On 16 Sep 2019, at 3:28, jason wrote:

I also really preferred this design (tabs only over active document), but
if I recall correctly, Allan said this was too hard to do after one of the
macOS AppKit upgrades.

Correct, it was Dark Mode support. The title bar has different colors/gradient based on OS release, dark/light mode, and what’s behind it inside the window, so if we want the active tab to bleed into the title bar, we need to have the tabs be part of the titlebar, as it’s no longer feasible to try and match the titlebar’s color/gradient.

With the tabs part of the title bar, the only way to have the file browser header in the tab bar is to also move that to the title bar, but from an implementation POV that would be rather messy, and it might also be hard to give it a distinct look, as now the tabs are effectively just rendered together with the title bar (using transparent colors to draw borders, inactive tabs, etc., so that they adopt nicely to changed title bar colors).