I've attached your test project, with the fixes to the problems I mentioned in my earlier comment. at runtime, things appear to be working as expected.
Here are some screenshots when I run the project.
1. At launch:
2. Clicking Button 1:
3. Clicking Button 2:
However, as you mentioned there is no sample data, so I cannot test that side of things. Please take my demo, read my notes and comments and apply these changes to your app.
If you still have an issue, please update my demo again, but with the actual data model you use and sample data that faithfully represents the real data. Ultimately, I need to be able to give the developers a project that shows the problem at runtime.
Important Note About UI Virtualization
I see you have other implementations in other forum threads where your code has a RadListView or a DataGrid inside a VerticalStackLayout. The same problem can affect you there.
My advice is to never use a StackLayout unless you are explicitly intending to "squish" the children to the bare minimum space and not allow forced measurement recalculation by the child. Always use a Grid or other layout parent that allows the child to expand into available space.
Lance | Manager Technical Support
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