bug#14567: Scrolling of large images

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 06 Jun 2013 20:32:15 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 14567@debbugs.gnu.org
> 
> > From: Thomas Wiecki <thomas.wiecki@gmail.com>
> > Date: Thu, 6 Jun 2013 13:16:43 -0400
> > Cc: 14567@debbugs.gnu.org
> > 
> > 1. Download a large image (e.g.
> > http://cdn.urbanislandz.com/wp-content/uploads/2011/10/MMSposter-large.jpg)
> > and save it as /tmp/test.jpg
> > 2. In an emacs buffer type "(insert-image (create-image "/tmp/test.jpg"))"
> > 3. Type some arbitrary lines of text below this line.
> > 4. In first line (containing insert-image), type C-x C-e at end of line to
> > load the image.
> > 5. Image should appear in buffer.
> > 6. Scroll down repeatedly until past the image.
> > 
> > Expected behavior: Scrolls down smoothly even once scrolled past image.
> > Observed behavior: Scrolls down smoothly until end of image is reached. At
> > end of image, the next scroll causes a huge jump in the window so that and
> > only the text is shown but not any of the residual image.
> > 
> > 7. scroll back up
> > 
> > Expected behavior: Scrolls upwards over image.
> > Observed behavior: Jumps over complete image and shows beginning of buffer.
> 
> Perfect, thanks.  I will take a look.

Can you try the changes below and see if they give good results,
including in the real-life use cases where you bumped into this?  Note
that you will have to rebuild Emacs to try this.

Thanks.

=== modified file 'lisp/simple.el'
--- lisp/simple.el	2013-06-05 18:10:27 +0000
+++ lisp/simple.el	2013-06-08 14:14:39 +0000
@@ -4738,20 +4738,35 @@ 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))))
 	(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)
+		   (<= this-height fch))
+	       (or (null rbot) (= rbot 0)))
 	  nil)
-	 ;; If cursor is not in the bottom scroll margin, move forward.
-	 ((and (> vpos 0)
+	 ;; If cursor is not in the bottom scroll margin, and the
+	 ;; current line is is not too tall, move forward.
+	 ((and (<= this-height fch)
+	       (> vpos 0)
 	       (< (setq py
 			(or (nth 1 (window-line-height))
 			    (let ((ppos (posn-at-point)))
@@ -4761,9 +4776,10 @@ lines."
 	  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 (> rbot 0)
+		    (> 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)
@@ -4808,7 +4824,14 @@ 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.

=== modified file 'src/xdisp.c'
--- src/xdisp.c	2013-06-06 16:35:31 +0000
+++ src/xdisp.c	2013-06-08 14:02:28 +0000
@@ -1217,6 +1217,24 @@ line_bottom_y (struct it *it)
   return line_top_y + line_height;
 }
 
+DEFUN ("line-pixel-height", Fline_pixel_height,
+       Sline_pixel_height, 0, 0, 0,
+       doc: /* Return height in pixels of text line in the selected window.
+
+Value is the height in pixels of the line at point.  */)
+  (void)
+{
+  struct it it;
+  struct text_pos pt;
+  struct window *w = XWINDOW (selected_window);
+
+  SET_TEXT_POS (pt, PT, PT_BYTE);
+  start_display (&it, w, pt);
+  it.vpos = it.current_y = 0;
+  last_height = 0;
+  return make_number (line_bottom_y (&it));
+}
+
 /* Subroutine of pos_visible_p below.  Extracts a display string, if
    any, from the display spec given as its argument.  */
 static Lisp_Object
@@ -28665,6 +28683,7 @@ syms_of_xdisp (void)
   defsubr (&Stool_bar_lines_needed);
   defsubr (&Slookup_image_map);
 #endif
+  defsubr (&Sline_pixel_height);
   defsubr (&Sformat_mode_line);
   defsubr (&Sinvisible_p);
   defsubr (&Scurrent_bidi_paragraph_direction);