When you add a custom column to the issue navigator, if there is a space in your data, then the column breaks across multiple lines.

For example, we have a column to track the name of the person that submitted an issue for use when the user creating the issue is not the user that requested the change. The first and last names are displayed on seperate lines in the issue navigator.

However, I can imagine another instance when you would want the columns to expand to multiple lines, so I'm not sure how this could be handled.