Code

e.Node.HandlesStyle = HandlesStyle.SquareHandles;
int handle = 8;
e.Node.HitTest(e.MousePosition, ref handle);
e.Node.HandlesStyle = HandlesStyle.Custom;

// additional custom testing
// ...

e.HitResult = handle; 

Code

private void diagram_HitTestAdjustmentHandles(object sender, HitTestEventArgs e)
{
    int handle = 8;

    DiagramNode node = (DiagramNode)e.Item;
    node.HandlesStyle = HandlesStyle.SquareHandles;
    node.HitTestHandle(e.MousePosition, ref handle);
    node.HandlesStyle = HandlesStyle.Custom;

    if (handle > 0 && handle <= 8)
    {
        // a resize or move handle was hit
        e.HitResult = handle;
        return;
    }

    if (node.ContainsPoint(e.MousePosition))
        e.HitResult = 8;
}

private void diagram_DrawAdjustmentHandles(object sender, MindFusion.Diagramming.DrawItemEventArgs e)
{
    DiagramNode node = (DiagramNode)e.Item;
    InternalUtils.DrawSelHandles(
        e.Graphics,
        diagram.SelectedItemHandlesStyle,
        diagram.DisabledHandlesStyle,
        node.Bounds, 0,
        node.EnabledHandles,
        false,
        HandlesStyle.SquareHandles2,
        diagram.AdjustmentHandlesSize);
}