Strange problem about OpenGL drawing in a QGraphicsView.

OpenGL rendering is done by reimplementing QGraphicsScene's drawBackground() function. I also add two QGraphicsRectItem in the QGraphicsScene, one embeded a dialog with four buttons in it; another is a yellow Solid circle.
when the opengl codes are as follows included in drawBackground() of QGraphicsScene,

Code:

---

MTList=glGenLists(1);
 
 	glEnableClientState(GL_VERTEX_ARRAY);
	Vertex_iterator vi = model.vertices_begin();
	glVertexPointer(3, GL_DOUBLE, sizeof(Vertex), &(vi->point()[0]));
 
    glDisable(GL_LIGHTING);
	glNewList(MTList, GL_COMPILE);
  	glColor3f(1.0,0.0,1.0);
 
	//glLineWidth(10.0);
	//glDisable(GL_CULL_FACE);
	//glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
 
  	glDrawElements(GL_POINTS, 3*model.size_of_facets(), GL_UNSIGNED_INT, &modelfaces[0]);
 	glEndList();
 
	glDisableClientState(GL_VERTEX_ARRAY);

---

at this time, the two items in QGraphicsScene display as expected:
Attachment 7920

However, when I uncomment //glPolygonMode(GL_FRONT_AND_BACK, GL_LINE):

Code:

---

glColor3f(1.0,0.0,1.0);
 
	//glLineWidth(10.0);
	//glDisable(GL_CULL_FACE);
	glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
 
  	glDrawElements(GL_POINTS, 3*model.size_of_facets(), GL_UNSIGNED_INT, &modelfaces[0]);

---

(I know I should also change GL_TRIAGNLES, however the result is the same.) then the two items as distorted like two slashes:
Attachment 7921

Why this weird display occur ?

Thanks a lot

I revised the line to : glPolygonMode(GL_BACK, GL_LINE); the problem still exists,

HHHOoooWWwwever, glPolygonMode(GL_FRONT, GL_LINE); sloved the problem and the two items come out to be as expected.

Why?