You mean like the preview space is shorter than the actual thread space.
Well small things like that can be tested in a actual thread. Find a post. Cause the thing is that there always has to be that side bar in the usercp
You could always think about small things like that.
Increasing the size of the preview box there would also lead to the problem that the page would look out of sync (the theme would be ruined)
There are also simulations like posts where the total length of your posts (post+sig height) was lesser than the info box on the left and when it is equal to or greater. The sig used to (i dont think it still does) behave a bit differently (atleast i think so).
So you cant test everything
But these are all rare cases which happen to a every specific size of sig. So for the minority, i just guess they will have to correct it as and when the notice it.
I for one find my post asap when i add a new sig code. I also tell users if their avatar, signature and/or their under avatar caption may have a problem.
So you see if there is a problem then you'll find out sooner rather than later
/ 
