QOpenGLWidget on Mac: update is automatically called?

[floatingWoods]

Hello,

I have an openGl application that runs on Windows, Linux and Mac. Until recently, I used QGLWidget, but since I upgraded to Qt 5.5, I am now using QOpenGLWidget.
Before, I had to swap buffers with QGLWidget::swapBuffers()
Now, I swap buffers (or update the image after rendering was done) with QOpenGLWidget::update()

This works fine on Windows and Linux, but on Mac (Yosemite), the rendering appears flickering, and incomplete. It is like update() is called automatically, also while a rendering is underway. Imagine, you want to display in every frame 3 cubes. So I do:

render cube 1 ( glBegin(GL_QUADS), etc..., glEnd() )
render cube 2
render cube 3
update()

Above works as expected on Linux and Windows, but on Mac, sometimes I have only one cube displayed, othertimes, 2 or 3 (or none). And the colors are very bright (as if the color buffer accumulates 2-3 frames)

This makes me think that internally, a swapBuffers() is called while I am still rendering (e.g. between cube 2 and cube 3).
If I explicitely set the swap behaviour of the surface format to double buffer (i.e. surface.setSwapBehavior(QSurfaceFormat::DoubleBuffer) ), then the colors look ok, but the missing cubes problem remains (i.e. incomplete rendering display).

Does anyone have a clue what is going on?

If I revert back to using QGLWidget on Qt5.5, everything looks normal.

Thanks for any insight

[floatingWoods]

I have a small update on this:

If I call glFinish() after my rendering is done, then I can make it work right (given that I specified double-buffering, otherwise the colors appear way too bright).
The downside is that with glFinish(), the rendering takes 30% longer..