The blue sign you're referring to is the undetermined state of the Tri-State CheckBox.
One way to get rid of it, if applicable, is to make the respective boolean property of your business object non-nullable so that a default value of false is set.
Your other option is to style the CheckBoxColumn
, and more specifically - the IndeterminatePath of its Indeterminate visual state, to your liking.
I hope you find these suggestions helpful.
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