Since very old symfony2 versions this issue still relevant, at least for me, I show it in two pictures:

1. What I currently see very often:
   
2. How it should look like:
   
   The only change required is adding width (for example min-width: 500px;) for CSS selector pre.sf-dump. Or make it configurable and use configurated option in dynamic styles part (<style> tag) at the end of the page, which is already starts with pre.sf-dump: