This seems like an issue with the way you attach the handler and the way different browsers interpret the returned value. Usually a postback is cancelled via adding return false; after the function call, yet you return true. If your function also returns true/false this may lead to a contradiction. I advise that you check this behavior with regular controls, as this does not seem related to the RadWinow in any way.
If you need the server-side functionality what you can do is:
1. add the server-side handler with the needed logic
2. remove (do not add) the client-side handler
3. declare the CloseAndRebind() function in the page you load in the RadWindow nevertheless
4. Inject the call to this function from the code-behind of the button;s Click. How to do this is explained in this help article
. The said function can also get a reference to the RadWindow and call its close() method.
the Telerik team
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