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DialogHost on MetroWindow causes rendering artifacts

See original GitHub issue

As you move your mouse around, or tab between different controls. Horizontal and vertical lines appear:

You can recreate the issue by adding a DialogHost to the MahAppsDragablzDemo MainWindow.xaml

<controls:MetroWindow x:Class="MahMaterialDragablzMashUp.MainWindow"
                      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                      xmlns:controls="http://metro.mahapps.com/winfx/xaml/controls"
                      xmlns:dragablz="clr-namespace:Dragablz;assembly=Dragablz"
                      xmlns:mahMaterialDragablzMashUp="clr-namespace:MahMaterialDragablzMashUp"
                      xmlns:dockablz="clr-namespace:Dragablz.Dockablz;assembly=Dragablz"
                      xmlns:wpf="http://materialdesigninxaml.net/winfx/xaml/themes"                      
                      WindowTransitionsEnabled="False"
                      TextElement.Foreground="{DynamicResource MaterialDesignBody}"
                      Background="{DynamicResource MaterialDesignPaper}"					  
                      GlowBrush="{DynamicResource AccentColorBrush}"					  
                      FontFamily="{StaticResource DefaultFont}"
                      Title="Material-MahApps-Dragablz Mash Up" Height="640" Width="800">
    
    <controls:MetroWindow.Flyouts>
        <controls:FlyoutsControl>
            <controls:Flyout x:Name="LeftFlyout" Position="Left" Header="Settings" wpf:FlyoutAssist.HeaderColorMode="Accent">
                <mahMaterialDragablzMashUp:FlyoutContent />
            </controls:Flyout>
        </controls:FlyoutsControl>
    </controls:MetroWindow.Flyouts>
    <controls:MetroWindow.LeftWindowCommands>
        <controls:WindowCommands>
            <wpf:PopupBox Style="{StaticResource WindowCommandsPopupBoxStyle}">
                <StackPanel>
                    <Button Content="Hello World"/>
                    <Button Content="Nice Popup"/>
                    <Button Content="Goodbye"/>
                </StackPanel>
            </wpf:PopupBox>
        </controls:WindowCommands>
    </controls:MetroWindow.LeftWindowCommands>

    <wpf:DialogHost IsOpen="True">
        <wpf:DialogHost.DialogContent>
            <DockPanel VerticalAlignment="Center" HorizontalAlignment="Center" Margin="20" FocusManager.IsFocusScope="True">
                <StackPanel DockPanel.Dock="Bottom" Orientation="Horizontal" HorizontalAlignment="Right">
                    <Button x:Name="Cancel" Style="{StaticResource MaterialDesignFlatButton}">Cancel</Button>
                    <Button x:Name="Add" Style="{StaticResource MaterialDesignFlatButton}">Add</Button>
                </StackPanel>
                <StackPanel Orientation="Vertical">
                    <TextBlock Margin="8" DockPanel.Dock="Top" Text="Dialog Test" TextWrapping="Wrap" Style="{StaticResource MaterialDesignHeadlineTextBlock}" />
                    <TextBox Text="Line 1" Margin="8,8,8,0" wpf:HintAssist.Hint="Test" Style="{StaticResource MaterialDesignFloatingHintTextBox}" />
                    <TextBox Text="Line 2" Margin="8,8,8,0" wpf:HintAssist.Hint="Test" Style="{StaticResource MaterialDesignFloatingHintTextBox}" />
                    <TextBox Text="Line 3" Margin="8,8,8,0" wpf:HintAssist.Hint="Test" Style="{StaticResource MaterialDesignFloatingHintTextBox}" />
                </StackPanel>
            </DockPanel>
        </wpf:DialogHost.DialogContent>


        <dockablz:Layout>
            <dragablz:TabablzControl BorderThickness="0"
                                 Margin="0,-1,0,1">
                <dragablz:TabablzControl.InterTabController>
                    <dragablz:InterTabController />
                </dragablz:TabablzControl.InterTabController>
                <TabItem Header="DIALOGS">
                    <mahMaterialDragablzMashUp:Dialogs Margin="8">
                        <mahMaterialDragablzMashUp:Dialogs.DataContext>
                            <mahMaterialDragablzMashUp:DialogsViewModel />
                        </mahMaterialDragablzMashUp:Dialogs.DataContext>
                    </mahMaterialDragablzMashUp:Dialogs>
                </TabItem>
                <TabItem Header="MAHAPPS">
                    <mahMaterialDragablzMashUp:Mah Margin="16" />
                </TabItem>
                <TabItem Header="PALETTE">
                    <mahMaterialDragablzMashUp:PaletteSelector Margin="16">
                        <mahMaterialDragablzMashUp:PaletteSelector.DataContext>
                            <mahMaterialDragablzMashUp:PaletteSelectorViewModel />
                        </mahMaterialDragablzMashUp:PaletteSelector.DataContext>
                    </mahMaterialDragablzMashUp:PaletteSelector>
                </TabItem>
            </dragablz:TabablzControl>
        </dockablz:Layout>
    </wpf:DialogHost>
</controls:MetroWindow>