I guess that’s the theory, but I suppose it’s up to each application developer to implement. Getting 100% scaling (no scaling) is easy, it most likely works out of the box. But deciding whether to display content actual or scaled size probably depends on context and it also requires the developer to get it right.
I scaled a Wayland display, and then opened the same PNG in several different applications. Only Gwenview displayed the PNG with no scaling. Oddly showfoto actually showed a scaled down image for 100% (bug?). Popular photo editing tools seemed scaling-unaware and just displayed scaled up images.
So it seems that if I want consistency for this PNG across all applications, it’s best not to scale.
If GUI text is too small, as described in my own solution above, I can set QT_FONT_DPI (and live with an unofficial/not-supported approach - which is fine, this is Linux).
Context is another issue, if I’m working on a photo I probably don’t want scaling, but if I’m working on an icon maybe I do.
Not having used scaled-displays, I would admit it’s possible I’m misinterpreting something. I can see that when coding (python/C/…), scaling displays is quite workable, I could see me using scaling in that scenario. I suppose I’m using HiDPI for more pixels, whereas others may be buying it for less-jaggy retina-display looking graphics.