How to make the width of a stackpanel the width of another stackpanel?

Question

I have two dock panels, each of which has a StackPanel.

The width of the bottom StackPanel is determined by the width of the text in it.

The width of the top StackPanel should be the same as the width of the bottom StackPanel.

I tried to bind the width of the top StackPanel to the width of the bottom StackPanel via ElementName, but this does not work.

How can I make the top width the same as the bottom?

<StackPanel>
    <DockPanel LastChildFill="True" Height="100" >
        <StackPanel Width="{Binding ElementName=LeftMenuText, Path=Width}" 
                    DockPanel.Dock="Left"
                    Background="Yellow">
            <TextBlock
                Text="This is some text."/>
        </StackPanel>
        <StackPanel DockPanel.Dock="Right"
                    Background="Orange">
        </StackPanel>
    </DockPanel>

    <DockPanel 
        Height="3"
        Background="Black"></DockPanel>

    <DockPanel LastChildFill="True" Height="100">
        <StackPanel Name="LeftMenuWrapper"
                    DockPanel.Dock="Left"
                    Background="Yellow">
            <TextBlock 
                    Text="This is some text that is longer."/>
        </StackPanel>
        <StackPanel DockPanel.Dock="Right"
                    Background="Blue">
        </StackPanel>
    </DockPanel>
</StackPanel>

Answer 1

Bind it to ActualWidth LeftMenuWrapper:

 <StackPanel>
    <DockPanel LastChildFill="True" Height="100" >
        <StackPanel Width="{Binding ElementName=LeftMenuWrapper, Path=ActualWidth}" 
                    DockPanel.Dock="Left"
                    Background="Yellow">
            <TextBlock
                Text="This is some text."/>
        </StackPanel>
        <StackPanel DockPanel.Dock="Right"
                    Background="Orange">
        </StackPanel>
    </DockPanel>

    <DockPanel 
        Height="3"
        Background="Black"></DockPanel>

    <DockPanel LastChildFill="True" Height="100">
        <StackPanel Name="LeftMenuWrapper"
                    DockPanel.Dock="Left"
                    Background="Yellow">
            <TextBlock 
                    Text="This is some text that is longer."/>
        </StackPanel>
        <StackPanel DockPanel.Dock="Right"
                    Background="Blue">
        </StackPanel>
    </DockPanel>
</StackPanel>

. Grid IsSharedScope:

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <StackPanel Grid.IsSharedSizeScope="True">
      <Grid Height="100">
         <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" SharedSizeGroup="TextHolder"/>
            <ColumnDefinition Width="*"/>
         </Grid.ColumnDefinitions>
         <Border Background="Yellow">
            <TextBlock Text="This is some text."/>
         </Border>
         <Border Grid.Column="1" Background="Orange"/>
      </Grid>
      <Border Height="3" Background="Black"/>
      <Grid Height="100">
         <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" SharedSizeGroup="TextHolder"/>
            <ColumnDefinition Width="*"/>
         </Grid.ColumnDefinitions>
         <Border Background="Yellow">
            <TextBlock Text="This is some text that is longer."/>
         </Border>
         <Border Grid.Column="1" Background="Blue"/>
      </Grid>
   </StackPanel>
</Page>

Answer 2

, Grid SharedSizeGroup DockPanel s. .

<StackPanel Grid.IsSharedSizeScope="True">
    <Grid Height="100" >

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" SharedSizeGroup="A"/>
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>

        <StackPanel Grid.Column="0" Width="{Binding ElementName=LeftMenuText, Path=Width}"
                                DockPanel.Dock="Left"
                                Background="Yellow">
            <TextBlock
                    Text="This is some text."/>
        </StackPanel>
        <StackPanel Grid.Column="1" DockPanel.Dock="Right"
                                Background="Orange">
        </StackPanel>
    </Grid>

    <DockPanel
      Height="3"
      Background="Black"></DockPanel>

    <Grid Height="100">

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" SharedSizeGroup="A"/>
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>

        <StackPanel Grid.Column="0" Name="LeftMenuWrapper"
                                DockPanel.Dock="Left"
                                Background="Yellow">
            <TextBlock
                            Text="This is some text that is longer."/>
        </StackPanel>
        <StackPanel Grid.Column="1" DockPanel.Dock="Right"
                                Background="Blue">
        </StackPanel>
    </Grid>
</StackPanel>

, , - a SharedSizeGroup ( "A" ) Grid.IsSharedSizeScope="True" Grid (StackPanel, Grid )