Main GUI thread serialises QOpenGLWidget::frameSwapped per VSync for multiple application windows

On an application with multiple Windows containing QOpenGLWidgets (for example, in the sample 'threadedqopenglwidget' project provided in Qt/Examples/Qt-5.4/opengl/threadedqopenglwidget/), it seems that the signal 'QOpenGLWidget::frameSwapped' is emitted on a per-window serialised VSync mechanism. E.g. in the scenario where the display frequency is at 60Hz:

Main app -> One window (two widgets each) - widgets update at 60Hz. QOpenGLWidget::frameSwapped is sent to Window 1's widgets.
Main app -> Two windows (two widgets each) - widgets update at 30Hz. QOpenGLWidget::frameSwapped is sent to Window 1's widgets. Vsync occurs. QOpenGLWidget::frameSwapped is sent to Window 2's widgets.
Main app -> Three windows (two widgets each) - widgets update at 15Hz. ...and so on

This means that an app cannot have multiple windows updating on the same VSync using this mechanism.
It looks like the main GUI thread is only emitting a QOpenGLWidget::frameSwapped per-window, per-VSync, not to all child windows per-VSync, causing them to visually stutter.

By running the 'threadedqopenglwidget' sample this can be reproduced by noticing that the OpenGL widgets update rate increases as each window is closed.