moving: /do/ redraw the prior line when the viewport hasn't changed

The complementary test on current_y should only be done when doing
a scroll-only, because only then the prior line can be offscreen.

This fixes https://savannah.gnu.org/bugs/?50658.
Reported-by: David Lawrence Ramsey <pooka109@gmail.com>

src/move.c

@@ -489,11 +489,12 @@ void do_up(bool scroll_only)
 				1 : editwinrows / 2 + 1);
 
     /* If the lines weren't already redrawn, see if they need to be. */
-    if (openfile->current_y > 0 && openfile->current_y < editwinrows - 1) {
+    if (openfile->current_y > 0) {
 	/* Redraw the prior line if it's not actually the same line as the
 	 * current one (which it might be in softwrap mode, if we moved just
          * one chunk) and the line was horizontally scrolled. */
 	if (openfile->current != was_current
+		&& (!scroll_only || openfile->current_y < editwinrows - 1)
 		&& line_needs_update(was_column, 0))
 	    update_line(openfile->current->next, 0);
 	/* Redraw the current line if it needs to be horizontally scrolled. */
@@ -545,11 +546,12 @@ void do_down(bool scroll_only)
 				1 : editwinrows / 2 + 1);
 
     /* If the lines weren't already redrawn, see if they need to be. */
-    if (openfile->current_y > 0 && openfile->current_y < editwinrows - 1) {
+    if (openfile->current_y < editwinrows - 1) {
 	/* Redraw the prior line if it's not actually the same line as the
 	 * current one (which it might be in softwrap mode, if we moved just
          * one chunk) and the line was horizontally scrolled. */
 	if (openfile->current != was_current &&
+		(!scroll_only || openfile->current_y > 0) &&
 		line_needs_update(was_column, 0))
 	    update_line(openfile->current->prev, 0);
 	/* Redraw the current line if it needs to be horizontally scrolled. */