parallax "glitch" in combination with bouncing

Hi there,

I have a small fix for the parallax in combination with bouncing for you. If parallax is enabled and the left/right side closes with bouncing or even when you drag the left/right side into the bouncing area, the mainView will become visible a bit and this does not look really nice. (even when the mainView's backgroundColor is set to black).

// Everything ok
![open](https://f.cloud.github.com/assets/1698387/250602/08ccf51c-8b4f-11e2-9264-d86602cb63f5.png)

// on the right side there is a white area (the mainWindow) - difficult to see on a white background … take a look at area of the status bar ;-)
![open-bounce](https://f.cloud.github.com/assets/1698387/250601/08ae01e8-8b4f-11e2-82f4-935b94153ad1.png)