Fix gnus-summary-recenter to properly handle invisible lines

* gnus-sum.el (gnus-forward-line-ignore-invisible): New function.
(gnus-summary-recenter): Use it instead of forward-line.  (Bug#5257)

2 files changed, 35 insertions, 7 deletions

diff --git a/lisp/gnus/ChangeLog b/lisp/gnus/ChangeLog
index c127239b164..1b418e3275a 100644
--- a/lisp/gnus/ChangeLog
+++ b/lisp/gnus/ChangeLog
@@ -1,3 +1,8 @@
+2010-01-05  Andreas Schwab  <schwab@linux-m68k.org>
+
+	* gnus-sum.el (gnus-forward-line-ignore-invisible): New function.
+	(gnus-summary-recenter): Use it instead of forward-line.  (Bug#5257)
+
 2010-01-02  Chong Yidong  <cyd@stupidchicken.com>
 
 	* message.el (message-exchange-point-and-mark): Rework last change to
@@ -13788,7 +13793,7 @@
 
 See ChangeLog.2 for earlier changes.
 
-    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
+    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc.
 
   This file is part of GNU Emacs.
 
diff --git a/lisp/gnus/gnus-sum.el b/lisp/gnus/gnus-sum.el
index 658193bd508..cd56cd1f4bb 100644
--- a/lisp/gnus/gnus-sum.el
+++ b/lisp/gnus/gnus-sum.el
@@ -6727,6 +6727,26 @@ Also do horizontal recentering."
 
 (put 'gnus-recenter 'isearch-scroll t)
 
+(defun gnus-forward-line-ignore-invisible (n)
+  "Move N lines forward (backward if N is negative).
+Like forward-line, but skip over (and don't count) invisible lines."
+  (let (done)
+    (while (and (> n 0) (not done))
+      ;; If the following character is currently invisible,
+      ;; skip all characters with that same `invisible' property value.
+      (while (invisible-p (point))
+	(goto-char (next-char-property-change (point))))
+      (forward-line 1)
+      (if (eobp)
+	  (setq done t)
+	(setq n (1- n))))
+    (while (and (< n 0) (not done))
+      (forward-line -1)
+      (if (bobp) (setq done t)
+	(setq n (1+ n))
+	(while (and (not (bobp)) (invisible-p (1- (point))))
+	  (goto-char (previous-char-property-change (point))))))))
+    
 (defun gnus-summary-recenter ()
   "Center point in the summary window.
 If `gnus-auto-center-summary' is nil, or the article buffer isn't
@@ -6742,16 +6762,19 @@ displayed, no centering will be performed."
 			     gnus-auto-center-summary
                            (/ (1- (window-height)) 2)))))
 	   (height (1- (window-height)))
-	   (bottom (save-excursion (goto-char (point-max))
-				   (forward-line (- height))
-				   (point)))
+	   (bottom (save-excursion
+		     (goto-char (point-max))
+		     (gnus-forward-line-ignore-invisible (- height))
+		     (point)))
 	   (window (get-buffer-window (current-buffer))))
       (when (get-buffer-window gnus-article-buffer)
 	;; Only do recentering when the article buffer is displayed,
 	;; Set the window start to either `bottom', which is the biggest
 	;; possible valid number, or the second line from the top,
 	;; whichever is the least.
-	(let ((top-pos (save-excursion (forward-line (- top)) (point))))
+	(let ((top-pos (save-excursion
+			 (gnus-forward-line-ignore-invisible (- top))
+			 (point))))
 	  (if (> bottom top-pos)
 	      ;; Keep the second line from the top visible
 	      (set-window-start window top-pos)
@@ -6760,12 +6783,12 @@ displayed, no centering will be performed."
 	    ;; visible, or revert to using TOP-POS.
 	    (save-excursion
 	      (goto-char (point-max))
-	      (forward-line -1)
+	      (gnus-forward-line-ignore-invisible -1)
 	      (let ((last-line-start (point)))
 		(goto-char bottom)
 		(set-window-start window (point) t)
 		(when (not (pos-visible-in-window-p last-line-start window))
-		  (forward-line 1)
+		  (gnus-forward-line-ignore-invisible 1)
 		  (set-window-start window (min (point) top-pos) t)))))))
       ;; Do horizontal recentering while we're at it.
       (when (and (get-buffer-window (current-buffer) t)