Just some observation with respect to the mysterious "empty" tag. In notmuch-show, if I press +<RET>, you can see the tag line changing, so it has been adding something (a newline?). You can remove it again with -<RET>.
IMHO we should protect against adding this from the emacs UI. I sometimes press accidentally +, and want to get out of the dialog by pressing return (rather than c-g). Sebastian
