At the moment, validation only appears when you hover over the cell that didn't pass validation. Is there a way to show the validation without the mouse hovering?
"Basically, you cannot force the tooltip to be shown constantly, because Windows is the one who decides when the tooltip hides (usually on MouseLeave or after some amount of time) to keep the look and feel of the applications consistent (The tooltip control is made to act this way).
If you want to display some helpful information to the user in a way that differs from the standard Windows tooltip way, you should consider using something else than a ToolTip, maybe a Popup."
With this said, you can predefine the control template of the GridViewEditorPresenter element and replace the tooltip with a Popup, for example:
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