alt Hacker News> macOS handles things on a per-section-of-window, per-application, per-display basis.
No, it does not. If you have two displays with different physical pixel densities, and especially if they are sufficiently different that Apple will consider one 'Retina' and 'not Retina' (this is usually the case if, for instance, you have your MacBook's display—which probably is 'Retina'—beneath a 2560 × 1440, 336 × 597 mm monitor, which is 'not Retina'), then the part of the window on the non-Retina display will be raster-scaled to account for the difference. This is how KDE Plasma on Wayland handles it, too.
In my opinion, any raster-scaling of vector/text UI is a deal-breaker.
I think the only case where raster scaling is not a deal breaker is a window spanning high and low DPI displays. That is unless the app delegates compositing to the OS compositor which could then raster the contents at the different scales correctly. Not all content can be delegated to the OS - video games for example.