1 files changed, 46 insertions, 13 deletions

diff --git a/src/window.c b/src/window.c
index d9ac2eb62bd..be5e9167d67 100644
--- a/src/window.c
+++ b/src/window.c
@@ -206,7 +206,7 @@ static int window_initialized;
 Lisp_Object Qwindow_configuration_change_hook;
 Lisp_Object Vwindow_configuration_change_hook;
 
-/* Nonzero means scroll commands try to put point
+/* Non-nil means scroll commands try to put point
    at the same screen height as previously.  */
 
 Lisp_Object Vscroll_preserve_screen_position;
@@ -4508,7 +4508,7 @@ window_scroll_pixel_based (window, n, whole, noerror)
          results for variable height lines.  */
       init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID);
       it.current_y = it.last_visible_y;
-      move_it_vertically (&it, - window_box_height (w) / 2);
+      move_it_vertically_backward (&it, window_box_height (w) / 2);
 
       /* The function move_iterator_vertically may move over more than
          the specified y-distance.  If it->w is small, e.g. a
@@ -4518,14 +4518,14 @@ window_scroll_pixel_based (window, n, whole, noerror)
       if (it.current_y <= 0)
         {
           init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID);
-          move_it_vertically (&it, 0);
+          move_it_vertically_backward (&it, 0);
           it.current_y = 0;
         }
 
       start = it.current.pos;
     }
 
-  /* If scroll_preserve_screen_position is non-zero, we try to set
+  /* If scroll_preserve_screen_position is non-nil, we try to set
      point in the same window line as it is now, so get that line.  */
   if (!NILP (Vscroll_preserve_screen_position))
     {
@@ -5187,7 +5187,7 @@ and redisplay normally--don't erase and redraw the frame.  */)
 
           SET_TEXT_POS (pt, PT, PT_BYTE);
           start_display (&it, w, pt);
-          move_it_vertically (&it, - window_box_height (w) / 2);
+          move_it_vertically_backward (&it, window_box_height (w) / 2);
           charpos = IT_CHARPOS (it);
           bytepos = IT_BYTEPOS (it);
         }
@@ -5195,29 +5195,62 @@ and redisplay normally--don't erase and redraw the frame.  */)
         {
           struct it it;
           struct text_pos pt;
-          int y0, y1, h, nlines;
+          int nlines = - XINT (arg);
+          int extra_line_spacing;
+          int h = window_box_height (w);
 
           SET_TEXT_POS (pt, PT, PT_BYTE);
           start_display (&it, w, pt);
-          y0 = it.current_y;
+
+          /* Be sure we have the exact height of the full line containing PT.  */
+          move_it_by_lines (&it, 0, 1);
 
           /* The amount of pixels we have to move back is the window
              height minus what's displayed in the line containing PT,
              and the lines below.  */
-          nlines = - XINT (arg) - 1;
+          it.current_y = 0;
+          it.vpos = 0;
           move_it_by_lines (&it, nlines, 1);
 
-          y1 = line_bottom_y (&it);
+          if (it.vpos == nlines)
+            h -= it.current_y;
+          else
+            {
+              /* Last line has no newline */
+              h -= line_bottom_y (&it);
+              it.vpos++;
+            }
+
+          /* Don't reserve space for extra line spacing of last line.  */
+          extra_line_spacing = it.max_extra_line_spacing;
 
           /* If we can't move down NLINES lines because we hit
              the end of the buffer, count in some empty lines.  */
           if (it.vpos < nlines)
-            y1 += (nlines - it.vpos) * FRAME_LINE_HEIGHT (it.f);
-
-          h = window_box_height (w) - (y1 - y0);
+            {
+              nlines -= it.vpos;
+              extra_line_spacing = it.extra_line_spacing;
+              h -= nlines * (FRAME_LINE_HEIGHT (it.f) + extra_line_spacing);
+            }
+          if (h <= 0)
+            return Qnil;
 
+          /* Now find the new top line (starting position) of the window.  */
           start_display (&it, w, pt);
-          move_it_vertically (&it, - h);
+          it.current_y = 0;
+          move_it_vertically_backward (&it, h);
+
+          /* If extra line spacing is present, we may move too far
+             back.  This causes the last line to be only partially
+             visible (which triggers redisplay to recenter that line
+             in the middle), so move forward.
+             But ignore extra line spacing on last line, as it is not
+             considered to be part of the visible height of the line.
+          */
+          h += extra_line_spacing;
+          while (-it.current_y > h)
+            move_it_by_lines (&it, 1, 1);
+
           charpos = IT_CHARPOS (it);
           bytepos = IT_BYTEPOS (it);
         }