Fix scrolling under scroll-preserve-screen-position and margins

* src/window.c (window_scroll_pixel_based): When setting point to
preserve screen coordinates, don't let cursor enter either of the
two scroll margins.  Fix incorrect usage of
WINDOW_WANTS_HEADER_LINE_P and use WINDOW_HEADER_LINE_HEIGHT
instead of CURRENT_HEADER_LINE_HEIGHT.  (Bug#22395)

1 files changed, 28 insertions, 15 deletions

diff --git a/src/window.c b/src/window.c
index bbe47c7255a..1a75342471f 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4981,27 +4981,34 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
 
   if (n > 0)
     {
+      int last_y = it.last_visible_y - this_scroll_margin - 1;
+
       /* We moved the window start towards ZV, so PT may be now
          in the scroll margin at the top.  */
       move_it_to (&it, PT, -1, -1, -1, MOVE_TO_POS);
-      if (IT_CHARPOS (it) == PT && it.current_y >= this_scroll_margin
+      if (IT_CHARPOS (it) == PT
+          && it.current_y >= this_scroll_margin
+          && it.current_y <= last_y - WINDOW_HEADER_LINE_HEIGHT (w)
           && (NILP (Vscroll_preserve_screen_position)
               || EQ (Vscroll_preserve_screen_position, Qt)))
         /* We found PT at a legitimate height.  Leave it alone.  */
         ;
-      else if (window_scroll_pixel_based_preserve_y >= 0)
-        {
-          /* If we have a header line, take account of it.
-             This is necessary because we set it.current_y to 0, above.  */
-          move_it_to (&it, -1,
-                      window_scroll_pixel_based_preserve_x,
-                      (window_scroll_pixel_based_preserve_y
-                       - WINDOW_WANTS_HEADER_LINE_P (w)),
-                      -1, MOVE_TO_Y | MOVE_TO_X);
-          SET_PT_BOTH (IT_CHARPOS (it), IT_BYTEPOS (it));
-        }
       else
         {
+          if (window_scroll_pixel_based_preserve_y >= 0)
+            {
+              /* Don't enter the scroll margin at the end of the window.  */
+              int goal_y = min (last_y, window_scroll_pixel_based_preserve_y);
+
+              /* If we have a header line, take account of it.  This
+                 is necessary because we set it.current_y to 0, above.  */
+              move_it_to (&it, -1,
+                          window_scroll_pixel_based_preserve_x,
+                          goal_y - WINDOW_HEADER_LINE_HEIGHT (w),
+                          -1, MOVE_TO_Y | MOVE_TO_X);
+            }
+
+          /* Get out of the scroll margin at the top of the window.  */
           while (it.current_y < this_scroll_margin)
             {
               int prev = it.current_y;
@@ -5025,7 +5032,7 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
       /* We moved the window start towards BEGV, so PT may be now
          in the scroll margin at the bottom.  */
       move_it_to (&it, PT, -1,
-                  (it.last_visible_y - CURRENT_HEADER_LINE_HEIGHT (w)
+                  (it.last_visible_y - WINDOW_HEADER_LINE_HEIGHT (w)
                    - this_scroll_margin - 1),
                   -1,
                   MOVE_TO_POS | MOVE_TO_Y);
@@ -5076,14 +5083,20 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
         ;
       else if (window_scroll_pixel_based_preserve_y >= 0)
         {
+          int goal_y = min (it.last_visible_y - this_scroll_margin - 1,
+                            window_scroll_pixel_based_preserve_y);
+
+          /* Don't let the preserved screen Y coordinate put us inside
+             any of the two margins.  */
+          if (goal_y < this_scroll_margin)
+            goal_y = this_scroll_margin;
           SET_TEXT_POS_FROM_MARKER (start, w->start);
           start_display (&it, w, start);
           /* It would be wrong to subtract CURRENT_HEADER_LINE_HEIGHT
              here because we called start_display again and did not
              alter it.current_y this time.  */
           move_it_to (&it, -1, window_scroll_pixel_based_preserve_x,
-                      window_scroll_pixel_based_preserve_y, -1,
-                      MOVE_TO_Y | MOVE_TO_X);
+                      goal_y, -1, MOVE_TO_Y | MOVE_TO_X);
           SET_PT_BOTH (IT_CHARPOS (it), IT_BYTEPOS (it));
         }
       else