We have the new Gmail compose window by default now. But how do we get the new interface while replying to an email?
Here is how!
Shift-click the “reply” link or the entire box that includes the message “Click here to Reply or Forward”. You can also Shift+click “forward” to open the forwarded message in a pop-out box.
Use keyboard shortcuts: Shift + r to reply in a pop-out box, Shift + a to reply to all, Shift + f to forward in a pop-out box.
If you’ve already writing the reply, click the arrow above your reply and pick “Pop out reply”.
Note: To go back to the regular interface, minimize the compose box, find this message “You are currently editing your reply in a separate window” and click “Show your draft here.”
Totally copied from The Google System.