Replying to a new message while typing a separate message

Consider this situation: User A asks a question and User B begins to type a response to it but it is a lengthy answer. While typing the reply to the first message, User A or another User C asks a separate question of makes a comment meriting a reply by User B. 

The current method of preserving a long-message you are working on would be to copy/cut it, paste it into an external notepad or some other text editor, reply, and then finish writing the original message. Especially in a scenario where the second reply is much shorter than the first that is being worked upon, it would be useful and less cumbersome to be able to open a secondary message box that does not delete the contents of the first.

Perhaps clicking reply on a message would select the message box element but slide a new message box over the original, obscuring but not entirely blocking it as if it were a new paper document sitting on top of an old one.