ChangeViewAsync.cs

public static async Task ChangeViewAsync(this ScrollViewer scrollViewer, double? horizontalOffset, double? verticalOffset, bool disableAnimation)
{
    var tcs = new TaskCompletionSource<object>();

    EventHandler<ScrollViewerViewChangedEventArgs> viewChanged = (s, e) => tcs.TrySetResult(null);
    try
    {
        scrollViewer.ViewChanged += viewChanged;
        scrollViewer.ChangeView(horizontalOffset, verticalOffset, null, disableAnimation);
        await tcs.Task;
    }
    finally
    {
        scrollViewer.ViewChanged -= viewChanged;
    }
}