Note that the problem isn't really with X11 (or at least Xorg) since it does provide the necessary information via RandR to perform per-monitor scaling.
The issue is really with window managers and toolkits. What needs to happen is that window managers ask the applications (the top level windows really) to scale themselves -this can be done not just for DPI purposes- and then the applications should apply that scaling.
(applications should expose that they can support this - like they expose that they support handling a 'close window' event - so that compositing window managers can do it themselves for applications that do not support it, which is also why this needs to go through the window manager)
Qt can do this but AFAIK window managers do not do this because there isn't any standard events for that - i remember an email about this exact topic when i took a look at the Xorg mailing list for WMs some time ago, but it didn't seem to be going anywhere (in that it barely had any replies).
So it really is about standardizing how it is to be done. I think the main reason it isn't done is that such setups are comparably rare (i mean hidpi displays themselves are very rare - if you check any desktop resolution stats they often barely are a blip - and having both a hidpi display and a regular dpi one is even less common than that) so developers aren't that interested in implementing such a thing.
One way it could be done however is also pushing the idea that this isn't just for multimonitor setups: as i wrote above, you can use that for generic scaling, which is useful even for single monitor regular dpi setups (if anything it can be very useful for low resolution setups - like 1366x768 and similar which are way more common than hidpi - when faced with applications with a lot of padding, etc).
The issue is really with window managers and toolkits. What needs to happen is that window managers ask the applications (the top level windows really) to scale themselves -this can be done not just for DPI purposes- and then the applications should apply that scaling.
(applications should expose that they can support this - like they expose that they support handling a 'close window' event - so that compositing window managers can do it themselves for applications that do not support it, which is also why this needs to go through the window manager)
Qt can do this but AFAIK window managers do not do this because there isn't any standard events for that - i remember an email about this exact topic when i took a look at the Xorg mailing list for WMs some time ago, but it didn't seem to be going anywhere (in that it barely had any replies).
So it really is about standardizing how it is to be done. I think the main reason it isn't done is that such setups are comparably rare (i mean hidpi displays themselves are very rare - if you check any desktop resolution stats they often barely are a blip - and having both a hidpi display and a regular dpi one is even less common than that) so developers aren't that interested in implementing such a thing.
One way it could be done however is also pushing the idea that this isn't just for multimonitor setups: as i wrote above, you can use that for generic scaling, which is useful even for single monitor regular dpi setups (if anything it can be very useful for low resolution setups - like 1366x768 and similar which are way more common than hidpi - when faced with applications with a lot of padding, etc).