These co-ordinates are floating-point numbers, and use the same relative co-ordinates as $Xpos and $Ypos. If a map view of a note is closed and later reopened, it will scroll to that map's most recently viewed position.
The scroll position is also reflected in the interior view of a note container viewport (as before an agent's contents are fixed). For a note container, data at $MapScrollX and $MapScrollY is drawn in the vertical/horizontal centre of the container viewport.
The "shadow" of the parent note, used in early versions of Tinderbox, is no longer drawn in map views, as this is always centred in the current display - i.e. current co-ordinates - and so is no longer informative.
The viewport within a container can be moved via click-drag within the viewport area - dragging the map background scroll the child map, dragging an icon moves that icon on its own map.