A sample page that shows how you can implement such functionality is attached.

As a workaround you can manually display the loading panel as per this KB - http://www.telerik.com/support/kb/aspnet-ajax/window/details/custom-loading-sign-for-radwindow

When the Lightweight render mode is enabled and you set large text in any of the dialogs of RadWindow, part of the text appears outside of the window.

You can workaround this issue by overriding the styles of the dialogs. Below you can check a possible approach for RadAlert:
    <style type="text/css">
        div.RadWindow {
            overflow: auto;
            height: auto !important;
        }

        .rwDialogMessage {
            padding-bottom: 30px;
        }
    </style>

There are several ways to work around this:
1) disable autosizing (set AutoSize to false)
2) move the RadWindow after autoziging finishes (see attached example for a demo):
			function OnClientAutoSizeEnd(sender, args)
			{
				var wndBounds = sender.getWindowBounds();
				//this is the case when the viewport is not sufficient for the RadWindow
				//so the RadWindow is as tall as the viewport
				if (wndBounds.height == $telerik.getClientBounds().height)
				{
					sender.moveTo(wndBounds.x - 9);
				}
			}

3) avoid autosizing for the subsequent page loads in the content page.
This can be done by setting AutoSize to false and additionally the following function attached to the OnClientPageLoad event will provide autosizing for the first load without positioning issues:
 			function OnClientPageLoad(sender, args)
			{
				if (!sender.hasBeenShown)
				{
					sender.autoSize(false);
					sender.hasBeenShown = true;
				}
			}

4)  use partial postbacks in the content page so that it is not fully reloaded and the autosizing logic is not fired again automatically.

There are two workarounds:
- set the EnableShadow property in the markup
- OR, for Lightweight RenderMode, add a CSS class:

<telerik:RadWindow ID="RadWindow_MyInfo" runat="server" RenderMode="Lightweight"></telerik:RadWindow>
<asp:Button ID="Button1" Text="show wnd" OnClientClick="test(); return false;" runat="server" />
<script>
	function test() {
		var myinfo = $find('<%=RadWindow_MyInfo.ClientID%>');
		myinfo.setUrl("../Dialogs/MyInfo.aspx");
		myinfo.set_enableShadow(true);
		myinfo.show();
		$telerik.$(myinfo.get_popupElement()).addClass("rwShadow");
	}
</script>

There are two possible workarounds:

- use RenderMode=Classic
- OR, add a CSS class to the dialog and a simple CSS rule to remove the built-in font icon:
			div.withCustomIcon .rwIcon:before
			{
				content: "";
			}

			<telerik:RadWindow ID="RadWindow1" runat="server" CssClass="withCustomIcon" IconUrl="~/images/icon_16x16.png" VisibleOnPageLoad="true" RenderMode="Lightweight"></telerik:RadWindow>

There are three possible workarounds until an official fix is available

- use RenderMode=Classic


- add a check for the object fields in the handler, in every handler

			function theCloseEventsHandler(sender, args) {
				var arg = args.get_argument();
				if (arg.target && arg.which) { //it is the mouse event
					arg = null;
				}
				alert(arg);
			}


- add the check by overriding the built-in function of the dialog. Place the following script at the end of the page that hosts the RadWindows:

			var oldClose = Telerik.Web.UI.RadWindow.prototype.close;
			Telerik.Web.UI.RadWindow.prototype.close = function (arguments) {
				if (arguments.target && arguments.which) { //it is the mouse event
					arguments = null;
				}
				var _oldClose = Function.createDelegate(this, oldClose);
				_oldClose(arguments);
			}

This causes some empty space to remain at the bottom of the dialog. Possible workarounds are:

- set the VIsibleStatusbar property of the RadWIndowManager to false, if all your RadWindows need no statusbar; or at least to the concrete instance.

- OR, use the following script that will improve hiding the status bar element and resizing the content:

oWnd = window.radopen(pageURL, winName);
//will work for classic rendermode
oWnd.set_visibleStatusbar(false);
//will work for lightweight. In classic jQuery will not throw exceptions because of the element it cannot find
$telerik.$(".rwStatusBar", oWnd.get_popupElement()).hide();
var contentElem = $telerik.$(".rwContent.rwExternalContent", oWnd.get_popupElement());
contentElem.height(contentElem.height() + 20);

At present, RadWindows do not have a z-index during animations so they may get hidden behind other elements on the page.
The animation should be performed with the final z-index being already set to the RadWindow's popup element.

