@selectedValue

<TelerikDropDownList Data="@myDdlData" TextField="MyTextField" ValueField="MyValueField" @bind-Value="@selectedValue">
</TelerikDropDownList>

@code {
    //in a real case, the model is usually in a separate file
    //the model type and value field type must be provided to the dropdpownlist
    public class MyDdlModel
    {
        public int MyValueField { get; set; }
        public string MyTextField { get; set; }
    }

    IEnumerable<MyDdlModel> myDdlData = Enumerable.Range(1, 20).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x });

    int selectedValue { get; set; }
}

<div style="height:80vh;background:linear-gradient(white,orange)">
    <ol>
        <li>Open a ComboBox</li>
        <li>Type a character to filter and reduce the visible data items</li>
        <li>Observe incorrect popup position that leaves a gap</li>
    </ol>
    <ol>
        <li>Focus a closed ComboBox</li>
        <li>Type a character to filter and display a reduced list of data items</li>
        <li>Remove the filter string to increase the visible data item count</li>
        <li>Observe incorrect popup position that overflows the screen</li>
    </ol>
</div>

WORKS:
<TelerikComboBox Data="@ListItems"
                 @bind-Value="@SelectedValue"
                 TextField="@nameof(ListItem.Text)"
                 ValueField="@nameof(ListItem.Id)"
                 Filterable="true"
                 FilterOperator="@StringFilterOperator.Contains"
                 Width="300px" />

BROKEN:
<TelerikComboBox Data="@ListItems"
                 @bind-Value="@SelectedValue"
                 TextField="@nameof(ListItem.Text)"
                 ValueField="@nameof(ListItem.Id)"
                 Filterable="true"
                 FilterOperator="@StringFilterOperator.Contains"
                 Width="300px">
    <ComboBoxSettings>
        <ComboBoxPopupSettings Height="auto" MinHeight="50px" MaxHeight="60vh" />
    </ComboBoxSettings>
</TelerikComboBox>

<div style="height:80vh;background:linear-gradient(orange, white)">

</div>

@code {
    private List<ListItem> ListItems { get; set; } = new();

    private int SelectedValue { get; set; }

    protected override void OnInitialized()
    {
        ListItems = new List<ListItem>();

        for (int i = 1; i <= 50; i++)
        {
            ListItems.Add(new ListItem()
            {
                Id = i,
                Text = $"Item {i} {(char)Random.Shared.Next(65, 91)}{(char)Random.Shared.Next(65, 91)}{(char)Random.Shared.Next(65, 91)}{(char)Random.Shared.Next(65, 91)}"
            });
        }

        base.OnInitialized();
    }

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

Selected value: @selectedValue
<br />
<label>some label for the dropdown, open it and test whether clicking outside closes it
    <TelerikDropDownList Data="@myDdlData" TextField="MyTextField" ValueField="MyValueField" @bind-Value="selectedValue">
    </TelerikDropDownList>
</label>

@code {
    //in a real case, the model is usually in a separate file
    //the model type and value field type must be provided to the dropdpownlist
    public class MyDdlModel
    {
        public int MyValueField { get; set; }
        public string MyTextField { get; set; }
    }

    IEnumerable<MyDdlModel> myDdlData = Enumerable.Range(1, 20).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x });

    int selectedValue { get; set; } = 3; //usually the current value should come from the model data
}