title: how to hide mat expansion panel header when in expanded mode
body: <p>I have multiple mat expansion panels on a page i want that when i expand a panel its header show hide.</p>
  
  <p><strong>HTML</strong></p>
  
  <pre><code>&lt;mat-expansion-panel 
    class="z-depth-5" 
    (opened)="panelOpenState = true" 
    (closed)="panelOpenState = false"&gt;
    &lt;mat-expansion-panel-header 
      [collapsedHeight]="customCollapsedHeight"       
      [expandedHeight]="customExpandedHeight"&gt;
    &lt;/mat-expansion-panel-header&gt;
  &lt;/mat-expansion-panel&gt;
  </code></pre>
  
  <p><strong>CSS</strong></p>
  
  <pre><code>.mat-expansion-panel.mat-expanded {
    position: fixed;
    padding: 0;
    top: 0px;
    left: 60px;
    right: 0px;
    bottom: 0px;
    z-index: 100;
  }
  </code></pre>