How to Launch pop up compose window in Gmail

Google has already introduced new Gmail compose window feature for users that would now be defaulted and that it'd be rolled out to all users.

You can launch pop-up resizable compose window by just hold down the Shift key on your keyboard while you click on the compose button. This also works when you click on the reply button.

Use the keyboard shortcut, Shift + C, to launch the compose window even faster. You can also reply in a new window (Shift + R), reply all in a new window (Shift + A), and forward in a new window (Shift + F).