QToolBar has incorrect layout if adding an expanding widget at the front

The steps will make QToolBar incorrect layout occur.

1. Create QWidget with SizePolicy-expanding
2. Add the widget to QToolBarra
3. Drag the main window to make it narrow, so toolbar's extension button raise
4. Click the extension button, hover on the background of the toolbar
5. The tool-button and extension button are crowded together (see attachment)