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>.

1 files changed, 13 insertions, 4 deletions

diff --git a/src/window.c b/src/window.c
index b00242311a6..6938ffb8cb8 100644
--- a/src/window.c
+++ b/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