You may change the behavior of this error dialog and to display it above the cell - all you need to do is to change the Margin of the Border:
However, if you place it above, you will probably not be able to edit the cell value afterwards. Still, you may define it so that it meets you particular requirements.
Considering the clipping, it cannot be removed when working with the above approach. What you may try is to still use a ToolTip, but a kind of specific one, whose Duration can be changed. Please look at this forum thread
for a reference on how to set the duration of the visibility of the ToolTip.
the Telerik team
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