When a RadWindow with RenderMode=Lightweight is shown the client-side equivalents of the Width and Height property change and the final dimensions of the popup are different than expected (larger if the popup is modal and smaller if not). These values should not change unless the set_width() or set_height() methods are called.

Possible workarounds:

- use RenderMode=Classic

- try adding the following CSS rule
	div.RadWindow
        {
                padding: 0 6px 5px;
        }

- use the following script together with the custom attributes in the markup:
            <telerik:RadWindow ID="m_printWindow" runat="server" RenderMode="Lightweight" Title="Print Settings"
			 Width="180" Height="330" origWidth="180" origHeight="330"
                Behaviors="Close,Move" VisibleStatusbar="False">
                <ContentTemplate>
                </ContentTemplate>
            </telerik:RadWindow>
			<asp:Button ID="Button1" Text="show Wnd" OnClientClick="showWnd(); return false;" runat="server" />

                <script type="text/javascript">
                	function showWnd() {
                		var wnd = $find("<%= m_printWindow.ClientID %>");
                		wnd.show();
                		var origWidth = wnd.get_element().getAttribute("origWidth");
                		var origHeight = wnd.get_element().getAttribute("origHeight");
                		wnd.setSize(origWidth, origHeight);
                		wnd.center();
                	}
	  </script>

- try the following approach:
			<script type="text/javascript">
				function OnClientShow(sender, args) {
					setTimeout(function () {
						sender.remove_show(OnClientShow);
						sender.set_modal(sender.get_element().getAttribute("ShouldBeModal"));
						var popupElem = sender.get_popupElement();
						popupElem.style.width = sender.get_width() + "px";
						popupElem.style.height = sender.get_height() + "px";
					}, 0);
				}
			</script>
			<telerik:RadWindowManager RenderMode="LightWeight" ID="rwmVessel" DestroyOnClose="false" runat="server" Skin="WebBlue" OnClientShow="OnClientShow">
				<Windows>
					<telerik:RadWindow RenderMode="LightWeight" 
									   AutoSize="false" 
									   ID="rwSendReceipt" 
									   ReloadOnShow="true" 
									   VisibleOnPageLoad="false" 
									   ShowContentDuringLoad="false"
									   Title="Submission Receipt" 
									   runat="server" 
									   Skin="WebBlue" 
									   Behaviors="Close" 
									   VisibleStatusbar="false" 
									   Width="760" 
									   Height="475" 
									   ShouldBeModal="true" 
									   IconUrl="images/application_16.png">
					</telerik:RadWindow>
				</Windows>
			</telerik:RadWindowManager>

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>

For the time being you can use the following workaround:

		<telerik:RadWindow ID="RadWindow1" runat="server" NavigateUrl="telerik_new-logo_thumb.png" VisibleOnPageLoad="true" RenderMode="Lightweight"></telerik:RadWindow>
		<script>
			var $W = Telerik.Web.UI.Window;
			$W.LightweightRenderer.prototype.createUI = function () {
				if (this.container) return;
				var wnd = this.window;
				var isRtl = wnd._isWindowRightToLeft();
				var addCssClass = Sys.UI.DomElement.addCssClass;

				var container = document.createElement("div");
				this._appendToDom(container);
				this.container = wnd._popupElement = container;
				container.id = "RadWindowWrapper_" + wnd.get_id();

				container.className = this._getSkinCssClass();
				var customCssClass = wnd.get_cssClass();
				if (customCssClass)
					addCssClass(container, customCssClass);
				if (isRtl)
					addCssClass(container, "rwRtl");
				if (!wnd._visibleTitlebar)
					addCssClass(container, "rwNoTitleBar");

				this.setShadowCssClass(wnd._enableShadow);

				container.setAttribute("unselectable", "on");

				var containerStyle = container.style;
				containerStyle.width = wnd._width;
				containerStyle.height = wnd._height;
				containerStyle.position = "absolute";

				var titlebar = this.titlebar = wnd._titlebarElement = document.createElement("div");
				titlebar.className = "rwTitleBar";
				container.appendChild(titlebar);

