Here's a small example of a CustomListView where the items can be expanded or collapsed using Erel's CLVExpandable class.
The main CLV contains 10 items. Each item is expandable or collapsible.
The expanded panel contains an inner CLV with a variable item count. Next to the inner CLV is a B4XSeekBar.
The scrolling of the inner CLV is done with the B4XSeekBar by tapping on it at a certain distance from the bottom.
I didn't find a good solution (yet) to do the scrolling of the inner CLV. There is a conflict with the main CLV over the touch actions.
Maybe you find it a real challenge and find a solution? Until then the B4XSeekBar does a fairly good scrolling job.
You can...