Fix bug #19060 with inaccurate pixel-based scrolling.

 src/window.c (window_scroll_pixel_based): Avoid truncation/rounding
 errors in computing the number of pixels to scroll.  Suggested by
 Kelly Dean <kelly@prtime.org>.

src/ChangeLog

+2014-11-15  Eli Zaretskii  <eliz@gnu.org>
+
+	* window.c (window_scroll_pixel_based): Avoid truncation/rounding
+	errors in computing the number of pixels to scroll.  Suggested by
+	Kelly Dean <kelly@prtime.org>.  (Bug#19060)
+
 2014-11-15  Jan Djärv  <jan.h.d@swipnet.se>
 
 	* nsmenu.m (update_frame_tool_bar): If tool bar changes height,

src/window.c

@@ -4955,9 +4955,14 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, int noerror)
 	{
 	  int px;
 	  int dy = frame_line_height;
+	  /* In the below we divide the window box height by the
+	     frame's line height to make the result predictable when
+	     the window box is not an integral multiple of the line
+	     height.  This is important to ensure we get back to the
+	     same position when scrolling up, then down.  */
 	  if (whole)
-	    dy = max ((window_box_height (w)
-		       - next_screen_context_lines * dy),
+	    dy = max ((window_box_height (w) / dy
+		       - next_screen_context_lines) * dy,
 		      dy);
 	  dy *= n;
 
@@ -5039,8 +5044,12 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, int noerror)
     {
       ptrdiff_t start_pos = IT_CHARPOS (it);
       int dy = frame_line_height;
-      dy = max ((window_box_height (w)
-		 - next_screen_context_lines * dy),
+      /* In the below we divide the window box height by the frame's
+	 line height to make the result predictable when the window
+	 box is not an integral multiple of the line height.  This is
+	 important to ensure we get back to the same position when
+	 scrolling up, then down.  */
+      dy = max ((window_box_height (w) / dy - next_screen_context_lines) * dy,
 		dy) * n;
 
       /* Note that move_it_vertically always moves the iterator to the