Filed by: cbrindusan [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer.html#?job_id=288308657&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/ay8Ln_0WSwKtxh7RC60Maw/runs/0/artifacts/public/logs/live_backing.log

[task 2020-02-11T05:11:02.421Z] 05:11:02 INFO - TEST-START | /intersection-observer/nested-cross-origin-iframe.sub.html
[task 2020-02-11T05:11:02.424Z] 05:11:02 INFO - Closing window 8589934658
[task 2020-02-11T05:11:02.753Z] 05:11:02 INFO - PID 12784 | [Child 12990, Main Thread] WARNING: Trying to request nsIHttpChannel from DocumentChannel, this is likely broken: file /builds/worker/workspace/build/src/netwerk/ipc/DocumentChannel.cpp, line 64
[task 2020-02-11T05:11:03.081Z] 05:11:03 INFO - PID 12784 | ### XPCOM_MEM_BLOAT_LOG defined -- logging bloat/leaks to /tmp/tmptnHljU.mozrunner/runtests_leaks_1023_tab_pid13132.log
[task 2020-02-11T05:11:03.158Z] 05:11:03 INFO - PID 12784 | [Child 12990, Main Thread] WARNING: Trying to request nsIHttpChannel from DocumentChannel, this is likely broken: file /builds/worker/workspace/build/src/netwerk/ipc/DocumentChannel.cpp, line 64
[task 2020-02-11T05:11:03.158Z] 05:11:03 INFO - PID 12784 | [Child 12990, Main Thread] WARNING: Trying to request nsIHttpChannel from DocumentChannel, this is likely broken: file /builds/worker/workspace/build/src/netwerk/ipc/DocumentChannel.cpp, line 64
[task 2020-02-11T05:11:03.159Z] 05:11:03 INFO - PID 12784 | [Child 12990, Main Thread] WARNING: NS_ENSURE_TRUE(request) failed: file /builds/worker/workspace/build/src/netwerk/base/nsLoadGroup.cpp, line 584
[task 2020-02-11T05:11:03.236Z] 05:11:03 INFO - PID 12784 | [Child 13132, Main Thread] WARNING: No CID found when attempting to map contract ID: file /builds/worker/workspace/build/src/xpcom/components/nsComponentManager.cpp, line 721
[task 2020-02-11T05:11:03.426Z] 05:11:03 INFO - PID 12784 | Couldn't convert chrome URL: chrome://branding/locale/brand.properties
[task 2020-02-11T05:11:03.629Z] 05:11:03 INFO - PID 12784 | [Child 13132, Main Thread] WARNING: could not set real-time limit at process startup: file /builds/worker/workspace/build/src/dom/ipc/ContentChild.cpp, line 1823
[task 2020-02-11T05:11:03.660Z] 05:11:03 INFO - PID 12784 | [Child 13132, Main Thread] WARNING: NS_ENSURE_TRUE(mPresShell) failed: file /builds/worker/workspace/build/src/layout/base/nsPresContext.cpp, line 847
[task 2020-02-11T05:11:03.800Z] 05:11:03 INFO - PID 12784 | [Child 13132, Main Thread] WARNING: '!topWindow', file /builds/worker/workspace/build/src/dom/html/HTMLIFrameElement.cpp, line 246
[task 2020-02-11T05:11:03.802Z] 05:11:03 INFO - PID 12784 | [Child 13132, Main Thread] WARNING: '!topWindow', file /builds/worker/workspace/build/src/dom/html/HTMLIFrameElement.cpp, line 246
[task 2020-02-11T05:11:03.863Z] 05:11:03 INFO - PID 12784 | [2020-02-11T05:11:03Z WARN rkv::backend::impl_safe::environment] Ignoring map_size=16777216
[task 2020-02-11T05:11:03.946Z] 05:11:03 INFO - PID 12784 | ### XPCOM_MEM_BLOAT_LOG defined -- logging bloat/leaks to /tmp/tmptnHljU.mozrunner/runtests_leaks_1023_tab_pid13160.log
[task 2020-02-11T05:11:03.974Z] 05:11:03 INFO - PID 12784 | [Child 13132, Main Thread] WARNING: Trying to request nsIHttpChannel from DocumentChannel, this is likely broken: file /builds/worker/workspace/build/src/netwerk/ipc/DocumentChannel.cpp, line 64
[task 2020-02-11T05:11:03.976Z] 05:11:03 INFO - PID 12784 | [Child 13132, Main Thread] WARNING: Trying to request nsIHttpChannel from DocumentChannel, this is likely broken: file /builds/worker/workspace/build/src/netwerk/ipc/DocumentChannel.cpp, line 64
[task 2020-02-11T05:11:03.977Z] 05:11:03 INFO - PID 12784 | [Child 13132, Main Thread] WARNING: NS_ENSURE_TRUE(request) failed: file /builds/worker/workspace/build/src/netwerk/base/nsLoadGroup.cpp, line 584
[task 2020-02-11T05:11:04.040Z] 05:11:04 INFO - PID 12784 | [Child 13160, Main Thread] WARNING: No CID found when attempting to map contract ID: file /builds/worker/workspace/build/src/xpcom/components/nsComponentManager.cpp, line 721
[task 2020-02-11T05:11:04.230Z] 05:11:04 INFO - PID 12784 | Couldn't convert chrome URL: chrome://branding/locale/brand.properties
[task 2020-02-11T05:11:04.439Z] 05:11:04 INFO - PID 12784 | [Child 13160, Main Thread] WARNING: could not set real-time limit at process startup: file /builds/worker/workspace/build/src/dom/ipc/ContentChild.cpp, line 1823
[task 2020-02-11T05:11:04.497Z] 05:11:04 INFO - PID 12784 | [Child 13160, Main Thread] WARNING: NS_ENSURE_TRUE(mPresShell) failed: file /builds/worker/workspace/build/src/layout/base/nsPresContext.cpp, line 847
[task 2020-02-11T05:11:04.805Z] 05:11:04 INFO -
[task 2020-02-11T05:11:04.805Z] 05:11:04 INFO - TEST-UNEXPECTED-FAIL | /intersection-observer/nested-cross-origin-iframe.sub.html | IntersectionObserver with implicit root in a nested cross-origin iframe works - assert_false: The target element is not intersecting in all ancestor viewports expected false got true

This started with landing of https://treeherder.mozilla.org/#/jobs?repo=autoland&selectedJob=288322877&resultStatus=testfailed%2Cbusted%2Cexception&revision=b8ad0267e88abff0a54a6f0f1b5de2ad3a018460 and it's tier 2 perma now.
:hiro can you take a look?

It's not clipped by scroll viewports at all.

https://hg.mozilla.org/mozilla-central/rev/73e691abcdb5