Merge branch 'master' of git.sv.gnu.org:/srv/git/emacs into feature/pgtk

1 files changed, 51 insertions, 5 deletions

diff --git a/src/window.c b/src/window.c
index 6cd3122b43b..8e75e460b2b 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5669,7 +5669,7 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
   if (whole)
     {
       ptrdiff_t start_pos = IT_CHARPOS (it);
-      int dy = frame_line_height;
+      int flh = frame_line_height;
       int ht = window_box_height (w);
       int nscls = sanitize_next_screen_context_lines ();
       /* In the below we divide the window box height by the frame's
@@ -5677,14 +5677,37 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
          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 = n * max (dy, (ht / dy - nscls) * dy);
+      int dy = n * max (flh, (ht / flh - nscls) * flh);
+      int goal_y;
+      void *it_data;
 
       /* Note that move_it_vertically always moves the iterator to the
          start of a line.  So, if the last line doesn't have a newline,
          we would end up at the start of the line ending at ZV.  */
       if (dy <= 0)
         {
+          goal_y = it.current_y - dy;
           move_it_vertically_backward (&it, -dy);
+          /* Extra precision for people who want us to preserve the
+             screen position of the cursor: effectively round DY to the
+             nearest screen line, instead of rounding to zero; the latter
+             causes point to move by one line after C-v followed by M-v,
+             if the buffer has lines of different height.  */
+          if (!NILP (Vscroll_preserve_screen_position)
+              && it.current_y - goal_y > 0.5 * flh)
+            {
+              it_data = bidi_shelve_cache ();
+              struct it it2 = it;
+
+              move_it_by_lines (&it, -1);
+              if (it.current_y < goal_y - 0.5 * flh)
+                {
+                  it = it2;
+                  bidi_unshelve_cache (it_data, false);
+                }
+              else
+                bidi_unshelve_cache (it_data, true);
+            }
           /* Ensure we actually do move, e.g. in case we are currently
              looking at an image that is taller that the window height.  */
           while (start_pos == IT_CHARPOS (it)
@@ -5693,8 +5716,25 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
         }
       else if (dy > 0)
         {
-          move_it_to (&it, ZV, -1, it.current_y + dy, -1,
-                      MOVE_TO_POS | MOVE_TO_Y);
+          goal_y = it.current_y + dy;
+          move_it_to (&it, ZV, -1, goal_y, -1, MOVE_TO_POS | MOVE_TO_Y);
+          /* See the comment above, for the reasons of this
+             extra-precision.  */
+          if (!NILP (Vscroll_preserve_screen_position)
+              && goal_y - it.current_y  > 0.5 * flh)
+            {
+              it_data = bidi_shelve_cache ();
+              struct it it2 = it;
+
+              move_it_by_lines (&it, 1);
+              if (it.current_y > goal_y + 0.5 * flh)
+                {
+                  it = it2;
+                  bidi_unshelve_cache (it_data, false);
+                }
+              else
+                bidi_unshelve_cache (it_data, true);
+            }
           /* Ensure we actually do move, e.g. in case we are currently
              looking at an image that is taller that the window height.  */
           while (start_pos == IT_CHARPOS (it)
@@ -8206,11 +8246,17 @@ is displayed in the `mode-line' face.  */);
   DEFVAR_LISP ("scroll-preserve-screen-position",
                Vscroll_preserve_screen_position,
                doc: /* Controls if scroll commands move point to keep its screen position unchanged.
+
 A value of nil means point does not keep its screen position except
 at the scroll margin or window boundary respectively.
+
 A value of t means point keeps its screen position if the scroll
 command moved it vertically out of the window, e.g. when scrolling
-by full screens.
+by full screens.  If point is within `next-screen-context-lines' lines
+from the edges of the window, point will typically not keep its screen
+position when doing commands like `scroll-up-command'/`scroll-down-command'
+and the like.
+
 Any other value means point always keeps its screen position.
 Scroll commands should have the `scroll-command' property
 on their symbols to be controlled by this variable.  */);