The edit tabs now have the word <undefined> (including the chevrons) next to them. see image.
This appeared when logged-in on Chrome, but NOT on Safari (logged in, or logged out).
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)?
This is not fixed, but it should not appear in production when it rolls out in the next few hours/days. Leaving as 'stalled' for us to re-visit in a few weeks' time when the train isn't such a mess.
I believe everything is done here, hopefully. Please reopen it if you see more issues with <undefined> showing up like this. Theoretically it shouldn't happen, but then it shouldn't've happened before, so...