When using a style library (like for example ModernWpf) that adds a default window style it can happen that the style interferes with the expected look of the window used when resizing controls in the DockManager.

Example

App.xaml

<Application.Resources>
    <ResourceDictionary 
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">        
        <Style TargetType="Window">
            <Setter Property="BorderBrush" Value="Black"/>
            <Setter Property="BorderThickness" Value="5"/>
        </Style>
    </ResourceDictionary>
</Application.Resources>

Result

Solution

I propose we make sure that the resizer window use an empty style and not the default style. We could define an empty style that could be restyled by themes but I cannot really think of a scenario where you would want this window to be visible. The simplest solution is rather to just apply a new empty style in code.