patch 9.2.0913: statusline: cell below the vertical separator keeps the old highlight

patch 9.2.0913: statusline: cell below the vertical separator keeps the old highlight

Problem:  When 'statusline' is set, the cell below a vertical separator keeps
          the highlight of the previous status line update and only catches
          up on the next key press (dougaak).
Solution: Also update that cell when the status line of the current window is
          redrawn while showing the ruler (Hirohito Higashi).

related: #20182
fixes:   #20948
closes:  #20949

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>