does it still show up if you try the page with uselang=qqx and then go back to normal?

That indeed fixed it. Don't know how or why...

Thanks to @Ijon for explaining to me that the instruction meant that needed to add "uselang=qqx" to the end of a wikipedia URL, and also to add a "?" before it.

An effect of this bug is that the tab bar jumps around distractingly if the window is below a certain width.

If I have the window set to a width of about 80% of my laptop screen or less (which I usually do as I find it easier to read and enables access to some of the desktop icons and gadgets such as clock and calendar), then the "Read", "Edit <Undefined>" and "Edit source <undefined>" tabs move constantly from side to side in a desperate attempt to display greater width than there is room for. The remaining tabs and the search box jump up and down, and the whole effect is very distracting.

I'm using Firefox and WIndows Vista.

I think there's some RL caching weirdness going on here. It's likely all related to https://gerrit.wikimedia.org/r/#/c/258357/6/modules/ve-mw/init/targets/ve.init.mw.DesktopArticleTarget.init.js - somehow clients are getting the new TabMessages data (which lacks the appendix keys used in the first section) but old ve.init.mw.DesktopArticleTarget.init.js (which wrongly relies on them existing)?

As the cluster's been re-re-rolled back to wmf.10 this probably has gone away at least for the next day or so, but it's likely to be worse when we re-re-re-deploy it.

Change 267551 had a related patch set uploaded (by Alex Monk):
Re-add null appendix tab messages to extension.json

https://gerrit.wikimedia.org/r/267551

Change 267552 had a related patch set uploaded (by Alex Monk):
Re-add null appendix tab messages to extension.json

https://gerrit.wikimedia.org/r/267552

Change 267551 merged by jenkins-bot:
Re-add null appendix tab messages to extension.json

https://gerrit.wikimedia.org/r/267551

Change 267552 merged by jenkins-bot:
Re-add null appendix tab messages to extension.json

https://gerrit.wikimedia.org/r/267552

Krinkle, what do you think about the RL caching aspect of this?

Based on chatting with Krinkle on IRC, we should be able to revert the patch and deploy, and then touch ve.init.mw.DesktopArticleTarget.init.js and sync that.

Change 269457 had a related patch set uploaded (by Alex Monk):
Revert "Re-add null appendix tab messages to extension.json"

https://gerrit.wikimedia.org/r/269457

Change 269457 merged by jenkins-bot:
Revert "Re-add null appendix tab messages to extension.json"

https://gerrit.wikimedia.org/r/269457

Change 271172 had a related patch set uploaded (by Alex Monk):
Revert "Re-add null appendix tab messages to extension.json"

https://gerrit.wikimedia.org/r/271172

Change 271173 had a related patch set uploaded (by Alex Monk):
Revert "Re-add null appendix tab messages to extension.json"

https://gerrit.wikimedia.org/r/271173

Change 271172 merged by jenkins-bot:
Revert "Re-add null appendix tab messages to extension.json"

https://gerrit.wikimedia.org/r/271172

Change 271173 merged by jenkins-bot:
Revert "Re-add null appendix tab messages to extension.json"

https://gerrit.wikimedia.org/r/271173

Mentioned in SAL [2016-02-17T01:36:11Z] <krenair@tin> Synchronized php-1.27.0-wmf.13/extensions/VisualEditor/modules/ve-mw/init/targets/ve.init.mw.DesktopArticleTarget.init.js: touch - https://phabricator.wikimedia.org/T125249#2012068 (duration: 01m 31s)