precedingScrollExtent property

@override double get precedingScrollExtent

The scroll extent of all slivers preceding the tree in its viewport, or 0.0 when isLaidOut is false.

Adding this converts sliver-local y to viewport scroll-space y (and subtracting, the reverse).

Implementation

@override
double get precedingScrollExtent {
  // `constraints` is only readable once layout has begun; before the
  // first layout there is no preceding extent to speak of.
  if (geometry == null) {
    return 0.0;
  }
  return constraints.precedingScrollExtent;
}