I can consistently reproduce this in latest stable Chrome, using the link https://vi.wikipedia.org/wiki/User:Krinkle?uselang=en. It happens both logged-in, and logged-out as well as in safemode (disables site scripts/styles).
The initial rendering shows the tabs are too wide to fit on one line, and just as before and as it has for years, this means it renders on the next line initially.
Then, the vector.js code is applied, which is meant to move some of the tabs to the "More" menu, and animate the width as it goes.
What actually happens is that this animation occurs correctly, but then it animates again in the opposite direction (undoing what it just did), and then it animates again for a third time, to re-apply the correction.
This whole show takes about two seconds during page load.