For example, lets say I have a 4k monitor that I set to 200% display scaling(For this example, lets assume the os is windows 10). Ideally this should render the UI elements to be the same size as it would be on a native 1080p monitor at 100% scaling, but twice the dpi, right?
How do programs see this? For example, if I was watching a 4k video full-screened in VLC, does VLC directly render the video at 4k or does it think I have a 1080p screen and downscale this 4k video to 1080p first which then gets up-scaled again by Windows?
Another example, can Photoshop render its UI with the 200% display scaling but render the canvas at the displays native resolution thereby taking full advantage of physical pixels on the display while keeping the UI to a usable size?