How to reproduce: CollapsiblePanelHeaderElement headerElement = this.radCollapsiblePanel1.CollapsiblePanelElement.HeaderElement; RadTextBoxElement txtBox = new RadTextBoxElement(); headerElement.Children.Add(txtBox); Workaround: use RadTextBoxControlElement
The issue is still present and it is related to the hosted standard Microsoft TextBox control in the RadTextBoxElement. It can be worked around by using the RadTextBoxControlElement.
Has this been corrected? I'm having the same issue.