title: Angular Material: mat-drawer-container not showing backdrop with multiple mat-drawer
body: <p>I'm trying to implement the <code>[hasBackdrop]=true</code> on my <code>mat-drawer-container</code> with multiple <code>mat-drawers</code>. My html structure is something like this:</p>
  <pre><code>&lt;mat-drawer-container [hasBackdrop]=&quot;true&quot;&gt;
  
    &lt;mat-drawer #drawer mode=&quot;over&quot;&gt;
      &lt;app-side-nav [drawer]=&quot;drawer&quot; [infoDrawer]=&quot;infoDrawer&quot; [contactDrawer]=&quot;contactDrawer&quot; &gt;&lt;/app-side-nav&gt;
    &lt;/mat-drawer&gt;
  
    &lt;mat-drawer #infoDrawer mode=&quot;over&quot;&gt;
      &lt;app-side-info [infoDrawer]=&quot;infoDrawer&quot;&gt;&lt;/app-side-info&gt;
    &lt;/mat-drawer&gt;
  
    &lt;mat-drawer #contactDrawer mode=&quot;over&quot;opened='true'&gt;
      &lt;app-side-contact [contactDrawer]=&quot;contactDrawer&quot;&gt;&lt;/app-side-contact&gt;
    &lt;/mat-drawer&gt;
  
    &lt;mat-drawer-content&gt;
      &lt;app-header [drawer]=&quot;drawer&quot;&gt;&lt;/app-header&gt;
  
      &lt;app-mensagem&gt;&lt;/app-mensagem&gt;
  
      &lt;div&gt;
        &lt;router-outlet&gt;&lt;/router-outlet&gt;
      &lt;/div&gt;
  
      &lt;app-footer&gt;&lt;/app-footer&gt;
    &lt;/mat-drawer-content&gt;
  &lt;/mat-drawer-container&gt;
  
  </code></pre>
  <p>With one single drawer the <code>hasBackdrop</code> works perfectly with the click outside the <code>mat-drawer-container</code> colapsing the drawer. But since i added the other two i no longer have the backdrop available.</p>
  <p>In the docs it says</p>
  <blockquote>
  <p>@Input()
  hasBackdrop: any | Whether the drawer container should have a backdrop while one of the sidenavs is open. If explicitly set to true, the backdrop will be enabled for drawers in the side mode as well.</p>
  </blockquote>
  <p>Anyone had a similar problem? Should i open a new issue?</p>
  <p>Thanks in advance.</p>
  <p>ps: bad english, sorry</p>