I just answered your support ticket on the matter.
The problem was due to that the OnClientShow="betterFixForEditor" property is set to RadEditor, but not to RadWindow.
Please set it to RadWindow and test again:
<telerik:RadWindow ID="RadWindow1" OnClientShow="betterFixForEditor" runat="server" Modal="True" VisibleOnPageLoad="True">
In case the problem is other please provide a reproduction project in the support ticket so that I can help you right away.
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