That's because you've added the following to your main stylesheet:
You'll notice that if you resize the popup window the text will display fine.
margin: 0 200px;
While your 200px margin might work for your regular pages, it won't suit your popups because the popups don't need that massive left/right margin you're imposing on your users.
You could use a different method of forcing those margins on your users, or you could add specific margin rules for those popup boxes that require something smaller like 5px.