Длина элемента в зависимости от других элементов

Наконец, после закрытия/открытия VS он начинает работать...

Вот реальный код, если кому-то нужно:

Логика взаимодействия

/// <summary>
    /// Take care of displaying correctly a list of <see cref = "CommentsListItemControl"/> by changing their size if needed
    /// </summary>
    /// <param name = "sender">And ItemsControl</param>
    /// <param name = "e"> A routed event</param>
    private void ItemsControl_Loaded(object sender, System.Windows.RoutedEventArgs e)
    {
        // Get the maximum grid colum size.
        List<double> maxSizeColumn = GetGridColumnSize(sender as ItemsControl, "Main_CommentsListItemControl_DO_NOT_CHANGE");
        if (maxSizeColumn == null)
            return;

        //initialize the list
        List<GridLength> gridLengths = new List<GridLength>(new GridLength[maxSizeColumn.Count]);

        for (int i = 0; i < maxSizeColumn.Count; i++)
        {
            if (i == 1)
                // Let the user be able to resize the content
                gridLengths[i] = new GridLength(1, GridUnitType.Star);

            gridLengths[i] = new GridLength(maxSizeColumn[i]);

        }

        SetColumSize(sender as ItemsControl, "Main_CommentsListItemControl_DO_NOT_CHANGE", gridLengths);

    }
    /// <summary>
    /// Search all the column width for the specific control <see cref = "CommentsListItemControl"/>
    /// </summary>
    /// <param name = "control">An itemsControl with <see cref = "CommentsListItemControl"/>datatemplate</param> 
    /// <param name = "containerName">The name of the main container of the <see cref = "CommentsListItemControl"/></param>
    /// <returns>The maximum width of all columnDefinition inside the control</returns>
    private List<double> GetGridColumnSize(ItemsControl control, string containerName)
    {
        if (control == null)
            throw new ArgumentNullException(nameof(control));

        List<double> gridSize = null;

        // Check the width for each item inside de control
        for (int i = 0; i < control.Items.Count; i++)
        {
            // get the container
            var itemControlItem = control.ItemContainerGenerator.ContainerFromIndex(i);
            if (itemControlItem == null)
                // if the container is not already loaded, return
                return null;

            // get the main grid

            Grid mainGrid = UIHelpers.FindChild<Grid>(itemControlItem, containerName);

            //Get the child grid only if visible
            var childrenGrid = (((Grid)mainGrid.Children[0]).Visibility == Visibility.Visible) ? (Grid)mainGrid.Children[0] : (Grid)mainGrid.Children[1];

            //initialize the list
            if (gridSize == null)
                gridSize = new List<double>(new double[childrenGrid.ColumnDefinitions.Count]);

            //Get the size of every column
            for (int j = 0; j < childrenGrid.ColumnDefinitions.Count; j++)
            {
                // find the size of the grid and if bigger than the previous one, record it
                gridSize[j] = Math.Max(gridSize[j], childrenGrid.ColumnDefinitions[j].ActualWidth);
            }
        }

        return gridSize;
    }

    /// <summary>
    /// Set the column width of a <see cref = "ItemsControl"/>
    /// </summary>
    /// <param name = "control">The control where the column width must be applied</param>
    /// <param name = "containerName">The name of the main container of the <see cref = "CommentsListItemControl"/></param>
    /// <param name = "gridLengths">The <see cref = "GridLength"/> to be set</param>
    private void SetColumSize(ItemsControl control, string containerName, List<GridLength> gridLengths)
    {
        for (int i = 0; i < control.Items.Count; i++)
        {
            // get the container
            var itemControlItem = control.ItemContainerGenerator.ContainerFromIndex(i);
            if (itemControlItem == null)
            {
                // if the container is not found, inform the user and return
                Debugger.Break();
                return;
            }

            // get the main grid
            Grid mainGrid = UIHelpers.FindChild<Grid>(itemControlItem, containerName);

            //Get the child grid only if visible
            var childrenGrid = (((Grid)mainGrid.Children[0]).Visibility == Visibility.Visible) ? (Grid)mainGrid.Children[0] : (Grid)mainGrid.Children[1];

            //Check if size are consistent
            if (childrenGrid.ColumnDefinitions.Count != gridLengths.Count)
            {
                // do nothing and inform the user
                Debugger.Break();
                return;
            }
            //Apply the size for every column
            for (int j = 0; j < childrenGrid.ColumnDefinitions.Count; j++)
            {
                // find the size of the grid and if bigger than the previous one, record it
                if (j == 1)
                    continue;
                childrenGrid.ColumnDefinitions[j].Width = gridLengths[j];
            }
        }
    }
}

Связанный XAML

<UserControl x:Class = "CCTT.CommentsListControl"
         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" 
         xmlns:local = "clr-namespace:CCTT"
         xmlns:design  = "clr-namespace:CCTT.ViewModels"
         mc:Ignorable = "d" 
         d:DesignHeight = "450" d:DesignWidth = "800">

<Grid DataContext = "{x:Static design:CommentsListDesignModel.Instance}" >
    <ItemsControl ItemsSource = "{Binding Items}" Loaded = "ItemsControl_Loaded">
        <ItemsControl.ItemTemplate >
            <DataTemplate>
                <local:CommentsListItemControl />
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>
</Grid>

Помощник

public static class UIHelpers
{
    /// <summary>
    /// Finds a Child of a given item in the visual tree. 
    /// </summary>
    /// <param name = "parent">A direct parent of the queried item.</param>
    /// <typeparam name = "T">The type of the queried item.</typeparam>
    /// <param name = "childName">x:Name or Name of child. </param>
    /// <returns>The first parent item that matches the submitted type parameter. 
    /// If not matching item can be found, 
    /// a null parent is being returned.</returns>
    public static T FindChild<T>(DependencyObject parent, string childName)
       where T : DependencyObject
    {
        // Confirm parent and childName are valid. 
        if (parent == null) return null;

        T foundChild = null;

        int childrenCount = VisualTreeHelper.GetChildrenCount(parent);
        for (int i = 0; i < childrenCount; i++)
        {
            var child = VisualTreeHelper.GetChild(parent, i);
            // If the child is not of the request child type child
            T childType = child as T;
            if (childType == null)
            {
                // recursively drill down the tree
                foundChild = FindChild<T>(child, childName);

                // If the child is found, break so we do not overwrite the found child. 
                if (foundChild != null) break;
            }
            else if (!string.IsNullOrEmpty(childName))
            {
                var frameworkElement = child as FrameworkElement;
                // If the child's name is set for search
                if (frameworkElement != null && frameworkElement.Name == childName)
                {
                    // if the child's name is of the request name
                    foundChild = (T)child;
                    break;
                }
            }
            else
            {
                // child element found.
                foundChild = (T)child;
                break;
            }
        }

        return foundChild;
    }
}