Access to the size of an items’ container

Can we get access to the size of an items’ container?

My use case: I have a horizontal stack group, containing a vertical stack group, a divider, and another vertical stack group.

The vertical stack groups can vary wildly in size, being filled dynamically from calendar events and notifications. I’d like the divider to scale vertically with the container, so it always spans the entire height.

For more complex applications, it’d be nice if we can select the amount of levels to go deep (like with si(mindex)) or even to select non-parent items, by refering to a unique id or with a nth-child approach.