trackBy with dropdown

[tim-wilkins]

I've tried using the virtualScrollOptions for the dropdown and setting the trackBy value to point to a suitable function but this function isn't being called. How do I do this? This is my attempt, but I don't get the logging to console. This is to work-around the jumping backwards bug and trackBy works well for that elsewhere, but I've not tried virtual scrolling with a dropdown before. appendOnly also avoids the bug and does work here, but I'd prefer the other fix.

public virtualScrollOptions : ScrollerOptions = {
appendOnly: true,
trackBy: this.trackBy
};

public trackBy(index: number): number {
console.log('trackBy');
return index;
}

        <p-dropdown [style]="{'width': '240px'}" [options]="logData | selectOperation : 'NozzleMeas' : dateRange : [] : JobNameNozzle" optionLabel="StartTime" [(ngModel)]="CurrentSnapshot"
                    [autoDisplayFirst]="true" [virtualScroll]="true" [virtualScrollItemSize]="35" [virtualScrollOptions]="virtualScrollOptions"></p-dropdown>