I've included a sample project below. What i would like to have is the "main" grid to be stretched to take up all the space it has. But even if every grid has HorizontalAlignment set to "Stretch" it still doesn't work. If you run the sample you can see the light blue color not being stretched. If you hardcode the width value however it works (but that is not an option).

Hi, 

So, you're not really going to be able to control this through a style. The width of each node is handled by the NodesPanel of the xamDataTree. 

I'm not 100% sure what you're looking for, but i guess you could potentially create your own NodesPanel that derives from outs and override the RenderNode method, and set the width of the node and re-measure it. 

Note: this would really be the only way to handle this, b/c we don't expose a way to override the Arranging of a node, without rewriting the entire arrange logic, which i wouldn't recommend doing:

public class CustomNodesPanel : NodesPanel

    {

        protected override Size RenderNode(Infragistics.Controls.Menus.XamDataTreeNode node, double availableWidth)

        {

            base.RenderNode(node, availableWidth);

            if (node.Control != null)

            {

                if(!double.IsPositiveInfinity(availableWidth))

                    node.Control.Width = availableWidth;

                node.Control.Measure(new Size(availableWidth, double.PositiveInfinity));

            }

            return node.Control.DesiredSize;

        }

    }

Then you can change the XamDataTree ControlTemplate to use your CustomNodesPanel

Here is an example of the xaml, and a modified version of your NodeTemplates