Cdk virtual-scroll inside mat-select for mat-option

The virtual scroll viewport needs a size in order to know, how big the scroll container must be. This can be done by specifying the [itemSize] property of <cdk-virtual-scroll-viewport> and its height.

In your example the height of one <option> item is 48px. If you want to show five items at once, the container size would be 5 * 48 = 240:

<mat-form-field>
    <mat-select placeholder="State">
        <cdk-virtual-scroll-viewport [itemSize]="48" [style.height.px]=5*48>
            <mat-option *cdkVirtualFor="let state of states" [value]="state"> 
                {{state}}
            </mat-option>
        </cdk-virtual-scroll-viewport>
    </mat-select>
</mat-form-field>

I think i have solved this:

https://stackblitz.com/edit/angular-gs4scp

The key things are when the mat select opens panel we trigger cdkVirtualScrollViewPort scroll and check view port size.

  openChange($event: boolean) {
    console.log("open change", $event);
    if ($event) {
      this.cdkVirtualScrollViewPort.scrollToIndex(0);
      this.cdkVirtualScrollViewPort.checkViewportSize();
    } else {
    }
  }

Where we get the reference to the virtual scroll viewport using @ViewChild

@ViewChild(CdkVirtualScrollViewport, { static: false })
  cdkVirtualScrollViewPort: CdkVirtualScrollViewport;

Other relevant pieces in the template are are pretty simple:-

<mat-form-field>
    <mat-select [formControl]="itemSelect"
  placeholder="Select Item"
  (openedChange)="openChange($event)">
    <mat-select-trigger>
      {{ itemTrigger }}
    </mat-select-trigger>
        <cdk-virtual-scroll-viewport itemSize="5" minBufferPx="200" maxBufferPx="400" class="example-viewport-select">
            <mat-option *cdkVirtualFor="let item of items" [value]="item"
                (onSelectionChange)="onSelectionChange($event)">{{item}}</mat-option>
        </cdk-virtual-scroll-viewport>
    </mat-select>
    <mat-hint>Justa hint</mat-hint>
</mat-form-field>

Add an additional mat-option using *ngFor as if you were not using cdk virtual scroll. This does not add any additional options to your select list, but does allow a default selected value, while also allowing cdk-virtual scroll to do its thing.

<mat-select placeholder="State">
  <cdk-virtual-scroll-viewport itemSize="10" [style.height.px]=10*100>
        <mat-option *cdkVirtualFor="let state of states" [value]="state">{{state}}</mat-option>
  </cdk-virtual-scroll-viewport>
  <mat-option *ngFor="let state of states" [value]="state"> 
  {{state}}</mat-option>
 </mat-select>