@using System.Collections.ObjectModel

<h4>Option Selected: 6 @selectedSix</h4>
<br />

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

<h4>Option Selected: 7 @selectedSeven</h4>
<br />

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

<TelerikButton OnClick="@AddOption">Add Item</TelerikButton>

<ul>
    @foreach (var item in myDdlData)
    {
        <li>@item.MyValueField</li>
    }
</ul>

@code {
    int selectedSix { get; set; } = 6;
    int selectedSeven { get; set; } = 7;

    ObservableCollection<MyDdlModel> myDdlData = new ObservableCollection<MyDdlModel>(Enumerable.Range(1, 5).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x }));

    protected override async Task OnInitializedAsync()
    {
        AddOption();

        await Task.Delay(TimeSpan.FromSeconds(1));

        AddOption();
    }

    void AddOption()
    {
        myDdlData.Add(new MyDdlModel { MyTextField = "item " + (myDdlData.Count + 1), MyValueField = myDdlData.Count + 1 });

        int origSelection = selectedSeven;
        selectedSeven = 0;
        StateHasChanged();
        //add this if it does not work
        //await Task.Delay(30);//wait a rendering frame
        selectedSeven = origSelection;
        //add this if it does not work
        //StateHasChanged();
    }

    public class MyDdlModel
    {
        public int MyValueField { get; set; }
        public string MyTextField { get; set; }
    }
}

<div class="box">
    <div class="row mt-3" style="align-items:center">
        <div class="col-12 col-lg-4">
            <span class="bold">ComboBox</span>
        </div>
        <div class="col">
            <TelerikComboBox Data="@myComboData" TextField="ComboTextField" ValueField="ComboValueField"
                             @bind-Value="selectedCombo" ClearButton="true" Filterable="true">
            </TelerikComboBox>
        </div>
    </div>

    <div class="row mt-3" style="align-items:center">
        <div class="col-12 col-lg-4">
            <span class="bold">DropDownList</span>
        </div>
        <div class="col">
            <TelerikDropDownList Data="@myDdlData" TextField="MyTextField" ValueField="MyValueField"
                                 @bind-Value="@selectedDropDown">
            </TelerikDropDownList>
        </div>
    </div>

    <div class="row mt-3" style="align-items:center">
        <div class="col-12 col-lg-4">
            <span class="bold">Autocomplete</span>
        </div>
        <div class="col">
            <TelerikAutoComplete Data="@Suggestions" @bind-Value="@AutoCompleteValue"
                                 ClearButton="true" />
        </div>
    </div>
</div>

@code {
    public int selectedDropDown { get; set; }
    public int selectedCombo { get; set; }
    public string AutoCompleteValue { get; set; }

    //ComboBox
    public class ComboModel
    {
        public int ComboValueField { get; set; }
        public string ComboTextField { get; set; }
    }

    IEnumerable<ComboModel> myComboData = Enumerable.Range(1, 20).Select(x => new ComboModel { ComboTextField = "item " + x, ComboValueField = x });

    //DropDownList
    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 });

    //Autocomplete
    List<string> Suggestions { get; set; } = new List<string> {
        "Manager", "Developer", "QA", "Technical Writer", "Support Engineer", "Sales Agent", "Architect", "Designer"
    };
}

<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
}