What is this gap, and how do I remove it?

Question

I'm changing the appearance of the tab-bar, and I have the following:

There's a gap present between tabs and between the left edge of the frame, despite setting the face attribute :box to nil on tab-bar-tab and tab-bar. What is this gap, what face does it belong to, and how can I remove it?

Edit: changing the backrgound color of the tab-bar face to match the background color of the tab-bar-tab face 'fixes' the issue in that I can no longer see the gap, but I'd still like to know how to fully remove the gap.

Answer 1

It's a separator. It separates tabs as well as the first tab from the window edge. You can either set tab-bar-separator to an empty string, thus losing the separator between tabs too. Or just remove the first separator with

(advice-add 'tab-bar-format-tabs :around
  (lambda (orig-fun)
    (cdr (funcall orig-fun))))