Thus, users can scroll down and see the content of the page behind maximized window.
Workaround for iOS and Android:

<telerik:RadWindow ID="RadWindow1" runat="server" VisibleOnPageLoad="true" OnClientCommand="OnClientCommand" />

<style>
        html, body {
            margin: 0px;
            padding: 0px;
            border: 0px;
        }
    </style>
    <script type="text/javascript">
        var currentDialog = null;

        function OnClientCommand(sender, eventArgs) {
            var commandName = eventArgs.get_commandName();

            if ($telerik.isTouchDevice) {
                if (commandName == "Maximize") {
                    document.body.style.position = 'fixed';
                    setTimeout(function () {
                        document.body.style.overflow = 'visible';
                    }, 100)

                }
                else if (commandName != "Pin") {
                    document.body.style.position = 'static';
                }
            }

            if ($telerik.isMobileSafari) {
                if (commandName == "Maximize") {
                    window.onscroll = centerDialog;
                    currentDialog = sender;
                }
                else if (commandName != "Pin") {
                    window.onscroll = null;
                    currentDialog = null;
                }
            }
        }

        function centerDialog() {
            if (currentDialog && currentDialog.center) {
                currentDialog.center();
            }
        }
    </script>