offset property
Current absolute offset, set by parent (and it's parent etc, to root).
That means, it is the offset from (0,0) of the canvas. There is only one canvas, managed by the top Container, passed to all children in the paint(Canvas, Size).
It is a sum of all offsets passed in subsequent calls to applyParentOffset during object lifetime.
Implementation
ui.Offset offset = ui.Offset.zero;