However, I have discovered a couple of options for if you find having your inbox in the background distracting:
1. Ctrl+click on the Compose button. This will open the compose window in a new tab, with no distracting inbox in the background.
2. Use Basic HTML view. You can get to Basic HTML view by clicking on the link at the bottom right of the Gmail loading page (the one with the horizontal blue bar) or by going to https://mail.google.com/mail/?