Yes I'm not really familiar with Trac but I will explore more to see if it's not something else that "breaks" Colorbox (time to time).
I thought at the begining it was my reset stylesheet but it occurs even without ... I've tested this on Safari and Chrome for mac.
By the way : I've tested this on my local server and I'm using MAMP (1.9).
I'm interresting to know if anyone has already notices this problem.
Thanks.