So maybe the problem is in set-window-configuration?
(that is used by tab switching)

But changing the default value of frame-inhibit-implied-resize
to '(tab-bar-lines tool-bar-lines)' fixes it.

> although I would prefer to (at least customizably) see the tab
> bar to appear below the tool bar but that's no great deal.

I have no preference, but in web browsers the tab bar is above the tool bar,
and this makes more sense because the tool bar depends more on the buffer so
it should located closer to the buffer, whereas the tab bar refers to elements
higher in the window/frame hierarchy.