- AFAIK, it's usually the toolkits that do the scaling. So, it's indeed possible that very old apps, if they're still using old versions of the toolkits, don't support scaling. They'll appear non-scaled, so "at 100%".
- I don't have a 4k display on a laptop so can't comment on battery life. But for "desktop use" (read: non gaming) GPU performance has been fine for a long time. I have an old desktop at work with a 4th gen i5 and whatever the integrated GPU was at the time. It can drive a 4k panel at 60 Hz just fine. A somewhat newer laptop, 8th gen i5 with a uhd620 integrated GPU could drive its internal FHD panel and an external UHD display without any issue.
- For your eyes comment: the small fonts may be illegible because they're blurry. On FHD screens, I've found that bitmap fonts are much more legible at small sizes. I've seen some Dell with a 4k display at work, probably 15", and the small text was much more legible than on my 14" FHD laptop (compared using Windows 11 - the guy was a Windows dev).
- TV has a blurriness to its movement, so it looks smooth enough because it's never actually sharp. The point of higher refresh rates is not "flickering", but a smooth movement. Try reading a scrolling page on a 30 Hz, 60 Hz, 120 Hz screen. I mostly look at static text on my screens, so 60 Hz works well enough for me, and I prefer higher resolution / better colors to higher refresh rates. Don't know how this affects GPU usage, though I don't expect it to be "free".
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.
- I don't have a 4k display on a laptop so can't comment on battery life. But for "desktop use" (read: non gaming) GPU performance has been fine for a long time. I have an old desktop at work with a 4th gen i5 and whatever the integrated GPU was at the time. It can drive a 4k panel at 60 Hz just fine. A somewhat newer laptop, 8th gen i5 with a uhd620 integrated GPU could drive its internal FHD panel and an external UHD display without any issue.
- For your eyes comment: the small fonts may be illegible because they're blurry. On FHD screens, I've found that bitmap fonts are much more legible at small sizes. I've seen some Dell with a 4k display at work, probably 15", and the small text was much more legible than on my 14" FHD laptop (compared using Windows 11 - the guy was a Windows dev).
- TV has a blurriness to its movement, so it looks smooth enough because it's never actually sharp. The point of higher refresh rates is not "flickering", but a smooth movement. Try reading a scrolling page on a 30 Hz, 60 Hz, 120 Hz screen. I mostly look at static text on my screens, so 60 Hz works well enough for me, and I prefer higher resolution / better colors to higher refresh rates. Don't know how this affects GPU usage, though I don't expect it to be "free".