display: don't compensate for chunks before firstcolumn

The number of rows to draw shouldn't be compensated for the chunks
of edittop that are before firstcolumn, because they are offscreen.

This completes the fix for https://savannah.gnu.org/bugs/?50621.

src/winio.c

@@ -2921,8 +2921,8 @@ void edit_scroll(scroll_dir direction, int nrows)
     while (i > 0 && line != NULL) {
 #ifndef NANO_TINY
 	/* If the first blank row is in the middle of a softwrapped line,
-	 * compensate for the earlier chunks of that line. */
-	if (ISSET(SOFTWRAP) && i == nrows)
+	 * compensate for the earlier onscreen chunks of that line. */
+	if (ISSET(SOFTWRAP) && i == nrows && line != openfile->edittop)
 	    i += strnlenpt(line->data, leftedge) / editwincols;
 #endif
 	i -= update_line(line, (line == openfile->current) ?