When using VisualEditor in Vector, I've been irritated by the strong border of the selected Vector tabs and VE's toolbar, which is counter-intuitive to “what's currently selected” connecting white space.
The blue separators were introduced in https://gerrit.wikimedia.org/r/#/c/210314/, my understanding is, the reason behind the change were the horizontal toolbar separators in VE toolbar, which would end in nil.
Before: