Apparently GMail leaks a pretty hefty chunk of web worker JS in its new design when closing the tab. This becomes detached and eventually ghost code that the CC will try to clean up. The same goes for ghost windows/documents of other websites that do not clean up properly.
This is potentially disastrous for performance if the CC “overworks” itself trying to clean up ghost windows that are likely going to be leaked forever anyway. Removing these leaked objects from the CC graph will reduce CC pressure considerably; we may want to re-include these objects once in a rare CC run afterwards to see if the ghost objects have been released yet or not. Say, one in 100 or so.