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 tap on items from each inner CLV to select them. Check the logs panel for the list of selected items.
I didn't create a B4J version yet.
You can find the B4A source code in the attachment (testenvironment44.zip).
The app was tested with an Android 8.1.0 device and an Android 14 device with One UI 6.0
I removed the starter module from the project to fix the blank screen at startup.