更新：
找到了一种适合我的解决方案。也许它可以帮助其他人寻找类似的东西。
<UserControl x:Class="Expander_test2.Overlap"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300">
<UserControl.Resources>
<Style x:Key="ExpanderLeftHeaderStyle"
TargetType="{x:Type ToggleButton}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border Padding="{TemplateBinding Padding}">
<Grid Background="Transparent"
SnapsToDevicePixels="False">
<Grid.RowDefinitions>
<RowDefinition Height="19"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Path x:Name="arrow"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Stroke="{TemplateBinding Foreground}"
StrokeThickness="1.5"
SnapsToDevicePixels="false"
Data="M 1,1.5 L 4.5,5 L 8,1.5">
<Path.LayoutTransform>
<RotateTransform Angle="90"/>
</Path.LayoutTransform>
</Path>
<ContentPresenter Grid.Row="1"
Margin="0,4,0,0"
HorizontalAlignment="Center"
VerticalAlignment="Top"
SnapsToDevicePixels="True"
RecognizesAccessKey="True"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsChecked"
Value="true">
<Setter Property="Data"
Value="M 1,4.5 L 4.5,1 L 8,4.5"
TargetName="arrow"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="{x:Type Expander}"
TargetType="{x:Type Expander}">
<Setter Property="Foreground"
Value="{DynamicResource {x:Static SystemColors.WindowTextBrushKey}}"/>
<Setter Property="Background"
Value="Transparent"/>
<Setter Property="HorizontalContentAlignment"
Value="Stretch"/>
<Setter Property="VerticalContentAlignment"
Value="Stretch"/>
<Setter Property="BorderBrush"
Value="Transparent"/>
<Setter Property="BorderThickness"
Value="1"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Expander}">
<Border BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Background="{TemplateBinding Background}"
CornerRadius="3"
SnapsToDevicePixels="true">
<DockPanel>
<Canvas>
<ToggleButton x:Name="HeaderSite"
Margin="1"
MinWidth="0"
MinHeight="0"
Style="{StaticResource ExpanderLeftHeaderStyle}"
IsChecked="{Binding Path=IsExpanded,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}"
Content="{TemplateBinding Header}"
ContentTemplate="{TemplateBinding HeaderTemplate}"
ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}"
Foreground="{TemplateBinding Foreground}"
Padding="{TemplateBinding Padding}"
FontFamily="{TemplateBinding FontFamily}"
FontSize="{TemplateBinding FontSize}"
FontStyle="{TemplateBinding FontStyle}"
FontStretch="{TemplateBinding FontStretch}"
FontWeight="{TemplateBinding FontWeight}"
HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Canvas>
<ContentPresenter x:Name="ExpandSite"
DockPanel.Dock="Bottom"
Visibility="Collapsed"
Focusable="false"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
Margin="{TemplateBinding Padding}">
</ContentPresenter>
</DockPanel>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsExpanded"
Value="true">
<Setter Property="Visibility"
Value="Visible"
TargetName="ExpandSite"/>
<Setter Property="Canvas.Left"
Value="{Binding ActualWidth, ElementName=ExpandSite}"
TargetName="HeaderSite" />
</Trigger>
<Trigger Property="ExpandDirection"
Value="Left">
<Setter Property="DockPanel.Dock"
Value="Left"
TargetName="ExpandSite"/>
<Setter Property="DockPanel.Dock"
Value="Right"
TargetName="HeaderSite"/>
<Setter Property="Style"
Value="{StaticResource ExpanderLeftHeaderStyle}"
TargetName="HeaderSite"/>
</Trigger>
<Trigger Property="IsEnabled"
Value="false">
<Setter Property="Foreground"
Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</UserControl.Resources>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Expander Canvas.ZIndex="1" Header="Details" ExpandDirection="Left">
<Grid Background="Blue">
<TextBlock Text="Details content" VerticalAlignment="Center" HorizontalAlignment="Center" Margin="5" Foreground="White"></TextBlock>
</Grid>
</Expander>
<ScrollViewer Background="Red" HorizontalScrollBarVisibility="Visible" Grid.Column="1">
<Grid Background="Red">
<TextBlock Text="Main content" VerticalAlignment="Center" HorizontalAlignment="Center" Foreground="White" />
</Grid>
</ScrollViewer>
</Grid>
</UserControl>
注意： IsExpanded 触发器中扩展器设置器属性 =“Canvas.Left” 的 ToggleButton Canvas.ZIndex="1" 周围的画布
结果：
