make div under div same width as parent even if its children larger its parent

Question

https://jsfiddle.net/h7j1wz9m/

I want this:

to behave like this:

The challenge I am facing is how to set the width of div#listWrapper in https://jsfiddle.net/h7j1wz9m/ so that it has the same width as div#right and does not grow with its contents.