alt Hacker NewsSigh. And now that it's been long enough, everyone will conveniently forget all of the reasons why this wound up being insufficient, and think that all of the desktop environment and toolkit developers are simply stupid. (Importantly, applications actually did do this by default at one point. I remember a wonky-looking nvidia-xsettings because of this.)
The thing X11 really is missing (at least most importantly) is DPI virtualization. UI scaling isn't a feature most display servers implement because most display servers don't implement the actual UI bits. The lack of DPI virtualization is a problem though, because it leaves windows on their own to figure out how to logically scale input and output coordinates. Worse, they have to do it per monitor, and can't do anything about the fact that part of the window will look wrong if it overlaps two displays with different scaling. If anything doesn't do this or does it slightly differently, it will look wrong, and the user has little recourse beyond searching for environment variables or X properties that might make it work.
Explaining all of that is harder than saying that X11 has poor display scaling support. Saying it "doesn't support UI/display scaling" is kind of a misnomer though; that's not exactly the problem.
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I sort of wanted Fresco (previously Berlin, inspired by InterViews) to succeed, because in their model the UI toolkits really were server-side and they could be changed out while the application was running. Because they were targeting an abstract device (could be a 1200 dpi printer and a 72 dpi display at the same time) they got the property you mentioned, for free.
> can't do anything about the fact that part of the window will look wrong if it overlaps two displays with different scaling
It's silly that people keep complaining about this. It's a very minor effect, and one that can be solved in principle only by moving to pure vector rendering for everything. Generally speaking, a window will only ever span a single screen. It's convenient to be able to drag a window to a separate monitor, but having that kind of overlap as a permanent feature of one's workflow is just crazy.
> The thing X11 really is missing (at least most importantly) is DPI virtualization.
Shouldn't that kind of DPI virtualization be a concern for toolkits rather than the X server or protocol? As long as X is getting accurate DPI information from the hardware and reporting that to clients, what else is needed?