"Ideally this should render the UI elements to be the same size as it
would be on a native 1080p monitor at 100% scaling"
No that's not how it works. All your examples and questions lead to me to believe your question should be something like this...
"Can windows upscale my software so that I can see them correctly in respect to my native monitor's default resolution?"
The answer is probably not. Windows is not in control if any specific software's interface, as far as I know. But I could be wrong.
Let's take steam for example, steam is a gaming platform that is mainly used by gamers. Gamers that are more than likely to want or seek out 4k monitors to play their games.
But steam's platform software took a long, long, long time before it finally supported 4k monitors. Steam basically had to do a major re-haul of their platforms sofwtare to make this happen. So, this could be a basic example that software doesn't magically upscale. So gamers had to suffer with super small chat windows until steam finally updated it's software.
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?
When it comes to media players, things don't really work the same. A media player has two aspects to consider.
- Is the actual software that allows a video to play.
- The play area itself.
With the actual software, the interface may look small and tiny on a 4k monitor. But video play area was designed to scale to whatever resolution you're viewing a video at.
Downscaling. If I have a 4k video file. But only have a 1080p monitor. The software recognizes this and attempts to fit the video with-in the scope of your monitor. So that begs the question, are you really watching a 4k video? Yes and no. The video is set at 3840 pixels x 2160 pixels. But 50% of the pixels will mesh together in order to fit into your 1920 x 1080 monitor. So it's really not actually 4k.
Upscaling: The flip side of that is a 4k Monitor attempting to play a 1080p movie. When you double click on the movie player, it's going to make the video play in full screen. Is the video now considered 4k? No it's not, it's just a blown up image which they will double up the pixels side by side (I don't know how this is exactly done, i'm just trying to paint a visual hypothetical example in order to explan how upscaling works) in order to make it seem like you're watching a 4k video.
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?
Photoshop is another great example of this being an issue. Adobe designed it's photoshop software to work with mainly 720p or 1080p monitors. I'm not sure of the newest software version of photoshop. But I can tell this could be an issue for people with 4k monitors.
Just like VLC, Photoshop has buttons and menus AND it has a project area, where you create your work of art or edit your images. The project area, can be any resolution you can think of (as long as your PC can handle that resolution). Your 4k monitor will show that many pixels without scaling. But the software interface (the buttons) will be built specifically for a 1080p monitor.
This isn't an option in windows to fix this interface scaling problem. You need to hope that adobe has fixed their software, or releases a patch in order to allow you to increase the size of the button to make your viewing experience on a 4k montior acceptable while playing with photoshop.
In essence, what does the display scaling option actually do? For my
question I used Windows as an example but the same setting is present
on OSX. If I set it to 200%, for most programs these days, the UI
elements are two times bigger.
It makes Windows or OSX UI elements appear two times larger than they would be if they were set to 100%. That doesn't guarantee that a 3rd party program will also be effected.
Judging from your answer, I assume this
is not magic and what ever UI library a program is built on top of has
to support display scaling.
Yes that is exactly what I stated above or tried to point out.
Is the play area of a media player or the
project area in Photoshop kept to 1:1 pixel mapping with the native
display resolution irrespective of scaling?
This question is a little bit over my head technically. For Photoshop I would say yes. As long as the Photoshop project area is set to 100%. For every pixel in Photoshop, the same will be related to your native display resolution. If you were to set the Photoshop scaling at 200%, then it would be 2:1 and so on.
Depending on your media player you can set the scaling with the media player itself. In VLC you can do this here:
That scaling will be respective of your native display resolution. At 1:1 Original each pixel in a video will be the same as your monitor. When you goto double or full screen, picture quality will visually decrease obviously to fill in the void.