Hello mariusz piatek,
Thank you for contacting us.
When you are using a WPF in the WebBrowser you are not allowed to suspend the main thread. We workaround this and making RadWindow to appear above all elements and we forbid clicking on the others elements. So, calling Close callback resolves this, as you find out already. We sorry for any inconvenience caused.
I hope this information helps. I will be glad to assist you further.
the Telerik team
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