title: Angular (7+) Material table with expandable rows - multiple expanded rows at the same time
body: <p>I am using <a href="https://stackblitz.com/angular/yoeqvrjkraq?file=app%2Ftable-expandable-rows-example.ts" rel="nofollow noreferrer">Material table with expandable row</a></p>
  
  <p>I would like to have multiple rows expanded on row clicks. The default behavior is to close the previously expanded row upon a new row click. I don't want the previous ones to be closed, each should be left open until clicked again. How can I do this?</p>