<telerik:RadComboBox runat="server" ID="CB" AllowCustomText="true" HighlightTemplatedItems="true" EnableViewState="false" CheckBoxes="true" EnableLoadOnDemand="true" OnItemsRequested="CB_ItemsRequested" ></telerik:RadComboBox>

Specifically, note that CheckBoxes is true.

In some cases, the combo box's  item's text may have HTML tags in them, but we don't want that HTML to be interpreted, and we're happy that RadComboBox automatically encodes the Item's name.  For example, the Item's text might be <img src=xxx onerror=alert(0)> , and of course we don't want that javascript to run. 

However, we have found a case where the text is not properly escaped, in which case the javascript does run and the alert popup shows.  The error is in RadComboBox.CheckBoxes.js, in the _checkedItemsTextsFitInputWidth function.  That function is passed the comma separated string of all the selected item's text.  This function wants to measure the width of the string, and so it creates a div off-screen, and then pushes the checkedItemTexts string into the div.  If the checkedItemTexts string contains any HTML tags that contain javascript, then that javascript is executed, which can be a XSS attack.

Problem:  Latest Telerik version of Combobox or RadCombobox reads "readonly" though it's not, when EnableAriaSupport is enabled.

Details: 

url to test with JAWS - https://demos.telerik.com/aspnet-ajax/combobox/examples/wai-aria-support/defaultcs.aspx

When I use JAWS to read the combobox present on this demo it reads as follows - 
              ComboBox readonly - Alice Mutton. To change the selection press the arrow keys.

Over here, why JAWS is reading "readonly". It's not readonly. It's making our screenreader blind users confusing.

I drilled down more into the problem and found that, the input element below has the aria-readonly=true set though the combobox is not readonly.
Here is the html generated for above combobox from telerik demo site -

<input name="ctl00$ContentPlaceholder1$RadComboBox2" class="rcbInput radPreventDecorate" id="ctl00_ContentPlaceholder1_RadComboBox2_Input" role="combobox" aria-busy="false" aria-disabled="false" aria-expanded="false" aria-haspopup="true" aria-readonly="true" aria-owns="ctl00_ContentPlaceholder1_RadComboBox2_listbox" aria-autocomplete="none" type="text" readonly="readonly" value="Alice Mutton" autocomplete="off">

I understand that this input control should be readonly so that aria users should not type and go haywire. But looks like to maintain the readonly=readonly property, telerik is generating aria-readonly=true for this _Input control and resulting this issue.

When you change this attribute aria-readonly="true" to aria-readonly="false", JAWS read it correctly as follows - 
              ComboBox - Alice Mutton. To change the selection press the arrow keys.

Let me know how to achieve that so that users will not be confused.

Please check the attached images for more details.

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.

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.

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.

For the time being the following CSS workaround can be used:
CSS:
    <style type="text/css">
        .RadButton.rbSkinnedButton input {
            padding-left: 6px;
            padding-right: 10px;
            margin: 0;
        }
    </style>
ASPX:
        <telerik:RadComboBox ID="RadComboBoxDateTimeFilter" runat="server">
            <ItemTemplate>
                <telerik:RadButton ID="RadButtonApply" Text="Apply" AutoPostBack="false" runat="server" />
                <telerik:RadButton ID="RadButtonClear" Text="Clear" AutoPostBack="false" runat="server" />
            </ItemTemplate>
            <Items>
                <telerik:RadComboBoxItem Text="" Value="" Selected="true" />
                <telerik:RadComboBoxItem Text="" Value="" />
            </Items>
        </telerik:RadComboBox>