WordPress visual editor in Add New and Edit post pages provide you a fullscreen button called as Distraction Free Writing Mode. When you switch to fullscreen mode your preview post button disappears and because of this you have to switch back to normal mode for previewing the blog post you were writing.
This tutorial explains you how you can easily add a preview post button in fulscreen mode of your WordPress blog’s visual text editor. This is very easy, all you need to do is install and activate wp-fullscreen-preview-button plugin, you are done. Enjoy by switching to fullscreen mode. One more thing if you don’t know, you can switch to fullscreen writing mode simply by pressing ALT + Shift + W key combination. For learning more about shortcut keys refer to this blog post.