> The win of nice clean window separation, without giving up significant percentage of my display space to window decoration, is -profound-.

What does this have to do with a tiling window manager? I'd expect the app decides how to display window decoration, not the window manager. In fact I can confirm that right now since Chrome has different window decoration than say TextEdit. Also, scrollbars, macOS has them invisible by default so they don't take any space.

As for 2-up. Both Windows and Mac have shortcuts for that case