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.

4 files changed, 76 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.
diff --git a/src/ChangeLog b/src/ChangeLog
index ec0fbc493cb..a26577c20a3 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,8 @@
+2013-06-15  Eli Zaretskii  <eliz@gnu.org>
+
+        * xdisp.c (Fline_pixel_height): New function, required for solving
+        bug #14567.
+
 2013-06-15  Paul Eggert  <eggert@cs.ucla.edu>
 
         * fns.c (Fcopy_sequence): Simplify XTYPE calculation.
diff --git a/src/xdisp.c b/src/xdisp.c
index e1d6b0c9a27..54ea325f642 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -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
@@ -28691,6 +28709,7 @@ 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);