delegate property Null safety
The delegate that provides the children for this widget.
The children are constructed lazily using this delegate to avoid creating more children than are visible through the Viewport.
Using more than one delegate in a Viewport
If multiple delegates are used in a single scroll view, the first child of each delegate will always be laid out, even if it extends beyond the currently viewable area. This is because at least one child is required in order to estimate the max scroll offset for the whole scroll view, as it uses the currently built children to estimate the remaining children's extent.
final SliverChildDelegate delegate;