AvalonDock fixed an issue in the default theme where the close button would upset the alignment on the tab headers.

ExtendedWPFToolkitSolution/Src/Xceed.Wpf.AvalonDock/Themes/generic.xaml

@@ -653,7 +653,7 @@
                                                   ContentTemplate="{Binding DocumentHeaderTemplate, Mode=OneWay, RelativeSource={RelativeSource AncestorType={x:Type avalonDock:DockingManager}, Mode=FindAncestor}}"
                                                   ContentTemplateSelector="{Binding DocumentHeaderTemplateSelector, Mode=OneWay, RelativeSource={RelativeSource AncestorType={x:Type avalonDock:DockingManager}, Mode=FindAncestor}}" />
                                 <!-- Close button should be moved out to the container style -->
-                                <Button x:Name="DocumentCloseButton" Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}" Grid.Column="1" Margin="5,0,0,0" Visibility="Hidden" 
+                                <Button x:Name="DocumentCloseButton" Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}" Grid.Column="1" Margin="1,-5,-3,-6" Visibility="Hidden" 
                                         Command="{Binding Path=LayoutItem.CloseCommand, RelativeSource={RelativeSource TemplatedParent}}"
                                         ToolTip="{x:Static avalonDockProperties:Resources.Document_Close}">
                                     <Image Source="/Xceed.Wpf.AvalonDock;component/Themes/Generic/Images/PinClose.png"/>