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In macOS, the entire display is rendered at 4K no matter what you pick from the resolution box. Most apps have learned to provide 2x-3x “retina” icons so that their bitmap resources look crisp alongside the system UI resources at user-selected “resolutions”.

The Windows UI scaling slider behaves in exactly the same way, though fewer apps include 2x or 3x bitmap resources.

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> In macOS, the entire display is rendered at 4K no matter what you pick from the resolution box.

This makes it sound as if macOS upscales a 4K render when displaying to (for example) 5K monitors, but on a 5K monitor everything is ultimately rendered at a full physical resolution of 5120x2880. But in the Displays Preference Pane, the logical resolution is set by default to 2560x1440 (2:1). One can choose a logical resolution of 5120x2880 (1:1), but I can't imagine anyone working like that.

Correct, in the 5K case the entire display is rendered at 5K, and ditto 2K etc. (I believe the internal canvas caps at 32K, but I don’t have the tools to find out for sure.)

Whatever-sized display viewports are just crop windows into it, and the crop window in internal canvas terms is scaled as necessary, and then it renders the vector canvas onto the raster viewport.

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