The width of the ComboBox is incorrect after showing if it has Label
<telerik:radcombobox id="RadComboBox1" runat="server" label="label" width="50%"> </telerik:radcombobox>
Call the repaint() method of the ComboBox client-side object to resolve the issue after you show it or its invisible container for the first time.
Clicks on the input of a ComboBox pass through the containing IFrame (RadWindow) into the owner document
You can resolve the issue by calling comboBox.repaint() after you show the ComboBox. The most common case of this issue is when the ComboBox is in a RadWindow.