MacOS 10.12.6 (16G1212)
Chrome Version 64.0.3282.186 (Official Build) (64-bit)
Also reproducible with Safari Version 11.0.3 (12604.5.6.1.1)
When the window isn't wide enough to fit all the tabs, some of the tabs get merged in a weird way. See the left pane in the attached screenshot. The right pane is exactly the same page, but with the window stretched wider so all the tabs fit.
When I check "Temporarily disable the visual editor while it is in beta" under Special:Preferences#mw-prefsection-editing, I can't reproduce the problem.