Selecting an item from the RadComboBox and then pressing Enter key, does not trigger the default button(if set) of the form/panel in IE and Chrome. The workaround is to handle the client-side OnClientKeyPressing event of the RadComboBox and manually trigger the clicking of the default button.For example:

ASPX:

    <asp:Panel ID="Panel1" runat="server" DefaultButton="RadButton1">
        <telerik:RadTextBox ID="RadTextBox1" runat="server">
        </telerik:RadTextBox>
        <asp:Label ID="Label1" Text="" runat="server" />
        <telerik:RadButton ID="RadButton1" runat="server" OnClick="OnClick1" Text="Click" />
        <telerik:RadComboBox ID="RadComboBox1" runat="server" OnClientKeyPressing="OnClientKeyPressing">
            <Items>
                <telerik:RadComboBoxItem Text="text1" Value="val1" />
                <telerik:RadComboBoxItem Text="text2" Value="val2" />
                <telerik:RadComboBoxItem Text="text3" Value="val3" />
            </Items>
        </telerik:RadComboBox>
    </asp:Panel>

JavaScript:
    <telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
        <script type="text/javascript">
            function OnClientKeyPressing(sender, args) {

                if (args.get_domEvent().keyCode == 13) {
                    $find("<%=RadButton1.ClientID %>").click();
                }
            }
        </script>
    </telerik:RadCodeBlock>

Customer feedback:

The first combobox below behaves as I would expect to, and resizes when the browser window is resized.  However, the second combobox does not resize when the browser window does.  They use the same skin.  The only real difference that I can see is that I use check boxes in the second one.

rad combobox on clientblur event setting custom text fine when we are going to press tab  key but we are getting issue when we are selecting item from rad combo box manually using mouse.on clicking suppose i have text :  ils - apple  i have called onclien

When RadCombobox is inside fixed div element and is expanded it doesn't scroll as expected. The dropdown container should stay fixed as the combo's input element.

If we dynamically remove highlighted item from a ComboBox at clientside, afterwards get_highlightedItem still returns that deleted item.
ComboBox takes care of deleting selected item at like 497 of RadComboBox.js. Highlighted item should be taken care of too.

In ComboBox If somehow focus is moved to dropdown element (e.g. when user clicks on a header, or on load more items button) then arrow keys and keyboard support won't work anymore.
It can be fixed by simply setting dropdown tabindex to say 1 and attaching ComboBox's own _onKeyDown handler to dropdown keydown too.

There is no RTL boundary detection in ComboBox, e.g. no _elementOverflowsLeftScreenBorder in RadComboBox.DropDown.js

ComboBox keeps the dropdown aligned to either sides of input field.
If due to content overflow e.g in a small frame input field is placed slightly out of viewport then it would be better to check the boundaries if ScreenBoundaryDetection is enabled and keep the dropdown in view and not necessarily aligned with input.

RadComboBox will open up in case that ScreenBoundaryDetection is enabled and there is not enough space below even if the space above is much less than below.
Like in a vertically narrow frame with the ComboBox being at the top of the page. 
It seems better to compare the space below with above and put it where there is more space.

We are using the combo box drop down:

                <telerik:RadComboBox ID="rcbAccounts" runat="server" Width="360" DropDownWidth="810" OffsetX="-200"
                    DataValueField="AcctNumber" DataTextField="DisplayString" AutoPostBack="True"
                    AllowCustomText="True" Filter="Contains" ExpandDelay="0" CollapseDelay="0" MaxHeight="300"
                    OnClientDropDownOpening="OnRcbAccountsOpening" Skin="CustomTelerikSkin" EnableEmbeddedSkins="false"
                    EnableViewState="false" EnableScreenBoundaryDetection="true" >
                    <HeaderTemplate>
                        <ul>
                            <li class="rcbColumn" style="width: 330px">Name</li>
                            <li class="rcbColumn" style="width: 70px">Acct #</li>
                            <li class="rcbColumn" style="width: 90px">Type</li>
                            <li class="rcbColumn" style="width: 150px">Address</li>
                            <li class="rcbColumn" style="width: 110px">City</li>
                        </ul>
                    </HeaderTemplate>
                    <ItemTemplate>
                        <ul>
                            <li class="rcbColumn" style="width: 330px">
                                <%#DataBinder.Eval(Container.DataItem, EC.ACCT_NAME).ToString()%></li>
                            <li class="rcbColumn" style="width: 70px">
                                <%#DataBinder.Eval(Container.DataItem, EC.ACCT_NO).ToString()%></li>
                            <li class="rcbColumn" style="width: 90px">
                                <%#DataBinder.Eval(Container.DataItem, EC.ACCT_TYPE).ToString()%></li>
                            <li class="rcbColumn" style="width: 150px">
                                <%#DataBinder.Eval(Container.DataItem, EC.ACCT_ADDR1).ToString()%></li>
                            <li class="rcbColumn" style="width: 110px">
                                <%#DataBinder.Eval(Container.DataItem, EC.ACCT_CITY).ToString()%></li>
                        </ul>
                    </ItemTemplate>
                </telerik:RadComboBox>

It looks like the attached at runtime.  My question is:  Can we align the dropdown area to be left-aligned with the control?  In some cases, the left part of the data gets cut off by the browser window.

From customer feedback:

Label display style is not taken into consideration when calculating the width of the combobox e.g. if the display style is block, the label and combo should be on separate lines. In terms they should be equally wide.

However currently the combo is squashed to a minimum width.

This feedback item has been raised from this forum thread: http://www.telerik.com/community/forums/aspnet-ajax/combobox/enablecheckallitemscheckbox-all-entry-doesnt-fire-onitemchecked-event.aspx

This issue was first raised back in Sep '11 as it was suggested that 'it was in the plan for a future release'.

I've added this item here in the feedback portal to ensure that it's not got lost

Based on customer feedback:

The RadFormDecorator decorates most native input controls, but does miss some, namely in ComboBox and ListBox.

As this [the lack of decoration] is something advertently done, due to various issues in different browsers, we should thread carefully.

Based on customer feedback:

In certain cases, the client side autowidth does not perform well enough or the results are not exactly what one would expect.

Instead, server side autowidth could do the job. True, there are drawbacks to this approach, like font name and font size, but it could be configurable.

Please check the attached images for more details.