[scrollable_positioned_list] Add ScrollController to be observed by Scrollbar

[vesuvi88]

Description

Hello everyone!
I found that the solution proposed in the following PRs (#305, #445) is incorrect, since when scrolling long lists, primary and secondary _ListDisplayDetails are swapped. Therefore, you need to track both Scrollcontrollers.
Therefore, I propose the following fix: attach position primary ScrollController if positions is empty, and detach when changing ScrollControllers. I checked in my project, everything works correctly.

Related Issues

#273
#303
#278
#235
#175
#407

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[marcorossitb]

I tried to use your fork together with a ScrollController linked with a LinkedScrollControllerGroup from 'linked_scroll_controller' package:

@override
void initState() {
  controllers = LinkedScrollControllerGroup();
  contentScroll = controllers.addAndGet();
  fixedScroll = controllers.addAndGet();

  super.initState();
}

// inside build method:
ScrollablePositionedList.separated(
  scrollController: contentScroll,
  ...
)

and it throws the following assertion: "_LinkedScrollControllers can only be used with _LinkedScrollPositions." from:

flutter.widgets/packages/linked_scroll_controller/lib/linked_scroll_controller.dart

Line 136 in 494c6ab

assert(

Any thoughts?