Such behavior is usually observer when the VisibleOnPageLoad property is used to open the RadWindow. This is a behavior property and it effect is quite self explanatory. This is the reason why such reopening should be expected if another element does a postback (i.e. a grid paging, for example).
There are several ways to work around it
to open the RadWindow
2) you can reset the VisibleOnPageLoad property back to false when needed (for example in the Page_Load event or in any other suitable event, depending on the place in the page lifecycle you set it to true)
3) set the EnableViewState for the RadWindow to false. This will prevent all other properties from maintaining their values between postbacks as well.
All the best,
the Telerik team
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