[ScrollablePositionedList] Scrollbar Drag does not work if initial index is end of list

[ankitkapoor2493]

Problem description

Scrollbar is not draggable when initial index is set as last item of list.

Steps to reproduce

1. Have a sizable list so that scrollbar appears.
2. Set initial index to be last item of list.

Expected behavior

1. Scrollbar should be draggable.

Actual behavior

1. Scrollbar is not draggable..

Environment

1. Tested on Windows