examples/qtbase/graphicsview/boxes demo doesn't work properly

The QGraphicsProxyWidget in the boxes demo behaves very strangely on Windows:

• To start with, it cannot be moved or resized.
• If you open a colour dialog, the dialog doesn't react to user input and can only be closed by pressing escape. This also happens on Linux.
• Immediately after closing the colour dialog, the mouse cursor doesn't change to the "resize" cursor when moving over the edge of the proxy widget.
• Immediately after closing the colour dialog you can move and resize the proxy widget once, and afterwards, the mouse cursor goes back to working normally, showing a "resize" cursor on the edge of the proxy widget.

The embedded dialogs example works fine, so maybe the bugs have something to do with QGraphicsProxyWidget on OpenGL?