GridSplitter Autosize

Question

You could use MultiBinding to achieve what you want.

For each RowDefinition you set a MinHeight which is bound to its contents ActualHeight's like so:

<RowDefinition Height="100">
    <RowDefinition.MinHeight>
        <MultiBinding Converter="{StaticResource ResourceKey=CalcAll}">
            <Binding ElementName="firstElementInThisRow" Path="ActualHeight"></Binding>
            <Binding ElementName="secondElementInThisRow" Path="ActualHeight"></Binding>
            <Binding ElementName="thirdElementInThisRow" Path="ActualHeight"></Binding>
            <Binding ElementName="fourthElementInThisRow" Path="ActualHeight"></Binding>
        </MultiBinding>
    </RowDefinition.MinHeight>
</RowDefinition>

Your Converter could look like:

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
    double result = 0.0;
    foreach (object item in values)
    {
        result += System.Convert.ToDouble(item);
    }
    return result;
}

public object[] ConvertBack(object values, Type[] targetType, object parameter, CultureInfo culture)
{
    return null;
}

Everytime you expand a control, its ActualHeight changes and the Binding gets update -> MinHeight of the parents RowDefinition changes.

But you cant set one if the controls VerticalAlignment to Stretch, because then the ActualHeight wouldn't change by expanding.

EDIT: Since the only property I can now think of is the DesiredSize.Height-property, you can't use Binding (the binding won't update, if the DesiredSize.Height-value changes). But perhaps you can use a property (let's call it MinHeightRowOne) of type double which raises the PropertyChanged event in it's setter and is bound to the first rows MinHeight (one property for each row):

public double _minHeightRowOne;
public double MinHeightRowOne
{
    get
    {
        return _minHeightRowOne;
    }
    set
    {
        _minHeightRowOne = value;
        OnPropertyChanged("MinHeightRowOne");
    }
}

public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged(string name)
{
    if (PropertyChanged != null)
    {
        PropertyChanged(this, new PropertyChangedEventArgs(name));
    }
}

<RowDefinition Height="100" MinHeight="{Binding Path=MinHeightRowOne}"/>

Now add this EventHandler to the SizeChanged-Event of every control in the first row (one handler for each row):

private List<KeyValuePair<string,double>> oldVals = new List<KeyValuePair<string,double>>();

private void ElementInRowOneSizeChanged(object sender, SizeChangedEventArgs e)
{
    FrameworkElement elem = (FrameworkElement)sender;
    MinHeightRowOne -= oldVals.Find(kvp => kvp.Key == elem.Name).Value;
    elem.Measure(new Size(int.MaxValue, int.MaxValue));
    MinHeightRowOne += elem.DesiredSize.Height;
    oldVals.Remove(oldVals.Find(kvp => kvp.Key == elem.Name));
    oldVals.Add(new KeyValuePair<string, double>(elem.Name, elem.DesiredSize.Height));
}

Through this the MinHeight of the Rows get updated everytime the Size of a control changes (which should include expanding or collapsing items).

Note that every control must have an unique name to make this work.