Filed by: mkmelin [at] iki.fi
Parsed log: https://treeherder.mozilla.org/logviewer?job_id=378602438&repo=comm-central
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/TR5VV2A1Q5eOqrS5M7ZeiQ/runs/0/artifacts/public/logs/live_backing.log

Mostly on debug, but apparently also windows shippable

Not sure that's all of it, but the more button width calculation won't really work since Fluent translation of the label hasn't happened yet...
https://searchfox.org/comm-central/rev/b7cc2508b3fa414260984118c8c66c21edd9ee3b/mail/base/content/widgets/header-fields.js#276

It's possible we should just give up on that whole more button and let layout handle it with an ellipsis. xref bug 77806

This bug is reproducible locally.

Great, fixed on Windows but now it fails on Linux: https://treeherder.mozilla.org/jobs?repo=try-comm-central&revision=1efec0930fd15cf46cad3c5f5c036d81c8c0a411

But the screenshot shows the correct UI: https://firefoxci.taskcluster-artifacts.net/RKrMKfMGQnuLFqQ18xpuYQ/0/public/test_info/mozilla-test-fail-screenshot_lgt7160y.png

Probably a timing issue.
I'll give it another try.

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/5b6993c290a5
Fix intermittent windows tests for browser_messageHeader.js. r=darktrojan

This seems to be back: https://treeherder.mozilla.org/logviewer?job_id=379407566&repo=comm-central&lineNumber=2453
It went away only for the revision in which this patch landed...amazing.