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)

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