@using Telerik.DataSource
@using Telerik.DataSource.Extensions

<p>ComboBox Value: @ComboBoxValue</p>

<TelerikComboBox ItemHeight="30"
                 OnRead="@OnComboBoxRead"
                 PageSize="20"
                 ScrollMode="@DropDownScrollMode.Virtual"
                 TItem="@ListItem"
                 TValue="@(int?)"
                 @bind-Value="@ComboBoxValue"
                 ValueMapper="@ComboBoxValueMapper"
                 Width="240px" />

@code {
    private List<ListItem>? ComboBoxData { get; set; }

    private int? ComboBoxValue { get; set; } = 3;

    private async Task OnComboBoxRead(ReadEventArgs args)
    {
        DataSourceResult result = await ComboBoxData.ToDataSourceResultAsync(args.Request);

        args.Data = result.Data;
        args.Total = result.Total;
    }

    private async Task<ListItem?> ComboBoxValueMapper(int? selectValue)
    {
        // Triggers the bug
        await Task.Yield();

        ListItem? result = ComboBoxData?.FirstOrDefault(x => selectValue == x.Value);

        return result;
    }

    protected override void OnInitialized()
    {
        ComboBoxData = Enumerable
            .Range(1, 1234)
            .Select(x => new ListItem()
                {
                    Value = x,
                    Text = $"Item {x}"
                })
            .ToList();
    }

    public class ListItem
    {
        public int Value { get; set; }
        public string Text { get; set; } = string.Empty;
    }
}