I belive this is a layout quirk. When editing, forwarding or replying to the message, the folder listing from the right disappears and gets replaced with the file attachment form. If one decides not to send the message and just jump to another folder, it requires 2 clicks. The appropriate way to do this would be to keep the folder listing where it always is and when someone tries to leave the message editing view, present a prompt to either send a message or save it in drafts.

ScriptHead