これは、FlipView コントロールの既定のスタイルによるものです。
それを削除するには、このデフォルト スタイル (以下に掲載) をコピーして、境界線の余白を削除する必要があります。
<Style x:Key="FlipViewStyle1" TargetType="FlipView">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderThickness" Value="0"/>
<Setter Property="TabNavigation" Value="Once"/>
<Setter Property="IsTabStop" Value="False"/>
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Hidden"/>
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Hidden"/>
<Setter Property="ScrollViewer.IsHorizontalRailEnabled" Value="False"/>
<Setter Property="ScrollViewer.IsVerticalRailEnabled" Value="False"/>
<Setter Property="ScrollViewer.IsHorizontalScrollChainingEnabled" Value="True"/>
<Setter Property="ScrollViewer.IsVerticalScrollChainingEnabled" Value="True"/>
<Setter Property="ScrollViewer.IsDeferredScrollingEnabled" Value="False"/>
<Setter Property="ScrollViewer.BringIntoViewOnFocusChange" Value="True"/>
<Setter Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<VirtualizingStackPanel AreScrollSnapPointsRegular="True" Orientation="Horizontal"/>
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="FlipView">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Focused">
<Storyboard>
<DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="FocusVisualWhite"/>
<DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="FocusVisualBlack"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Unfocused"/>
<VisualState x:Name="PointerFocused"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Margin="3">
<Border.Resources>
<ControlTemplate x:Key="HorizontalNextTemplate" TargetType="Button">
<Border x:Name="Root" BorderBrush="{StaticResource FlipViewButtonBorderThemeBrush}" BorderThickness="{StaticResource FlipViewButtonBorderThemeThickness}" Background="{StaticResource FlipViewButtonBackgroundThemeBrush}">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverBorderThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="Arrow">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedBorderThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="Arrow">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Path x:Name="Arrow" Data="M4.12,0 L9.67,5.47 L4.12,10.94 L0,10.88 L5.56,5.47 L0,0.06 z" Fill="{StaticResource FlipViewButtonForegroundThemeBrush}" HorizontalAlignment="Center" Height="10.94" Stretch="Fill" UseLayoutRounding="False" VerticalAlignment="Center" Width="9.67"/>
</Border>
</ControlTemplate>
<ControlTemplate x:Key="HorizontalPreviousTemplate" TargetType="Button">
<Border x:Name="Root" BorderBrush="{StaticResource FlipViewButtonBorderThemeBrush}" BorderThickness="{StaticResource FlipViewButtonBorderThemeThickness}" Background="{StaticResource FlipViewButtonBackgroundThemeBrush}">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverBorderThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="Arrow">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedBorderThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="Arrow">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Path x:Name="Arrow" Data="M5.55,0 L9.67,0.06 L4.12,5.47 L9.67,10.88 L5.55,10.94 L0,5.48 z" Fill="{StaticResource FlipViewButtonForegroundThemeBrush}" HorizontalAlignment="Center" Height="10.94" Stretch="Fill" UseLayoutRounding="False" VerticalAlignment="Center" Width="9.67"/>
</Border>
</ControlTemplate>
<ControlTemplate x:Key="VerticalNextTemplate" TargetType="Button">
<Border x:Name="Root" BorderBrush="{StaticResource FlipViewButtonBorderThemeBrush}" BorderThickness="{StaticResource FlipViewButtonBorderThemeThickness}" Background="{StaticResource FlipViewButtonBackgroundThemeBrush}">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverBorderThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="Arrow">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedBorderThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="Arrow">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Path x:Name="Arrow" Data="M0.06,0 L5.47,5.56 L10.88,0 L10.94,4.12 L5.48,9.67 L0,4.12 z" Fill="{StaticResource FlipViewButtonForegroundThemeBrush}" HorizontalAlignment="Center" Height="9.67" Stretch="Fill" UseLayoutRounding="False" VerticalAlignment="Center" Width="10.94"/>
</Border>
</ControlTemplate>
<ControlTemplate x:Key="VerticalPreviousTemplate" TargetType="Button">
<Border x:Name="Root" BorderBrush="{StaticResource FlipViewButtonBorderThemeBrush}" BorderThickness="{StaticResource FlipViewButtonBorderThemeThickness}" Background="{StaticResource FlipViewButtonBackgroundThemeBrush}">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverBorderThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="Arrow">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPointerOverForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Root">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedBorderThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="Arrow">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource FlipViewButtonPressedForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Path x:Name="Arrow" Data="M5.63,0 L11.11,5.55 L11.05,9.67 L5.64,4.12 L0.23,9.67 L0.17,5.55 z" Fill="{StaticResource FlipViewButtonForegroundThemeBrush}" HorizontalAlignment="Center" Height="9.67" Stretch="Fill" UseLayoutRounding="False" VerticalAlignment="Center" Width="10.94"/>
</Border>
</ControlTemplate>
</Border.Resources>
<Grid>
<ScrollViewer x:Name="ScrollingHost" BringIntoViewOnFocusChange="{TemplateBinding ScrollViewer.BringIntoViewOnFocusChange}" HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}" HorizontalSnapPointsType="MandatorySingle" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" IsTabStop="False" IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}" IsHorizontalScrollChainingEnabled="{TemplateBinding ScrollViewer.IsHorizontalScrollChainingEnabled}" IsVerticalScrollChainingEnabled="{TemplateBinding ScrollViewer.IsVerticalScrollChainingEnabled}" IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}" IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" Padding="{TemplateBinding Padding}" TabNavigation="{TemplateBinding TabNavigation}" VerticalSnapPointsType="MandatorySingle" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}" ZoomMode="Disabled">
<ItemsPresenter/>
</ScrollViewer>
<Button x:Name="PreviousButtonHorizontal" HorizontalAlignment="Left" Height="40" IsTabStop="False" Template="{StaticResource HorizontalPreviousTemplate}" VerticalAlignment="Center" Width="70"/>
<Button x:Name="NextButtonHorizontal" HorizontalAlignment="Right" Height="40" IsTabStop="False" Template="{StaticResource HorizontalNextTemplate}" VerticalAlignment="Center" Width="70"/>
<Button x:Name="PreviousButtonVertical" HorizontalAlignment="Center" Height="40" IsTabStop="False" Template="{StaticResource VerticalPreviousTemplate}" VerticalAlignment="Top" Width="70"/>
<Button x:Name="NextButtonVertical" HorizontalAlignment="Center" Height="40" IsTabStop="False" Template="{StaticResource VerticalNextTemplate}" VerticalAlignment="Bottom" Width="70"/>
</Grid>
</Border>
<Rectangle x:Name="FocusVisualWhite" IsHitTestVisible="False" Opacity="0" StrokeDashOffset="1.5" StrokeEndLineCap="Square" Stroke="{StaticResource FocusVisualWhiteStrokeThemeBrush}" StrokeDashArray="1,1"/>
<Rectangle x:Name="FocusVisualBlack" IsHitTestVisible="False" Opacity="0" StrokeDashOffset="0.5" StrokeEndLineCap="Square" Stroke="{StaticResource FocusVisualBlackStrokeThemeBrush}" StrokeDashArray="1,1"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
border の margin プロパティを変更するだけです:
<Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Margin="3">
に :
<Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Margin="0">