				var titleWrap = document.createElement("div");
				titleWrap.className = "rwTitleWrapper";
				titlebar.appendChild(titleWrap);

				titleWrap.appendChild(this.getIconNode());
				titleWrap.appendChild(this.getTitleNode());
				wnd.set_title(wnd._title);
				titleWrap.appendChild(this.getTitleCommandsContainer());

				wnd._registerTitlebarHandlers(true);
				wnd.set_iconUrl(wnd.get_iconUrl());

				var content = this.content = $get(wnd.get_id() + "_C") || this.pendingContent || document.createElement("div");
				if (content) {
					content.style.display = "none";
					content.className = "rwContent";
					this.setContent(content);
				}

				if (!wnd._dockMode) {
					var contentFrames = content.getElementsByTagName("iframe");
					//Create content IFRAME. Due to a bug in IE regarding setting the name attribute, the following ugly code needs to be used
					var frame = contentFrames.length > 0 ?
									contentFrames[0] :
									document.createElement(($telerik.isIE && !$telerik.isIE9Mode) ? "<iframe name='" + name + "'>" : "iframe");

					var name = this.window.get_name();

					frame.name = name;
					/*jshint scripturl:true*/
					frame.src = "javascript:'<html></html>';";
					frame.style.width = "100%";
					frame.style.height = "100%";
					frame.style.border = "0px"; //set to 0
					frame.frameBorder = "0";

					//Only under IE8 it is necessary to set display = "block" for the IFRAME - otherwise it will not occupy 100% of its parent element
					if ($telerik.isIE8)
						frame.style.display = "block";

					this.contentFrame = wnd._iframe = frame;

					//FIX for IFRAME overflowing outside the RadWindow under mobile device
					if (($telerik.isMobileSafari || wnd._isiPhoneiPadAppleWebkit) && !wnd._isPredefined) {
						var iframeWrapper = document.createElement('div');
						$(iframeWrapper).addClass('rwIframeWrapperIOS');
						iframeWrapper.appendChild(this.contentFrame);
						this.content.appendChild(iframeWrapper);
						//in iOS5 having a wrapper with only overflow hidden does not resolve the frame height problem
						//we need to have explicit pixel height for that wrapper as well !!!
						if (wnd._isiOS5Safari) wnd.setContentFixedHeight(wnd.get_height(), iframeWrapper);
						wnd._iframeWrapper = iframeWrapper;
					} else {
						this.content.appendChild(this.contentFrame);
					}

					Sys.UI.DomElement.addCssClass(this.content, "rwExternalContent");

					//Create a back reference to parent RadWindow
					wnd._createBackReference();
				}

				if (wnd._visibleStatusbar) {
					var statusbar = this.statusbar = document.createElement("div");
					statusbar.className = "rwStatusBar";
					container.appendChild(statusbar);

					statusbar.appendChild(this.getStatusMessageNode());
					if (wnd.isBehaviorEnabled(Telerik.Web.UI.WindowBehaviors.Resize))
						statusbar.appendChild(this.createStatusbarResizer());
				}


				wnd._addWindowToDocument();

				if (!$telerik.isTouchDevice) //fix various issues with the control when hardware acceleration is enabled with CSS
				{
					this.container.style["Transform"] = "none";
					this.container.style["BackfaceVisibility"] = "visible";
					this.container.style["webkitTransform"] = "none";
					this.container.style["webkitBackfaceVisibility"] = "visible";
					this.container.style["OTransform"] = "none";
					this.container.style["OBackfaceVisibility"] = "visible";
					this.container.style["MozTransform"] = "none";
					this.container.style["MozBackfaceVisibility"] = "visible";
					this.container.style["msTransform"] = "none";
					this.container.style["msBackfaceVisibility"] = "visible";
				}

				//Create the popup if it has not been created
				if (!wnd._popupBehavior) {
					//Set behaviors (move, resize,etc etc) - do it here, so that the IFRAME is created and can be passed to be skipped
					//should be done only once!
					wnd.set_behaviors(wnd._behaviors);
					this.popupBehavior = wnd._popupBehavior = $create(Telerik.Web.PopupBehavior, {
						'id': ((new Date() - 100) + 'PopupBehavior'),
						'parentElement': null, 'overlay': wnd._overlay, 'keepInScreenBounds': wnd._keepInScreenBounds
					}, null, null, this.container);
				}

			};
		</script>

A workaround is to disable animations. You can do this in the control markup/code-behind or JavaScript depending on your case.

