I tried everything I could to get rid of that flash!    We're going back a long time ago but I recall it having to do with the DOM tree and when it was being loaded within the extension, I believe caching helped with this as you clicked through, but refreshes would cause it and sometimes it would happen without a refresh, as the CSS was loaded/injected. The CSS code itself should work under other extensions like Stylus, if you wanted to try something like that but I could never get the flash fully eliminated from the BPIP extension itself. If you want the direct code, I had posted it here as well.