A common reason for such behavior is focus being set to the textbox too early. Sometimes browsers do not calculate the absolute position of this textbox in the RadWindow's iframe and consider it to be the beginning of the form, so they attempt to scroll it into view and this pushes the RadWIndow titlebar out of the viewport.
To avoid this, you can try focusing the textbox with a small timeout on the client, instead of the server Focus() method. For example:
where you can try increasing the timeout. Another option would be to call the center() method of the RadWindow after focusing the textbox if this does not help as-is.
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