I'm suddenly unable to drag and drop mail folders to rearrange their sequence. The blue indicator bar appears and moves when I drag but the folder doesn't move when I attempt to drop it in the new location. I can drag and drop a folder into another folder to nest it but when I drag it out and attempt to place it, it is instead placed at the top of the folder list. From there I can only nest the folder - can't place it elsewhere in the folder list. I can create new mail folders but they behave the same as existing ones. I'm accessing a Gmail account in Mail 10.3 on a MacBook Pro running Sierra 10.12.6. I have rebooted and re-opened Mail but problem persists. Thanks in advance for any help.