title: How to prevent mat-expansion-panel from closing?
body: <p>I have matAccordion in my page in which the number of panels are dynamic. Each of these panels contain a form. I want to prevent the closing of my panel until the form is duly filled and is valid.</p>
  
  <p>I am not able to find any event that will prevent the panel from closing. The "(closed)" event fires after the panel close animation has happened.</p>
  
  <p>Is there some logic to control the close?</p>