Perhaps I should have added "... unless you've managed to disable the new HiDPI mechanism in which case none of it will work right... Oh, I think I see the problem..."
The long answer follows:
After reading through your reply I think I have some idea of why you are seeing the results you are seeing, but I would like to get more information on how you've configured your system. What display panel are you using to change the scaling? (i.e. how do you get there and if you could post a window snapshot of the control panel that would be great). Also, what do you see when you go to "Start icon -> Settings -> System tab -> Display tab"?
In particular, you can choose settings which disable all of the new HiDPI mechanisms on the computer. They don't really tell you that is what is happening, but they at least try to recommend that you don't use those settings (and this is the reason why they recommend not to use those settings). It's not clear all of the reasons why they provided this ability to disable the new mechanism entirely, but it may have been that they couldn't be 100% sure that older-style applications would all work properly when they were being run in the compatibility mode (basically the backwards compatibility mechanism will lie to older applications that don't report that they are aware of the new mechanism and tell them that there is one DPI for all monitors and then the system will pixel stretch them on the secondary monitors if they need to adjust for a different DPI). It may also be that some users might not like the fuzziness that comes from such pixel stretching and if most of their programs (or one particular important application that they use a lot) have not been re-coded for the new APIs and are still running in the compatibility mode then they may prefer to disable the new mechanism and just use old-style scaling which does not have the ability to deal with monitors of different resolutions, but at the same time it has fewer cases in which it will decide to pixel stretch an app to make it fit. In any case, they provided the setting and they tried to put it behind dialog boxes that said "You can do that if you really want to, but we don't recommend it" and didn't really say "we don't recommend it because you will be turning off important new display capabilities that make radically high DPI monitors work well".
For the record, you can also disable all HiDPI support on Mac systems, and they will not work well if you have a retina monitor, but they hide it better and so fewer users end up triggering it and getting frustrated by the option.
When you choose those settings and disable the new mechanisms, then you end up with limitations like:
- No more dynamic DPI for any application, they've turned it off at the system level and all changes of the scale require a logout.
- No more per-monitor DPI. A single DPI is reported for all displays even if they have radically different actual resolutions and there are no APIs that will ever return an accurate value so no application can adjust for this.
Basically, if you choose that option, you've set your system back to the Windows Vista days of how to deal with monitor resolutions - with respect to display scaling you might as well install Vista instead as you will lose no sophistication in how to deal with mixed DPI cases - that's how ancient the mechanisms are.
If you choose to undo that setting and go back to the modern way of scaling applications, then handling of multi-monitor DPI differences will be restored and old and new applications will work properly across monitors of a wide range of DPIs, but you might see some blurriness in some cases using older applications that haven't been updated and so they will experience the pixel-stretching to accommodate their older DPI awareness. Most applications I use have been updated so I don't tend to see any pixel stretching any more (except, embarrassingly for some of Microsoft's own control panels - D'oh!).
To undo the "kill the new HiDPI support" setting simply go to "Start icon -> Settings -> System Tab -> Display tab" and it should say in red text "A custom scale factor is set", followed immediately by a link to "Turn off custom display scaling and sign out". You will need to click on that link and it will log you out (immediately, no confirm dialog!) and when you log back in you should have the modern display scaling again. The modern display scaling will have the following properties:
- Super old applications may be pixel stretched on nearly all displays. Those applications are using system queries that pre-date Vista and other than some ancient Windows Control Panels, these applications should be very very rare.
- Old applications that haven't been updated in the past couple of years may look fine on the primary monitor but be pixel stretched on secondary monitors.
- New applications that have been updated in the past year or so should be updated for the new HiDPI system queries and should be scaled appropriately on every display individually, but you may see some size discrepancies if they span multiple monitors - but only during the time frame when they span more than one monitor and they should then re-adjust when they are fully dragged to the new monitor. As soon as they are mostly on the new monitor they should rescale to the new monitor.
- Changes to the display scaling using the new mechanism (Start -> Settings -> System -> Display) will not require a reboot, but they will say that some apps might not respond to the new setting until you log out. If all of your applications have been recently updated then you can ignore this recommendation. If you have some applications in the "ancient" and "old" categories that I mention above then they may be blurrier than usual until you log out.