* lisp/xt-mouse.el (xterm-mouse--read-event-sequence-1000):

Always store button numbers in the same way in xterm-mouse-last;
Don't burp is xterm-mouse-last is not set as expected.
Never return negative indices.

Fixes: debbugs:17378

lisp/ChangeLog

+2014-05-05  Stefan Monnier  <monnier@iro.umontreal.ca>
+
+	* xt-mouse.el (xterm-mouse--read-event-sequence-1000): (bug#17378)
+	Always store button numbers in the same way in xterm-mouse-last;
+	Don't burp is xterm-mouse-last is not set as expected.
+	Never return negative indices.
+
 2014-05-05  Dmitry Gutov  <dgutov@yandex.ru>
 	* progmodes/ruby-mode.el (ruby-syntax-propertize-function):

lisp/xt-mouse.el

@@ -161,10 +161,10 @@ http://invisible-island.net/xterm/ctlseqs/ctlseqs.html)."
 	   (cond ((>= code 64)
 		  (format "mouse-%d" (- code 60)))
 		 ((memq code '(8 9 10))
-		  (setq xterm-mouse-last code)
+		  (setq xterm-mouse-last (- code 8))
 		  (format "M-down-mouse-%d" (- code 7)))
 		 ((= code 11)
-		  (format "M-mouse-%d" (- xterm-mouse-last 7)))
+		  (format "M-mouse-%d" (+ 1 (or xterm-mouse-last 0))))
 		 ((= code 3)
 		  ;; For buttons > 5 xterm only reports a
 		  ;; button-release event.  Avoid error by mapping
@@ -174,8 +174,8 @@ http://invisible-island.net/xterm/ctlseqs/ctlseqs.html)."
 		  (setq xterm-mouse-last code)
 		  (format "down-mouse-%d" (+ 1 code))))))
 	;; x and y coordinates
-	(- (read-event) 33)
-	(- (read-event) 33)))
+	(max 0 (- (read-event) 33))
+	(max 0 (- (read-event) 33))))
 
 ;; XTerm's 1006-mode terminal mouse click reporting has the form
 ;; <BUTTON> ; <X> ; <Y> <M or m>, where the button and ordinates are