Google has just revealed that it's working on a brand-new compose window for Gmail. The new feature is already in testing, but Google is opting for a very slow roll-out, so it may be a while before it's enabled for you.
Most people don't like change and are going to be annoyed when the new compose window is enabled for them.
That's partly why Google is doing it slowly. However, there are some out there who, gasp, actually like change and who try out new things.
While Google has said that it will take things slow, there's a chance that you'll be asked to take part of the preview phase for the new compose window.
If you accept, you get the new feature in all its glory. If you don't, you can still try it out later as you'll see a link to enable it if you change your mind at any time.