Right scrolling on the selection expanding

The selection in namelist area for MCA and MSA should provide visibility for the boundary that is 'moving'. For example:

1. Open an alignment that cannot fit into the screen
2. Select a sequence
3. Shift + Down
   Expected state: the bottom sequence that is added into the selection on each Down press should become visible

The same logic should be applied to the upper bound and work as this for both MCA and MSA.