Save the current page with PageStorage and restore it if this controller's scrollable is recreated.
If this property is set to false, the current page is never saved and initialPage is always used to initialize the scroll offset. If true (the default), the initial page is used the first time the controller's scrollable is created, since there's isn't a page to restore yet. Subsequently the saved page is restored and initialPage is ignored.
- PageStorageKey, which should be used when more than one scrollable appears in the same route, to distinguish the PageStorage locations used to save scroll offsets.
final bool keepPage;