Using v3.0, here's the result I get with and without your PropertyGrid.EditorDefinitions :
https://postimg.org/image/k5p8yqwvp
So everything looks good.
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Get more controls, features, updates and technical support with Xceed Toolkit Plus for WPF
https://postimg.org/image/k5p8yqwvp
So everything looks good.
――――
Get more controls, features, updates and technical support with Xceed Toolkit Plus for WPF