去除后：

样式：

<Window.Resources>

<BorderGapMaskConverter x:Key="BorderGapMaskConverter"/>

<Style x:Key="GroupboxWithoutWhiteBorder" TargetType="{x:Type GroupBox}">

<Setter Property="BorderBrush" Value="#D5DFE5" />

<Setter Property="BorderThickness" Value="1" />

<Setter Property="Template">

<Setter.Value>

<ControlTemplate TargetType="{x:Type GroupBox}">

<Grid SnapsToDevicePixels="true">

<Grid.ColumnDefinitions>

<ColumnDefinition Width="6" />

<ColumnDefinition Width="Auto" />

<ColumnDefinition Width="*" />

<ColumnDefinition Width="6" />

</Grid.ColumnDefinitions>

<Grid.RowDefinitions>

<RowDefinition Height="Auto" />

<RowDefinition Height="Auto" />

<RowDefinition Height="*" />

<RowDefinition Height="6" />

</Grid.RowDefinitions>

<Border CornerRadius="4" Grid.Row="1" Grid.RowSpan="3" Grid.Column="0" Grid.ColumnSpan="4" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="Transparent" Background="{TemplateBinding Background}" />

<Border Name="Header" Padding="3,1,3,0" Grid.Row="0" Grid.RowSpan="2" Grid.Column="1">

<ContentPresenter ContentSource="Header" RecognizesAccessKey="true" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />

</Border>

<ContentPresenter Grid.Row="2" Grid.Column="1" Grid.ColumnSpan="2" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />

<Border Grid.Row="1" Grid.RowSpan="3" Grid.ColumnSpan="4" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" CornerRadius="3">

<Border.OpacityMask>

<MultiBinding Converter="{StaticResource BorderGapMaskConverter}" ConverterParameter="7">

<Binding ElementName="Header" Path="ActualWidth" />

<Binding Path="ActualWidth" RelativeSource="{RelativeSource Self}" />

<Binding Path="ActualHeight" RelativeSource="{RelativeSource Self}" />

</MultiBinding>

</Border.OpacityMask>

</Border>

</Grid>

</ControlTemplate>

</Setter.Value>

</Setter>

</Style>

</Window.Resources>