The desired effect can be achieved easily with CSS and this approach will even allow for finer tuning of the desired effect.

The RadWindpw's ContentTemplate is a simple div element that has dimensions set in pixels by the RadWindow scripts, so you can control the way its contents flow just like any other generic div element on your page. Below follows an example.

In case an external page is loaded in the RadWindow (e.g., via the NavigateUrl property), it will be loaded with an iframe that has its width and height set to 100% of the RadWIndow content area. Thus, it is up to the content page to control its scrolling.

<%--A sample RadWindow declaration you can use for quick testing--%>
<telerik:RadWindow ID="RadWindow1" runat="server" VisibleOnPageLoad="true">
	<ContentTemplate>
		<div class="desiredOverflow">
			Add your content here
			<div style="height: 1000px; width: 1000px; background-color: yellow;">
				some dummy content so you can test out the overflow CSS rules behavior
			</div>
		</div>
	</ContentTemplate>
</telerik:RadWindow>


/* some sample CSS that you can use to control the scrolling behavior of your content */
.desiredOverflow
{
	width: 100%;
	height: 100%;
	margin: 0;
	padding: 0;
	overflow: scroll; /* set the desired value here */
	/* you can also use the other overflow rules for fine control */
	overflow-x: hidden; /* set the desired value here */
	overflow-y: scroll; /* set the desired value here */
}

This will be an enumerator just like the Behaviors property. It should, however, list the behaviors that are not allowed for the control. The common use case is that the developer does not want his popup to be resizable, so he/she needs only DisabledBehaviors="Resize" instead of listing all others that should be allowed in the Behaviors property. 

A ShowOnTopWhenModal property would have the effect that any modal RadWindow (Modal=true) will increase greatly its z-index, just like it does when it is maximized. This property should take precedence over a maximized RadWindow (i.e. it should increase the z-index to more than 100 000).

A possible workaround:

<style>
    .topWindow {
        z-index: 100001 !important;
    }
</style>

for a markup of:
<telerik:RadWindow ID="topWindow" runat="server"
    Width="500"
    Height="300"
    VisibleOnPageLoad="true"
    CssClass="topWindow"
    Modal="true"></telerik:RadWindow>

Implement the ability to set the size of RadWindow in percents out of the box. The following knowledge base article describes how it could be achieved for the time being using the RadWindow client-side API:

http://www.telerik.com/support/kb/aspnet-ajax/window/setting-size-in-percent-for-the-radwindow.aspx

When a RestrictionZone is used the RadWindows are not positioned correctly in all cases.
When the number of windows is low (e.g. below approximately 6) the dimensions set to the RadWindows are not optimal. In some cases not all the available viewport is filled even if the number of popups matches the needed (e.g. 6 instances should be able to cover the viewport, but they cover about 2/3 of its height).

Currently setSize() works for popups that are already shown. If setSize() is called before show() (or radopen() or open() from a RadWindowManager) the new size will not be respected on subsequent iterations.

After added this code window close button stop working.

if we remove these three properties it works fine
        oWnd.SetWidth(width);
        oWnd.SetHeight(height);
        oWnd.center();


function openWin(url) {
        var oWnd = radopen(url, "RadWindow22");
        oWnd.SetWidth(width);
        oWnd.SetHeight(height);
        oWnd.center();
    }

It should be similar to the RadAjaxManager.GetCurrent(Page) method and return the first RadWindowManager instance on the page. Note that, unlike most other manager type controls there may be multiple RadWindowManagers on the page in a valid scenario.

In IE only (not Chrome), beginning with the Q2 2013 release, an error is thrown on a RadWindow with the EnableShadow="true" property if .hide() is called on the window before it is made visible.  This is new behavior with Q2 2013.  The affected line is:

}else{Sys.UI.DomElement.removeCssClass(this.container,"rwShadow");

where .container = nothing

Current workarounds are:

1. Set EnableShadow="false"
or
2. Make sure that radwindow.isVisible() === true prior to calling .hide()

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. 

Unable to resolve the server's DNS address.

this is error when I try to open telerik window in google chrome browser.

Unable to resolve the server's DNS address.

this is error when I try to open telerik window in google chrome browser.

Opening a RadWindow containing a text input and selecting the input brings up the onboarrd keyboard. If you then zoom the input, the textbox becomes uneditable until the keyboard is manually hidden or the page is refreshed. I believe this is a focus issue, but cannot discern the details.

The following CSS will override the built-in rules to improve the icons positioning:

            div.RadWindow_Glow .rwTable .rwTitlebarControls .rwIcon
            {
                margin: 3px 5px 0 0;
            }

            div.RadWindow_Silk .rwTable .rwTitlebarControls .rwIcon
            {
                margin: 2px 5px 0 0;
            }

            div.RadWindow_Bootstrap .rwTable .rwTitlebarControls .rwIcon
            {
                margin: 6px 5px 0 0;
            }

I was using a RadWindowManager with PreserveClientState=True for the purpose of maintaining the size of the RadWindow following postbacks to the server (the user was allowed to resize the RadWindow).  I had javascript to adjust the layout of the controls inside of the RadWindow based on the RadWindow size.  

Following a postback, there was no way to know when the RadWindow's size had been adjusted back to the user's setting because there are no events that indicate when the client state has been restored.  I would like to request that you add an OnClientStateRestored event to the RadWindowManager so that we can execute code once the layout of the RadWindow is finalized following a postback.