The word "beta" in superscript is drawn after the tabs have rendered, causing the tabs to shift and flicker.
@matmarex, this is not quite the same phenomenon I observed in T89892. On my setup (Chrome/Ubuntu), the "beta" suffix (both on the tab and the section links) shows up initially in regular font size, and then is replaced with the superscript version. I don't see the edit tab jumping around as in the animation. Probably same root cause, but wanted to point out the difference, and clarify that it applies to the section edit links as well.