An iterator over browsing contexts, returning all descendant
contexts in depth-first order. Note that this iterator returns all
contexts, not just the fully active ones.
The constellation’s view of a browsing context.
Each browsing context has a session history, caused by navigation and
traversing the history. Each browsing context has its current entry, plus
past and future entries. The past is sorted chronologically, the future is
sorted reverse chronologically: in particular prev.pop() is the latest
past entry, and next.pop() is the earliest future entry.
Because a browsing context is only constructed once the document that’s
going to be in it becomes active (i.e. not when a pipeline is spawned), some
values needed in browsing context are not easily available at the point of
constructing it. Thus, every time a pipeline is created for a browsing
context which doesn’t exist yet, these values needed for the new browsing
context are stored here so that they may be available later.