Layout stuttering on resize when QOpenGLWidget insde

I'm doing the following.
I make a QWidget , set it as the central widget.
To this QWidget I add a QVBoxLayout. And in this layout I add a QPushButton and a QOpenGLWidget. Then I resize vertically, and the visual effect is a quite heavy stuttering of the layout.
In fact, when i resize down, the window-bar (which should stay the same) is increasing and decreasing in height, and creating this stutter of the whole layout. (I'm using ubutnu 14.10).
On the other had, if I set the QOpenGLWidget directly as the central widget, resizing is smooth/normal.
(The QOpenGLWidget is empty/black , that's why I'm a little surprized about the stuttering).