title: Vertical scroll in overlay creating using angular-cdk
body: <p>I've used cdk's Overlay to show some content when a button is clicked. Everything working as expected except the vertical scroll mechanism.</p>
  
  <p>Content inside the overlay is getting the vertical scrollbar when I'm reducing the height of the browser. Given <code>max-height: 600px</code> and <code>height: 600px</code> to both <code>panelClass</code> and content too. But still the same behaviour.</p>
  
  <p>Tried to look into angular-material dialog code but unable to understand how scrolling is achieved.</p>
  
  <p>Due to lack of clear documentation on <code>Overlay.scrollStrategies</code> i'm not clear If I have to look in this direction.</p>
  
  <p>Below is code extract, which I'm using for making the Overlay.</p>
  
  <pre><code>.help-menu-overlay-panel-class {
      overflow-y: auto;
      box-sizing: border-box;
      border-radius: 5px;
      background-color: #fff;
      max-height: 600px;
      height: 600px;
  }
  
  .help-overlay-container {
     overflow-y: auto;
     height: 600px;
  }
  
  
  
  &lt;button [helpMenuOverlayTrigger]="helpMenuOverlayTemplate"&gt;&lt;/button&gt;
  
  
  &lt;ng-template #helpMenuOverlayTemplate&gt;
      &lt;div class="help-overlay-container"&gt;
          ...
      &lt;/div&gt;
  &lt;/ng-template&gt;
  
  
  
  const overlayConfig: OverlayConfig = new OverlayConfig(&lt;OverlayConfig&gt;{
          hasBackdrop: true,
          direction: this.dir.value,
          backdropClass: 'cdk-overlay-dark-backdrop',
          maxHeight: '600px',
          maxWidth: '400px',
          panelClass: ['help-menu-overlay-panel-class']
      });
  
  overlayConfig.positionStrategy = this.overlay
      .position()
      .connectedTo(
          this.elementRef,
          {
              originX: 'start',
              originY: 'bottom'
          },
          {
              overlayX: 'start',
              overlayY: 'top'
          }
      )
      .withOffsetY(10)
      .withOffsetX(-4)
      .withDirection(this.dir.value);
  
  // overlayConfig.scrollStrategy = this.overlay.scrollStrategies.reposition();
  
  this._overlayRef = this.overlay.create(overlayConfig);
  
  this._overlayRef.backdropClick().subscribe(
      () =&gt; {
          this._overlayRef.detach();
      }
  );
  </code></pre>