(mouse-choose-completion):

Include words separated by single spaces in the completion.

lisp/mouse.el

@@ -1189,10 +1189,20 @@ and selects that window."
 	  (setq buffer completion-reference-buffer))
       (save-excursion
 	(goto-char (posn-point (event-start event)))
-	(skip-chars-backward "^ \t\n")
-	(let ((beg (point)))
+	(let (beg end)
 	  (skip-chars-forward "^ \t\n")
-	  (setq choice (buffer-substring beg (point))))))
+	  (while (looking-at " [^ \n\t]")
+	    (forward-char 1)
+	    (skip-chars-forward "^ \t\n"))
+	  (setq end (point))
+	  (skip-chars-backward "^ \t\n")
+	  (while (and (= (preceding-char) ?\ )
+		      (not (and (> (point) (1+ (point-min)))
+				(= (char-after (- (point) 2)) ?\ ))))
+	    (backward-char 1)
+	    (skip-chars-backward "^ \t\n"))
+	  (setq beg (point))
+	  (setq choice (buffer-substring beg end)))))
     (let ((owindow (selected-window)))
       (select-window (posn-window (event-start event)))
       (bury-buffer)