I'd like to get the status of the group "Details" toggle item (+/- Symbol) in Power BI Report Builder (Paginated Reports) as shown in the screenshot:
The toggle item is located in Textbox A.
The formula
IIF(InScope("Details"), "Visible", "Hidden")
works as long as it is in the shown/hidden group rows, but I would like to display the visibility status in the report header. If I place the formula there, the status does not change when the group is shown or hidden.

Create a new measure with the following DAX formula:
You can use this measure in the report header to display the status of the "Details" toggle item
Use
and it will update as you toggle the group visibility.