Each time a window is re-opened its size is decreased with several pixels.

video: https://www.screencast.com/t/kxVyE0U2

Steps to reproduce:
1. Open http://demos.telerik.com/aspnet-ajax/window/examples/modalpopup/defaultcs.aspx
2. Close and re-open RadWindow multiple times

Result: The window's height is decreased each time.

Recommended Workaround: Apply the following CSS class to the page with RadWindow (***  Updated on 22 Jan 2018 ***)

<style>
	.RadWindow {
		padding: 0 6px 6px !important;
	}
</style>


Optional Workaround: Manually resize the Window at the stage of opening it. This example is re-using the width and height set in the markup.

JavaScript
<script>
	function openFixedSize(sender, args) {
		var oWindow = radopen("WindowContent.aspx", "RadWindow1");
		oWindow.__oWidth = oWindow.__oWidth ? oWindow.__oWidth : oWindow.get_width();
		oWindow.__oHeight = oWindow.__oHeight ? oWindow.__oHeight : oWindow.get_height();
		oWindow.setSize(oWindow.__oWidth, oWindow.__oHeight);
	}
</script>

Markup
<telerik:RadButton ID="RadButton1" runat="server" Text="Open - Custom" AutoPostBack="false" OnClientClicked="openFixedSize"></telerik:RadButton>

<telerik:RadWindowManager RenderMode="Lightweight" ID="RadWindowManager1" runat="server">
	<Windows>
		<telerik:RadWindow RenderMode="Lightweight" ID="RadWindow1" runat="server" Title="" Height="500px"
			Width="500px" Left="1px" Top="1px" ReloadOnShow="true" ShowContentDuringLoad="false" Behaviors="None"
			Modal="true" ShowOnTopWhenMaximized="True" VisibleStatusbar="False" VisibleTitlebar="false">
		</telerik:RadWindow>
	</Windows>
</telerik:RadWindowManager>