In case this is not an option, you can disable the animations prior to closing so that autosizing does not use them:
function CloseAndRebind() {
	GetRadWindow().BrowserWindow.refreshGrid();
	setTimeout(function () {
		var wnd = GetRadWindow();
		wnd.set_animation(0);//disable animations
		wnd.close();
	}, 0);
}

You can find the detailed discussion here http://www.telerik.com/forums/issue-with-radwindow-(2014-2-618-45)

The content of the RadWindows overflows the available area when the control is used in LightWeight render mode in IE. Also the width of the window is increased with 14px.

image - http://screencast.com/t/vQqM9jLRy6uy

We recently upgraded Telerik Dlls from 2011 to 2013, 
We are facing some issue
1: We are receiving compilation error The 'ClientCallBackFunction' property cannot be set declaratively.

Code snippet is like 

<telerik:RadWindowManager Visible="true" Modal="true" Behavior="Close,Move" OffsetElementID="OffsetElement"
            ID="RWindow" runat="server" ClientCallBackFunction="ClientCallBackFunction" KeepInScreenBounds="true"
            ReloadOnShow="true">

-------
  <telerik:RadCodeBlock>
   <script type="text/javascript">
                function ClientCallBackFunction(sender, eventArgs) {
                    if (eventArgs == ‘close it’) {
                        window.location.href = 'Somepage.aspx';
                    }
                }
            </script>
        </telerik:RadCodeBlock>

2:  In RadGrid’s OnItemCommand event  code snippet e.Item.Cells[0..to n].Text always returns &nbsp;. 

Problem is this is live site and we need to fix these as soon as possible, looking forward for your response. 

A JavaScript error is thrown when a dialog that has animations enabled is closed if unobtrusive validation is used on the page.

Workarounds are:
- avoid animations
- OR, add the following script at the end of the form and remove it when this issue is fixed internally
Telerik.Web.UI.RadWindow.prototype._hide = function() {
	if (!this.get_animation() || this.get_animation() == 0) {
		this._afterHide();
	} else {
		if (this._enableShadow && $telerik.isIE) {
			this._setShadowCSSClass(false);
		} 
		var fnc = Function.createDelegate(this, this._afterHide),
			isMaximized = this.isMaximized(),
			duration = this.get_animationDuration();
		var popupElem = $telerik.$(this._popupElement);
		if (popupElem.length > 0 && popupElem.stopTransition) {
			$telerik.$(this._popupElement).stopTransition().transition({ opacity: 0 }, duration, "linear", function () {
				fnc(isMaximized);
			});
		} else {
			fnc(isMaximized);
		}
	}
}

A possible workaround is to remove the wrong skin-specific CSS class and add the correct one, for example:
<telerik:RadWindow ID="first" runat="server" Skin="Silk" Height="200px" Width="200px" VisibleOnPageLoad="true" RenderMode="Lightweight"
				   OnClientShow="applyProperSkinClass">
	<ContentTemplate>
		<asp:Label runat="server" Text="Silk"></asp:Label>
	</ContentTemplate>
</telerik:RadWindow>
<telerik:RadWindow ID="second" runat="server" Skin="MetroTouch" Height="200px" Width="200px" VisibleOnPageLoad="true" RenderMode="Lightweight"
				   OnClientShow="applyProperSkinClass">
	<ContentTemplate>
		<asp:Label runat="server" Text="MetroTouch"></asp:Label>
	</ContentTemplate>
</telerik:RadWindow>
<script type="text/javascript">
	function applyProperSkinClass(sender, args) {
		var classesArray = $telerik.$(sender.get_popupElement()).attr('class').split(' ');
		for (var i = 0; i < classesArray.length; i++) {
			if (classesArray[i].indexOf("RadWindow_") > -1) {
				$telerik.$(sender.get_popupElement()).removeClass(classesArray[i]);
			}
		}
		$telerik.$(sender.get_popupElement()).addClass("RadWindow_" + sender.get_skin());
		var wndBounds = sender.getWindowBounds();
		sender.setSize(wndBounds.width, wndBounds.height);
	}
</script>