I also tried to put new UXWindow element into UXDesktop, but there was no change. Here is XAML:

<Intersoft:UXPage x:Class="ClientUIApplication1.MainPage" 
	xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
	xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
	xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
	mc:Ignorable="d"
	xmlns:Intersoft="http://intersoft.clientui.com/schemas"
    xmlns:my="clr-namespace:ClientUIApplication1"
	Title="MainPage Page"
	d:DesignWidth="640" d:DesignHeight="480">

    <Intersoft:UXDesktop>
        <Intersoft:UXWindow Header="UXWindow Title" Height="157" Width="336" IsClientVisible="True" CanMaximize="True">
            <Grid />
        </Intersoft:UXWindow>
    </Intersoft:UXDesktop>
</Intersoft:UXPage>