P: Support all Windows DPI scaling options (125%, 150%, ...)

You're pretty close. That's why we are continuing to work with Microsoft on the issues.

Allow me to elaborate. From an outsiders perspective, it would seem trivial, but obviously it isn't (thought I had made this clear in my first comment, apparently not). The current scaling support in Windows is fairly limited, overall making it difficult to effectively implement scaling. Heck, when on 150% scaling, a number of programs become blurred, so clearly the scaling system in Windows needs some work.

So here is my understanding of the situation with scaling (just to be sure I've got it right). The scaling system in Windows is flawed. Getting a program to work effectively with the current scaling system would either require an excessive amount of effort, or is simply not going to be achieved with the current system.

Right?

It is not a trivial problem, and we have been working with Microsoft for a couple of years to solve the issues inside Windows that make it fail to work correctly.

We are continuing to work with Microsoft on the issues needed to allow more flexible UI scaling on Windows.

Seems like such a trivial problem, but I can understand why this is such a difficult issue to resolve. Big program, lots of modules that will have to be redesigned to enable successful scaling.

Ideally, in the future I'd like to see scaling done in a similar way to Illustrator. I was surprised to find that Illustrator even works well with Microsoft's Windows10 continuum feature by switching into the touch mode, which is a bonus.

This is so crucial for me... I simply do not understand why this isn't supported? Indesign, Illustrator - they do it perfectly... Although 125% would be even better.

Yes, there are serious problems in the Windows UI Scaling APIs. We worked with Microsoft to fix the major issues to allow 200% UI Scaling. We are continuing to work with Microsoft to address the issues that still prevent more flexible UI scaling on Windows.
We are also in contact with the Office team about the UI scaling issues they have seen and the remaining work to be done in Windows.

There are no "Windows issues" (other than the fact that it takes more work!). You just have to scale every draw call to the system DPI and supply images at the four most common scaling percentages. Every Microsoft Office application is scaled correctly, even InDesign is. If they can do it, so can you. It's just a matter of wanting to do so. If you didn't design your custom UI layer with High DPI in mind, well that is your problem and not Microsoft's. Windows has had all the necessary API's for High DPI since Vista.

We spent a lot of time working with Microsoft to even allow 200% scaling. We are continuing to work with Microsoft to address the Windows issues needed to enable more flexible UI scaling on Windows.

On the Surface Pro 3 200% scaling looks rather large. The system is running at 150% scaling - why can't Photoshop simply respect that? InDesign does (though they forgot 125% scaling). Why is it so hard for you to just RESPECT user preferences? Windows is not a Mac - we have a lot more choice! It is not 100% or 200%.