Fix child object x,y set to 0,0 when dragged out of parent onto diagram

* If a child object is dragged out of its parent into a new parent we want to ensure a minimum x,y of 0,0
* But we don't want to do this if the parent is the diagram itself
* This is because a diagram cna have negative space

com.archimatetool.editor/src/com/archimatetool/editor/diagram/policies/DiagramLayoutPolicy.java

@@ -21,6 +21,7 @@
 import com.archimatetool.editor.diagram.commands.SetConstraintObjectCommand;
 import com.archimatetool.editor.diagram.editparts.IConstrainedSizeEditPart;
 import com.archimatetool.editor.diagram.editparts.INonResizableEditPart;
+import com.archimatetool.model.IDiagramModel;
 import com.archimatetool.model.IDiagramModelContainer;
 import com.archimatetool.model.IDiagramModelObject;
 import com.archimatetool.model.ILockable;
@@ -111,13 +112,17 @@ protected AddObjectCommand createAddCommand(ChangeBoundsRequest request, EditPar
         IDiagramModelContainer parent = (IDiagramModelContainer)getHost().getModel();
         IDiagramModelObject child = (IDiagramModelObject)childEditPart.getModel();
 
-        // Keep within box
         Rectangle bounds = (Rectangle)constraint;
-        if(bounds.x < 0) {
-            bounds.x = 0;
-        }
-        if(bounds.y < 0) {
-            bounds.y = 0;
+
+        // Keep within the parent box
+        // Fixed 2019-06-11 to check that the parent is not a diagram model (which can have negative space)
+        if(!(parent instanceof IDiagramModel)) {
+            if(bounds.x < 0) {
+                bounds.x = 0;
+            }
+            if(bounds.y < 0) {
+                bounds.y = 0;
+            }
         }
 
         return new AddObjectCommand(parent, child, bounds);