The parallax effect depends on the list item’s
current position within its ancestor Scrollable.
As the list item’s scroll position changes, the position
of the list item’s background image must also change.
This is an interesting problem to solve. The position
of a list item within the Scrollable isn’t 
available until Flutter’s layout phase is complete.
This means that the position of the background image
must be determined in the paint phase, which comes after 
the layout phase. Fortunately, Flutter provides a widget
called Flow, which is specifically designed to give you
control over the transform of a child widget immediately
before the widget is painted. In other words,
you can intercept the painting phase and take control
to reposition your child widgets however you want.

Note:
  To learn more, watch this short Widget of the Week video on the Flow widget:

Note:
  In cases where you need control over what a child paints,
  rather than where a child is painted,
  consider using a CustomPaint widget.