dom/base/test/test_intersectionobservers.html fails in fission world. A problem I can see is to obtain the root scroll frame for the presshell of the document. In fission world, PresShell::GetRootScrollFrame returns an nsIFrame but it's not what we want there. We, instead, check whether the given |aDocument| is cross-origin or not there.

Hsin-Yi, is there anybody wants to work on this in your team? I believe this can be fixed without any layout knowledge, so this would be a good first bug for a person who want to jump in fission world. If nobody there, I can of course take this.

One caveat is that I haven't read the IntersectionObserver spec carefully, so there might be other works we should do for fission.

Hi :hiro, thank you for filing this bug. My team members are currently working on other fission mochitest failures that we probably won't get to this before the investigation/fix on others is done.

I will take this. I realized there is another issue in fission world, that is that we walk up prescontext tree to find the implicit root but it fails if there is a cross-origin document in between the top level document and same-origin descendant documents in fission world. So, a better way I can think of is to try to obtain the top level document via BrowsingContext::Top() and if we can't get the top level document, we can consider it's a cross-origin document in fission world.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=de1ead54305bdc3ddcc878e25a43c02225964f3f

So that we can avoid 1) obtaining a wrong root in cross-process documents
and also avoid 2) not being able to obtain the proper root in same-origin
documents in the case where there is a cross-process document in between
the top level document and the same-origin documents.

dom/base/test/test_intersectionobservers.html is a test of case 1).
testing/web-platform/tests/intersection-observer/same-origin-grand-child-iframe.sub.html
is a test case of case 2).

https://hg.mozilla.org/mozilla-central/rev/676ba571526a

Retroactively moving fixed bugs whose summaries mention "Fission" (or other Fission-related keywords) but are not assigned to a Fission Milestone to an appropriate Fission 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