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

At present, when a RadWindow is pinned, it still uses position: absolute and an interval is initiated to recalculate its position every 100ms. This causes it to visibly "jump" when the user scrolls the page because of the interval update in its location.

Instead, position: fixed should be used to avoid the position calculation on an interval.

Currently, when a maximized RadWindow is closed the OnClientCommand event handler is called with a command "Restore" because when a maximized RadWindow is being closed it is first restored.



A flag can be used to avoid it, see the attachment.