Why am I getting weird line breaks in my sidebar nav outline?

I just noticed something. In some of my courses, there are some outline listings show up with line breaks (only not very elegantly because there's a horizontal line between the lines) and others not, with no seeming logic. 

I am familiar with having to edit the titles in the Slide Properties dialog but here I see some lines wrapping even though they could clearly fit without it, and others that are not wrapping even though they are longer. 

How can I make those shorter lines not wrap?

Can anyone offer any help with this?

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