The validation message is always visible if the control is in focus. Initially, you could edit the template of the RadComboBox in Blend and extract the ValidationToolTip template. In that way you will be able to edit/remove the animation for showing the tooltip. You can edit the ComboBox in Blend by dragging the control on the design surface, right-clicking it and choosing from the menu Edit Template-> Edit a Copy
. In that way you will have all the resources need for the change.
Also, I can suggest you to use the Summary label as demonstrated in this online demo
Hope this information helps.
the Telerik team
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