These tests are marked as skipped/failed for cross-origin and Fission, and need to be fixed
layout/base/tests/test_bug603550.html
layout/base/tests/test_bug842853-2.html
layout/base/tests/test_bug842853.html
layout/base/tests/test_bug849219.html
layout/base/tests/test_bug851445.html
layout/base/tests/test_bug851485.html
layout/generic/test/test_scroll_position_iframe.html
layout/base/tests/test_frame_reconstruction_scroll_restore.html

https://docs.google.com/spreadsheets/d/16G5AZhHWWow3rBgim4QBHzWXMIIJiky2SzXYgDMTTKY/edit#gid=1354562828&range=59:69

More info: https://wiki.mozilla.org/Project_Fission/Enabling_Tests_with_Fission#Cross-Origin_Mochitests

Mats: Can you start taking a look at some of these?

I debugged layout/base/tests/test_bug842853-2.html a bit and it seems the root cause is that scroll position restoration isn't working for the embedded <iframe> when running with --enable-xorigin-tests --enable-fission.
Here's what I get with MOZ_LOG=scrollrestore:5 normally:

0:17.96 TEST_START: layout/base/tests/test_bug842853-2.html
0:18.97 GECKO(5157) [Child 5250: Main Thread]: D/scrollrestore 0x7f2e125fc260: Clearing mRestorePos (cur=0, dst=0)
0:21.31 GECKO(5157) [Child 5250: Main Thread]: D/scrollrestore 0x7f2e10577260: Clearing mRestorePos (cur=0, dst=301640)
0:21.31 GECKO(5157) [Child 5250: Main Thread]: D/scrollrestore 0x7f2e10577260: Clearing mRestorePos (cur=294280, dst=30000)
0:21.86 GECKO(5157) [Child 5250: Main Thread]: D/scrollrestore 0x7f2e10577260: SaveState, pt.y=30000, mLastPos.y=-1, mRestorePos.y=-1
0:22.56 GECKO(5157) [Child 5250: Main Thread]: D/scrollrestore 0x7f2e1257c260: RestoreState, set mRestorePos.y=30000 mLastPos.y=0
0:22.57 GECKO(5157) [Child 5250: Main Thread]: D/scrollrestore 0x7f2e1257c260: SaveState, pt.y=0, mLastPos.y=0, mRestorePos.y=30000
0:22.57 GECKO(5157) [Child 5250: Main Thread]: D/scrollrestore 0x7f2e1257c260: ScrollToRestoredPosition (mRestorePos.y=30000, mLastPos.y=0, layoutRestorePos.y=30000, visualRestorePos.y=30000, logicalLayoutScrollPos.y=0, GetLogicalVisualViewportOffset().y=0)
0:22.57 GECKO(5157) [Child 5250: Main Thread]: D/scrollrestore 0x7f2e1257c260: ScrollToRestoredPosition (mRestorePos.y=30000, mLastPos.y=30000, layoutRestorePos.y=30000, visualRestorePos.y=30000, logicalLayoutScrollPos.y=30000, GetLogicalVisualViewportOffset().y=30000)
0:22.58 PASS undefined assertion name

and here's what I get with --enable-xorigin-tests --enable-fission:

0:21.00 TEST_START: http://mochi.test:8888/tests/layout/base/tests/test_bug842853-2.html
0:22.02 GECKO(5508) [Child 5699: Main Thread]: D/scrollrestore 0x7f00b8ee8260: Clearing mRestorePos (cur=0, dst=0)
0:23.88 GECKO(5508) ### XPCOM_MEM_BLOAT_LOG defined -- logging bloat/leaks to /tmp/tmpTJYkgG.mozrunner/runtests_leaks_tab_pid5823.log
0:28.65 GECKO(5508) [Child 5748: Main Thread]: D/scrollrestore 0x7faf6a54c260: Clearing mRestorePos (cur=0, dst=301640)
0:28.65 GECKO(5508) [Child 5748: Main Thread]: D/scrollrestore 0x7faf6a54c260: Clearing mRestorePos (cur=294280, dst=30000)
0:30.51 GECKO(5508) [Child 5748: Main Thread]: D/scrollrestore 0x7faf6a56e260: Clearing mRestorePos (cur=0, dst=301640)
0:30.51 GECKO(5508) [Child 5748: Main Thread]: D/scrollrestore 0x7faf6a56e260: Clearing mRestorePos (cur=294280, dst=301640)
0:30.51 PASS undefined assertion name - got 4904.66650390625, expected 500

(it says PASS b/c I didn't update mochitest.ini -- it's really a FAIL as you can see from the numbers)

It appears that ScrollFrameHelper::SaveState/RestoreState isn't called at all with fission...
Is that a known bug?

I am supposing that it's due to the lack of proper handling OOP iframes in session restore (bug 1572084).

Thanks hiro, I'll move this to DOM/Navigation for now then. Let us know if there's anything Layout-related that needs fixing here.

Are all of these tests related to scroll position restoration? (A quick glance through a couple of them makes it seem so.) If so, then I think we should update the bug title accordingly.

Yes, I think all of them are, except the first one, but that one already PASS.

Do we expect scroll restoration to work with ship now? If we do, this bug may be worth revisiting now.

The remaining Layout tests that are still skipped for Fission are:
layout/base/tests/test_bug603550.html (Bug 1677543) --> Fission-Future
layout/base/tests/test_bug607529.html (Bug 1689677) --> Depends on BFCache for Fission and tracked with fission-bfcache meta bug
layout/base/tests/test_zoom_restore_bfcache.html (Bug 1654427) --> Depends on BFCache for Fission and tracked with fission-bfcache meta bug

Closing this meta as the rest are fixed.