Hello,
I think to get the proper behavior you are after, it would need to be a new, separate control. You certainly could have a set of full-width stacked expanders. That would work fine as-is, since you could put them in a StackPanel. However you mentioned you wanted the expanded item's content to fill the available height, which would require custom layout logic.
Some questions with that layout logic would be:
- What happens when you expand a second expander item?
- What if the expander headers themselves take up more height than is available in the overall control?