If you need to move your compose window around or reference different browser windows without losing sight of your draft, compose your email in a new browser window.
Compose a message in a new browser window
Hold Shift and click COMPOSE.
If you already have a compose window open that you want to pop out into a new window, hold Shift and click Expand .
Reply to a message in a new browser window
In the top right of the message, click Expand .