Hi, I have tried a few hack but I cant find the solution.
The widget width change when he start scrolling;
So I have added a width of 225px to the class of the shortcode who is in the widget :
Uncheck it to see the problem. The problem come with firefox only..
You can try it on this page : http://wp-infinity.com, the fixed widget start from middle of page.
The problem with my solution is that the widget is no more responsive, and over all on that page for exemple the widget should be larger : http://wp-infinity.com/themes-wordpress-magazine/
I also added a class to the widget :
Thank you if you can help, and sorry for my english. ;-)