Despite the fact that the elements of RadTextBox have ToolTipText and AutoToolTip properties, one can't set the tooltip at design-time. We should consider whether we should continue using these properties or introduce a new method to allow for setting tooltips at design-time. Resolution: Open 'Edit UI elements' and set the Tooltip property of RadTextBoxItem
For me Tooltips are a must. I use them quite often, especially in a "Non Ribbon App". Please make them available at design time.