraph. If you don’t want this, tick the option off in the Screen Options panel.
Compatibility: WordPress 4.0