WPF share column width between separate grids

You need to make sure your usercontrol hasn't set it's width in the usercontrol's xaml file. Just delete the Width="..." from it and you're good to go!

EDIT: This is the code I tested it with:

SOUserAnswerTest.xaml:

<UserControl x:Class="WpfApplication1.SOAnswerTest"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="300">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Name="LeftSideMenu" Width="100"/>
            <ColumnDefinition Name="Middle" Width="*"/>
            <ColumnDefinition Name="RightSideMenu" Width="90"/>
        </Grid.ColumnDefinitions>
        <TextBlock Grid.Column="0">a</TextBlock>
        <TextBlock Grid.Column="1">b</TextBlock>
        <TextBlock Grid.Column="2">c</TextBlock>
    </Grid>
</UserControl>

Window1.xaml:

<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WpfApplication1"
    Title="Window1" Height="300" Width="415">
    <Grid>

        <local:SOAnswerTest Grid.Column="0" Grid.Row="5" Grid.ColumnSpan="2"/>
    </Grid>
</Window>

<ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto" Name="sv">
    <Grid MinWidth="{Binding ElementName=sv, Path=ViewportWidth}" MinHeight="{Binding ElementName=sv, Path=ViewportHeight}" />
</ScrollViewer>

The problem is setting up a correct dependency between the ScrollViewer container size and the grid size. Let us consider just the width: the binding above allows the ScrollViewer to expose the horizontal scrollbar only if necessary:

enlarging a single column of the grid (eg. with a splitter) does not provoke the resizing of the other columns (which would happen if you bind Width instead MinWidth).

Using the ViewportWidth instead of ActualWidth takes into account the scrollbar width, if there is one.

Just try