Moving a QMainWindow by dragging the Toolbar moves window over wrong distance

When moving a QMainWindow by dragging an empty portion (e.g. the space between two buttons) the window is moved by a multiple of the distance the mouse has moved (at first look I'ld say the difference is exponential)

The problem only occurs when dragging the toolbar, not the titlebar.

• create a QToolBar and add a button, add an empty qwidget with its horizontal size policy set to expanding and add another button. I had the toolbar set to only show icons (24x24) but that probably doesn't matter.
• Add the toolbar to the QMainWindow (top location, not movable by user). When the window is big enough one of the buttons should stick to the left margin while the other sticks to the right.
• Moving the window by dragging the space between the buttons results in the described behavior.