(In reply to Jared Wein [:jaws] (please needinfo? me) from comment #2)
Can you please test again? I just tested with latest m-c build and in both the overflow and underflow states the bottom edge of the New Tab button is equal to the bottom edge of the selected tab.
I still see it. I suspect it's a problem with rounding, I'm on Windows 10 at 125% DPI.
For both #2 and #3, the tab hover color needs to be a different shade than the button hover color, otherwise the hover color of the close button on top of the selected tab would need to be different.
I understand that, I think my concern is mostly that there's too many grays and differ just a little bit, causing a patchwork effect. This is less important than the misalignment anyway.