FF 52 and Chrome58 seem to have started reading some CSS from 2010 differently. 

This affects controls such as RadMenu, RadTreeView, RadPanelBar, RadEditor, RadListBox, RadTabStrip that are in the ContentTemplate of a RadWindow. The issue can manifest as bad element alignment or extra padding.

Internal fix will be available as of R2 2017.

There are two workarounds:

- use the Lightweight Render Mode (helps in some cases)

- add a CSS override similar to the following

			/* FireFox 52+ and Chrome 58+  */
			html:first-child .RadWindowFixed ul {
				float: none !important;
			}

<telerik:RadWindow ID="RadWindow2" runat="server" Title="Rad Window with RadTreeView - Fixed" Width="400px" MinWidth="230px" MaxWidth="600px" CssClass="RadWindowFixed"  
							   Height="610px" VisibleOnPageLoad="true" Behaviors="Minimize, Move, Maximize, Resize" VisibleStatusbar="false" KeepInScreenBounds="true" Left="406">