We recently added this to notmuch-hello mode so we might as well
support it in this mode as well for consistency.
(let ((map (make-sparse-keymap)))
(define-key map "?" 'notmuch-help)
(define-key map "q" 'kill-this-buffer)
+ (define-key map (kbd "<C-tab>") 'widget-backward)
(define-key map (kbd "M-TAB") 'notmuch-show-previous-button)
(define-key map (kbd "<backtab>") 'notmuch-show-previous-button)
(define-key map (kbd "TAB") 'notmuch-show-next-button)