I too second this feature. It is there on the old interface and it comes in handy. Maybe it is not for everyone, but as storyboard mentioned, perhaps add it as a custom feature for those who prefer it. i would add one other possibility to what was requested. A “remember” feature. So that if i select it once it will remember for future messages, so i would not have to force it to pop-out every time. Or have a setting in preferences to allow pop out composing/replying/forwarding by default.