mAncestorIsLoading is used to track if some ancestor is loading and if so, window's setTimeout/Interval will be deferred (this leads to better page load performance).
To make this all work in Fission, we'll need to probably track the state in BrowsingContext to handle multiple processes.

Tentatively moving all bugs whose summaries mention "Fission" (or other Fission-related keywords) but are not assigned to a Fission Milestone to the "?" triage milestone.

This will generate a lot of bugmail, so you can filter your bugmail for the following UUID and delete them en masse:

0ee3c76a-bc79-4eb2-8d12-05dc0b68e732

I initially said this should probably go in WindowContext rather than BrowsingContext, but thinking about it again, a document should probably set the flag on all of its child BrowsingContexts when its load state or an ancestor's load state changes, since the state of the BC's ancestors won't change depending on the document that's currently loaded in the BC.

Tracking for Fission Nightly (M6)

blocks fission perf

Tracking for Fission Beta (M7) because this perf bug doesn't need to block Fission Nightly.

Haven't we been tracking this in the BrowsingContext since bug 1596784?

Is there any more work we need to do here?

bug 1596784 should have been enough