I only have experience with this on Linux, where at least it supports scaling, so that if you use a constant one (my case), then it's OK.

Older ones will simply ignore the scaling settings and draw the interface 1-to-1. One such application that comes to mind is VMWare's remote console (for esx). I haven't used it in a few years, but I remember at the time it was painful to run on a 24" UHD screen.

On the windows side, I think things are somewhat better than on Linux, but there still is confusion, including Windows 11 22h2's start menu. If you start the computer in 100% mode, then plug an external screen scaled at 200%, it works OK for the app list (what it shows on first click) but if you start typing everything becomes a blurry mess.

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Edit: I actually think QT is one of the better toolkits, at least on Linux, in the case of scaling. IIRC it's able to adapt the scaling based on the screen DPI reported by X, so a full qt desktop should be able to handle situations like a high-DPI laptop connected to a low-dpi monitor.

It's not that it ignores it, it's that it tried to handle it and gets confused. I move fairly frequently between my 3x laptop screen and a 1x external monitor, and at this point I've got used to either the app logo randomly being a third the size it should be in the start bar or the text rendering three times as big as it should in the app.