scrollController property
ScrollController to get the current scroll position. Important for calculations!
This controller has to be assigned if the returned widget of builder is scrollable. Every GridView is scrollable by default.
You have assign the controller to the ReorderableBuilder and to your GridView.
If the scrollable widget is outside ReorderableBuilder, then you don't add scrollController to this widget because the calculations also works by using the scroll value in the context.
Implementation
final ScrollController? scrollController;