InvalidOperationException when closing a RadWindow opened on a separated UI thread. The exception message is: "Cannot use a DependencyObject that belongs to a different thread than its parent Freezable."

To work this around, you can merge the Telerik.Windows.Controls.Navigation.xaml dictionary in the Resources of all RadWindow instances open on another thread and then set their Style property to the RadWindowStyle from the merged .xaml dictionary.

<telerik:RadWindow x:Class="WpfApp84.Window1"
        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"
        xmlns:local="clr-namespace:WpfApp84" xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
        mc:Ignorable="d"  Height="450" Width="800" Style="{DynamicResource RadWindowStyle}">
    <telerik:RadWindow.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/Telerik.Windows.Controls.Navigation;component/Themes/GenericWindows11.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </telerik:RadWindow.Resources>
</telerik:RadWindow>

To resolve this remove the MinWidth setting. You can set it back when the window is restored.

private double myMinWidth;
private void Window_WindowStateChanged(object sender, System.EventArgs e)
{
            var window = (RadWindow)sender;
            if (window.WindowState == WindowState.Minimized)
            {
                this.myMinWidth = window.MinWidth;
                window.MinWidth = 0;
            }
            else
            {
                window.MinWidth = this.myMinWidth;
            }
}

Popup shows behind RadWindow when using two monitors in Windows XP.

As a workaround you can set:  window.CaptionHeight= 0;

Maximizing RadWindow from the header button shows a window with the size of the content if SizeToContent = true. If you maximize with double click on header it is ok.

This can be observed only when AllowTransparance is set to false.

It can be reproduced with the Show method of the window or with Dock, ToolWindows and the save - load functionality.

When the Window is opening or resizing and Winforms controls are placed inside its content the content flickers.