I'd like to see a possibly optional hairline drawn to separate the menubar from the terminal area, at least when the tab bar isn't being shown at the top.

This is a feature that could be provided by the style, evidently, but not all styles have such a feature, and it also wouldn't be appropriate or desirable in all situation.

Reproducible: Always

Steps to Reproduce:
NA

Actual Results:  
See screenshot


Black hairlines is what fresco artists used to improve the perceived contrast between coloured areas in their paintings. You'll see them when looking up close; at the intended viewing distance they become invisible, but they continue to have an effect. I presume the same will be true on current-day ultra-high resolution displays where a 1-pixel wide hairline might become invisible itself between 2 areas of much brighter colour.