The content of the header will "stretch" as much as is needed.... What's the use case/scenario that isn't working?
@skendrot if you have the expander stretch to its parent size, but then want to right-align anything within the header itself, you can't as the header content is left-aligned and not able to be stretched.
I was trying to use Expander for this and found it very hard to customise the header. Eventually ended up with custom control rather than expander
You could easily change the HeaderStyle of the Expander to stretch the content. Don't need a new control.
I submitted a PR for this but it is a breaking visual change