Description:
The tab-bar height is wrong (bigger than expected) at startup. It only
happens when tab-bar is enabled in init.el. After emacs is loaded, the
tab-bar height can be "fixed" by toggling the tab-bar.

A 'git bisect' indicates that the commit
52d4c98cec0901ef5cc1c55d5b3b33ac9d9c519f introduced this bug.
 
Steps:
1) Use the following in init.el:
(tab-bar-mode 1)
2) Run emacs
   - tab-bar is shown, notice the height
3) M-x tab-bar-mode RET (hide tab-bar)
4) M-x tab-bar-mode RET (show tab-bar)
   - tab-bar height is smaller than it was at startup

The video below demonstrates the issue: