It looks to me like the hidden window is created on startup, and lasts until shutdown. Thus we should not traverse it. This contains things (looks like maybe 500 DOM nodes?) and is traversed on OSX. It contains things on Windows, even though it is a different type of document, but I don't know if it is really traversed there. Hopefully this is doable by modifying nsCCUncollectableMarker. I'm not sure why it isn't caught by that already.
Created attachment 576929 [details] [diff] [review]
Does the patch help+
Comment on attachment 576929 [details] [diff] [review]
This prevents the hidden window from being traversed, thanks!
Pushed to try: https://tbpl.mozilla.org/?tree=Try&rev=d1c5a9775853
It looked like that all documents were being traversed in the CC before the shutdown CC, which is odd, but I can't imagine this patch is at fault. I'll have to keep an eye on that.