;;; To reproduce the problem:
;;;  * emacs -Q
;;;  * visit this text in a buffer
;;;  * M-x eval-buffer
;;;  * C-n through the buffer until Emacs freezes, on the long "xxx...xxx" 
line

;;; short line
;;; long line long line long line long line long line long line long line 
long line long line long line
;;; short line
;;; 
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
;;; short line

(progn
   ;; (find-file "~/tmp/longline.txt-BAD-1")
   (visual-line-mode 1)
   ;; wrap-prefix for second line is 100 spaces
   (put-text-property 199 302 'wrap-prefix (make-string 100 32))
   ;; wrap-prefix for fourth line is 100 spaces
   (put-text-property 318 422 'wrap-prefix (make-string 100 32))
   )

;;; Emacs freezes, uses 100% of CPU, and is unresponsive to C-g keystrokes.

;;; From cursory experimentation, the problem seems to occur when both the
;;; wrap-prefix, and a single word, are longer than the Emacs window is 
wide.