Fix bug #14567 with jumpy scrolling of tall images.

 src/xdisp.c (Fline_pixel_height): New function, required for solving
 bug #14567.

 lisp/simple.el (line-move-partial): Don't jump to the next screen
 line as soon as it becomes visible.  Instead, continue enlarging
 the vscroll until the portion of a tall screen line that's left on
 display is about the height of the frame's default font.

2 files changed, 52 insertions, 17 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index cced6eb0043..fadbabf808e 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,11 @@
+2013-06-15  Eli Zaretskii  <eliz@gnu.org>
+
+	* simple.el (line-move-partial): Don't jump to the next screen
+	line as soon as it becomes visible.  Instead, continue enlarging
+	the vscroll until the portion of a tall screen line that's left on
+	display is about the height of the frame's default font.
+	(Bug#14567)
+
 2013-06-15  Glenn Morris  <rgm@gnu.org>
 
 	* vc/vc-dispatcher.el (vc-compilation-mode): Avoid making
diff --git a/lisp/simple.el b/lisp/simple.el
index 3fd94e96d33..34ebc8f122f 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -4738,42 +4738,62 @@ lines."
 	   (vpos (nth 1 lh))
 	   (ypos (nth 2 lh))
 	   (rbot (nth 3 lh))
+	   (this-lh (window-line-height))
+	   (this-height (nth 0 this-lh))
+	   (this-ypos (nth 2 this-lh))
+	   (fch (frame-char-height))
 	   py vs)
       (when (or (null lh)
-		(>= rbot (frame-char-height))
-		(<= ypos (- (frame-char-height))))
+		(>= rbot fch)
+		(<= ypos (- fch))
+		(null this-lh)
+		(<= this-ypos (- fch)))
 	(unless lh
 	  (let ((wend (pos-visible-in-window-p t nil t)))
 	    (setq rbot (nth 3 wend)
 		  vpos (nth 5 wend))))
+	(unless this-lh
+	  (let ((wstart (pos-visible-in-window-p nil nil t)))
+	    (setq this-ypos (nth 2 wstart)
+		  this-height (nth 4 wstart))))
+	(setq py
+	      (or (nth 1 this-lh)
+		  (let ((ppos (posn-at-point)))
+		    (cdr (or (posn-actual-col-row ppos)
+			     (posn-col-row ppos))))))
 	(cond
-	 ;; If last line of window is fully visible, move forward.
-	 ((or (null rbot) (= rbot 0))
+	 ;; If last line of window is fully visible, and vscrolling
+	 ;; more would make this line invisible, move forward.
+	 ((and (or (< (setq vs (window-vscroll nil t)) fch)
+		   (null this-height)
+		   (<= this-height fch))
+	       (or (null rbot) (= rbot 0)))
 	  nil)
-	 ;; If cursor is not in the bottom scroll margin, move forward.
-	 ((and (> vpos 0)
-	       (< (setq py
-			(or (nth 1 (window-line-height))
-			    (let ((ppos (posn-at-point)))
-			      (cdr (or (posn-actual-col-row ppos)
-				       (posn-col-row ppos))))))
+	 ;; If cursor is not in the bottom scroll margin, and the
+	 ;; current line is is not too tall, move forward.
+	 ((and (or (null this-height) (<= this-height fch))
+	       vpos
+	       (> vpos 0)
+	       (< py
 		  (min (- (window-text-height) scroll-margin 1) (1- vpos))))
 	  nil)
 	 ;; When already vscrolled, we vscroll some more if we can,
 	 ;; or clear vscroll and move forward at end of tall image.
-	 ((> (setq vs (window-vscroll nil t)) 0)
-	  (when (> rbot 0)
-	    (set-window-vscroll nil (+ vs (min rbot (frame-char-height))) t)))
+	 ((> vs 0)
+	  (when (or (and rbot (> rbot 0))
+		    (and this-height (> this-height fch)))
+	    (set-window-vscroll nil (+ vs fch) t)))
 	 ;; If cursor just entered the bottom scroll margin, move forward,
 	 ;; but also vscroll one line so redisplay won't recenter.
-	 ((and (> vpos 0)
+	 ((and vpos
+	       (> vpos 0)
 	       (= py (min (- (window-text-height) scroll-margin 1)
 			  (1- vpos))))
 	  (set-window-vscroll nil (frame-char-height) t)
 	  (line-move-1 arg noerror to-end)
 	  t)
 	 ;; If there are lines above the last line, scroll-up one line.
-	 ((> vpos 0)
+	 ((and vpos (> vpos 0))
 	  (scroll-up 1)
 	  t)
 	 ;; Finally, start vscroll.
@@ -4808,7 +4828,14 @@ lines."
 	       ;; display-based motion doesn't make sense (because each
 	       ;; logical line occupies exactly one screen line).
 	       (not (> (window-hscroll) 0)))
-	  (line-move-visual arg noerror)
+	  (prog1 (line-move-visual arg noerror)
+	    ;; If we moved into a tall line, set vscroll to make
+	    ;; scrolling through tall images more smooth.
+	    (let ((lh (line-pixel-height)))
+	      (if (and (< arg 0)
+		       (< (point) (window-start))
+		       (> lh (frame-char-height)))
+		  (set-window-vscroll nil (- lh (frame-char-height)) t))))
 	(line-move-1 arg noerror to-end)))))
 
 ;; Display-based alternative to line-move-1.