Hello. In my project I am sometimes unable to select the first option in a combo box unless I select another option first and then go back and pick the first one. After a lot of experimentation I was finally able to duplicate a similar issue in your demo system.
Follow these steps to duplicate the problem:
1. Start out here: http://demos.telerik.com/silverlight/#ComboBox/Configurator
2. Check "IsFilteringEnanbled", "CanKeyboardNavigation...", and "OpenDropDownOnFocus". Leave the rest at default (IsEditable should be checked already and the others should be unchecked).
3. Click in the box and type the letter "p", then try to select "Pavlova, Ltd.". Sometimes it works, sometimes it doesn't and you are stuck with just p in the box unless you clear it out or select something else. If it works, clear out the box and try doing the same thing again, it usually fails on the second try for me.
The screenshot I attached illustrates the issue. If I click on "Pavlova, Ltd.", it will collapse and the menu the text in the box wont change. Essentially "Pavlova, Ltd." is not selectable at this point unless the box is cleared out or another item is selected first.
I know this is pretty obscure but it came up during QA in my application. Please give me an explanation or let me know this is something which will be fixed in a future build. Thanks in advance.