The content scrollbars of RadWindow are not shown in Lightweight if ShowContentDuringLoad="false". The bug is specific to Chrome.
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Workarounds:

1. Use the control in RenderMode=Classic

2. Enable the ShowContentDuringLoad="true" property

3. or attach a handler to the RadWindow's OnClientPageLoad proeprty and call the restore() method of the control in it:

        <telerik:RadWindow RenderMode="Lightweight" ID="RadWindow1" ShowContentDuringLoad="false" VisibleOnPageLoad="true" Width="600px" Height="400px" Title="Wikipedia" OnClientPageLoad="onLoad"
            NavigateUrl="https://www.wikipedia.org" runat="server">
        </telerik:RadWindow>
        <script>
            function onLoad(sender, args) {
                sender.restore();
            }
        </script>


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Steps to reproduce:

1. Run the following code:

        <telerik:RadWindow RenderMode="Lightweight" ID="RadWindow1" ShowContentDuringLoad="false" VisibleOnPageLoad="true" Width="600px" Height="400px" Title="Wikipedia"
            NavigateUrl="https://www.wikipedia.org" runat="server">
        </telerik:RadWindow>

Result: No scroll bars are shown, they apper after moving the window