Smart down: scroll one row inside a tall expanded row, else jump.
If the selected row's bottom edge is already inside the viewport,
jump to the next row (header or tool), wrapping to the first row from
the final one. For rows taller than the viewport, pin the new
selection's top to the viewport; otherwise just ensure the row is
visible. Tab / Shift+Tab jump between server headers (see
action_jump_down).
