If you're using an old version of Gmail, your Inbox may look a little different.
Gmail’s compose format makes it easy to see your Inbox while writing an email, but there may still be times when having your drafted message stuck in one place just won’t work. Here are a couple of ways to compose your message in a new browser window.
Shift-click the Compose link to author a new message in a separate window, instead of as a pop-up in your Inbox. (If keyboard shortcuts are enabled, type Shift + c.)
If you are replying to a message in your Inbox, click the new window icon to author your reply in a new window. Now you can reference another email or keep an eye on your Inbox while replying to a message.
Clicking the new window icon opens the message you’re currently responding to in a new browser window. You can then go back to the original browser window and click Inbox (or type u on your keyboard if you have keyboard shortcuts enabled) to return to your Inbox.
Now you can move your message freely, making it easier to see your whole Inbox or reference another email. When you're ready, go ahead and send your message, then close the window.