WPF TabControl header border - wpf

Question

how can I remove the border arround the TabControll headers?
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<Style TargetType="{x:Type TabItem}" x:Key="tabStyle" BasedOn="{StaticResource baseStyle}">
<Setter Property="Padding" Value="5" />
<Setter Property="FontSize" Value="14px" />
<Setter Property="FontWeight" Value="Bold" />
<Setter Property="Typography.Capitals" Value="AllSmallCaps" />
</Style>
Markup
<TabControl x:Name="tabControl" Grid.Row="1" BorderThickness="0">
<TabItem Header="TabItem" Style="{StaticResource tabStyle}">
<Grid Background="white" Margin="-3" />
</TabItem>
<TabItem Header="TabItem">
<Grid Background="white"/>
</TabItem>
</TabControl>

Answer 1

The value of the BorderThickness property is hardcoded in the default ControlTemplate of the TabItem. You will have to modify the template in order to be able to change it.
You could copy the default template into your XAML markup by right-click on the TabItem in design mode in Visual Studio or in Blend and then choose Edit Template->Edit a Copy. You will then have to change the the BorderThickness property of the Border elements in the template. Remember to get rid of or change the MultiDataTriggers that sets the BorderThickness of the "innerBorder" and "mainBorder" elements of the TabItem when certain conditions.
Here is something that should get you started:
<Style x:Key="FocusVisual">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<Rectangle Margin="2" SnapsToDevicePixels="true" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<LinearGradientBrush x:Key="TabItem.Static.Background" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#F0F0F0" Offset="0.0"/>
<GradientStop Color="#E5E5E5" Offset="1.0"/>
</LinearGradientBrush>
<SolidColorBrush x:Key="TabItem.Static.Border" Color="#ACACAC"/>
<LinearGradientBrush x:Key="TabItem.MouseOver.Background" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#ECF4FC" Offset="0.0"/>
<GradientStop Color="#DCECFC" Offset="1.0"/>
</LinearGradientBrush>
<SolidColorBrush x:Key="TabItem.MouseOver.Border" Color="#7EB4EA"/>
<SolidColorBrush x:Key="TabItem.Disabled.Background" Color="#F0F0F0"/>
<SolidColorBrush x:Key="TabItem.Disabled.Border" Color="#D9D9D9"/>
<SolidColorBrush x:Key="TabItem.Selected.Border" Color="#ACACAC"/>
<SolidColorBrush x:Key="TabItem.Selected.Background" Color="#FFFFFF"/>
<Style x:Key="TabItemStyle1" TargetType="{x:Type TabItem}">
<Setter Property="FocusVisualStyle" Value="{StaticResource FocusVisual}"/>
<Setter Property="Foreground" Value="Black"/>
<Setter Property="Background" Value="{StaticResource TabItem.Static.Background}"/>
<Setter Property="BorderBrush" Value="{StaticResource TabItem.Static.Border}"/>
<Setter Property="Margin" Value="0"/>
<Setter Property="Padding" Value="6,2,6,2"/>
<Setter Property="HorizontalContentAlignment" Value="Stretch"/>
<Setter Property="VerticalContentAlignment" Value="Stretch"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TabItem}">
<Grid x:Name="templateRoot" SnapsToDevicePixels="true">
<Border x:Name="mainBorder" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="0" Background="{TemplateBinding Background}" Margin="0">
<Border x:Name="innerBorder" BorderBrush="{StaticResource TabItem.Selected.Border}" BorderThickness="0" Background="{StaticResource TabItem.Selected.Background}" Margin="-1" Opacity="0"/>
</Border>
<ContentPresenter x:Name="contentPresenter" ContentSource="Header" Focusable="False" HorizontalAlignment="{Binding HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{Binding VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
</Grid>
<ControlTemplate.Triggers>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Left"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Border}"/>
<!--<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,0,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,0,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Bottom"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Border}"/>
<!--<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,0,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,0,1,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Right"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Border}"/>
<!--<Setter Property="BorderThickness" TargetName="innerBorder" Value="0,1,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="0,1,1,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Top"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Background}"/>
<!--<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Border}"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,1,0"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,1,0"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Left"/>
</MultiDataTrigger.Conditions>
<Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Border}"/>
<!--<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,0,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,0,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Bottom"/>
</MultiDataTrigger.Conditions>
<Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Border}"/>
<!--<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,0,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,0,1,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Right"/>
</MultiDataTrigger.Conditions>
<Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Border}"/>
<!--<Setter Property="BorderThickness" TargetName="innerBorder" Value="0,1,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="0,1,1,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Top"/>
</MultiDataTrigger.Conditions>
<Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Border}"/>
<!--<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,1,0"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,1,0"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Left"/>
</MultiDataTrigger.Conditions>
<!--<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,0,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,0,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Left"/>
</MultiDataTrigger.Conditions>
<Setter Property="Panel.ZIndex" Value="1"/>
<Setter Property="Margin" Value="-2,-2,0,-2"/>
<!--<Setter Property="Opacity" TargetName="innerBorder" Value="1"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,0,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,0,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Bottom"/>
</MultiDataTrigger.Conditions>
<!--<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,0,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,0,1,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Bottom"/>
</MultiDataTrigger.Conditions>
<Setter Property="Panel.ZIndex" Value="1"/>
<Setter Property="Margin" Value="-2,0,-2,-2"/>
<!--<Setter Property="Opacity" TargetName="innerBorder" Value="1"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,0,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,0,1,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Right"/>
</MultiDataTrigger.Conditions>
<!--<Setter Property="BorderThickness" TargetName="innerBorder" Value="0,1,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="0,1,1,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Right"/>
</MultiDataTrigger.Conditions>
<Setter Property="Panel.ZIndex" Value="1"/>
<Setter Property="Margin" Value="0,-2,-2,-2"/>
<!--<Setter Property="Opacity" TargetName="innerBorder" Value="1"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="0,1,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="0,1,1,1"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Top"/>
</MultiDataTrigger.Conditions>
<!--<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,1,0"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,1,0"/>-->
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Top"/>
</MultiDataTrigger.Conditions>
<Setter Property="Panel.ZIndex" Value="1"/>
<Setter Property="Margin" Value="-2,-2,-2,0"/>
<!-- <Setter Property="Opacity" TargetName="innerBorder" Value="1"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,1,0"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,1,0"/>-->
</MultiDataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
I am afraid that there is no easier way to get rid of the borders than to modify the control template.