Can you rephrase your question? I don't quite follow.

Adithya Sreyaj looks like asking for initially open items.

I've update the code to add support for initial expand conditions:

merge(this.items.changes, of(this.items))
      .pipe(map(() => this.items.toArray()))
      .subscribe((items) => {
        items.forEach((item, index) => {
          if (item.expanded) {
            this.expanded.add(index);
          }
        });
        this.cdr.detectChanges();
      });