Right now I'm sitting behind 4K (Dell P2715Q) monitor connected to rMBP (13", 2015). What I'm supposed to miss? What exactly is not displaying correctly and I didn't notice?

Also, exactly what applications do not use Cocoa? Apple had made extremely difficult to talk directly do display server, the two non-private-frameworks-without-stable-abi ways were Carbon and Cocoa. Carbon is already dead.

> Right now I'm sitting behind 4K (Dell P2715Q) monitor connected to rMBP (13", 2015). What I'm supposed to miss? What exactly is not displaying correctly and I didn't notice?

I use the same monitor as well. Until recently, I had the 15" version of that laptop, and now I have the latest 15" Macbook Pro. Both use 2560x1440 on the external monitor even though it's capable of 3840x2160, because that's what OS X defaults to, and every time I try increasing the resolution, I only make it a few minutes before I can't stand how terrible the experience is and end up switching back.

> Also, exactly what applications do not use Cocoa?

I'm saying that native Mac applications look really bad on OS X when set to 3840x2160. The UI and text don't scale consistently, and that's true both of the system-level UI and of application UI. The situation gets even worse when you're talking about applications designed for X11[0], which I could forgive, but on the other hand, those applications scale just fine on Linux.

By contrast, all of this works out-of-the-box in (for example) Unity. I don't have to deal with dialog boxes that are out of proportion with the system bar, or text that's impossible to read unless I squint on some applications but perfectly readable on others.

[0] We could get into a debate about whether XQuartz really counts as "cocoa" or not, but I'd rather not.

> I use the same monitor as well. Until recently, I had the 15" version of that laptop, and now I have the latest 15" Macbook Pro. Both use 2560x1440 on the external monitor even though it's capable of 3840x2160, because that's what OS X defaults to, and every time I try increasing the resolution, I only make it a few minutes before I can't stand how terrible the experience is and end up switching back.

I would suggest to try using different DisplayPort cable.

OSX defaults to "best" resolution for the given display. In case of this monitor, it is 3840x2160, but - it will tell you is 1920x1080 and use it in HiDPI mode. Like this: http://imgur.com/a/infej

Once you change resolution, it will remember, what you changed it to on that specific display, and will use it next time you connect to it.

> I'm saying that native Mac applications look really bad on OS X when set to 3840x2160. The UI and text don't scale consistently, and that's true both of the system-level UI and of application UI.

In my experience, they scale perfectly. The ones that are non-HiDPI aware, are scaled by the system to the proper size (and thus are pixelated).

XQuartz is another matter. It should be taken behind the barn and shot long time ago. Running X11 apps is better done using a linux vm.

However, what I don't understand, why you have wrong scale. XQuartz is a not DPI-aware app, thus it is scaled by the system.

> I would suggest to try using different DisplayPort cable.

It has nothing to do with the cable. I've had this issue on every single 4K monitor I've used with these laptops - which is at least three different monitors, with different cables - and it even manifests with HDMI 4K output (at 30Hz). All of these setups work perfectly fine with my XPS 13 running at 4K on Linux. I don't understand why you think that the OS's inability to scale the system UI or applications in a reasonable way would have anything to do with the cable.

Try it with a Linux laptop running Unity, and you'll see the difference.

> I don't understand why you think that the OS's inability to scale the system UI or applications in a reasonable way would have anything to do with the cable.

Because that's not the experience everyone else is having. That leaves possibility, that something is broken on your end.

> Try it with a Linux laptop running Unity, and you'll see the difference.

I know how Linux handles HiDPI. See my comment about Fedora elsewhere in the thread.

Okay I have no idea what you're talking about either, and I've used OS X on a 4K monitor. It scales to a virtual 1920x1080p by default; everything just looks sharper. What did you expect to happen?

> Okay I have no idea what you're talking about either, and I've used OS X on a 4K monitor. It scales to a virtual 1920x1080p by default; everything just looks sharper. What did you expect to happen?

If it's scaling to 1080p for you, that's because either your monitor or your cable doesn't support 2560x1440 at 60Hz. If it does, it should default to 1440, assuming you're on an rMBP.

In terms of "what I expected to happen" - well, I expected an actual 4K resolution (like 3840x2160) to work. Maybe not by default, but at least when I enable it, it should be able to auto-scale the appropriate parts of the system and application UI in a consistent manner. That means that I'd be able to actually view a 3840x2160 image at its full intended resolution (not 1080p or 1440p) without scrolling, while also not having to squint in order to read the system bar or do anything else on the OS.

That works pretty much out-of-the-box on Linux with Unity. Even with tweaking configuration everywhere, I can't find any way to do that on OS X and have it look halfway decent.

> In terms of "what I expected to happen" - well, I expected an actual 4K resolution (like 3840x2160) to work. Maybe not by default, but at least when I enable it, it should be able to auto-scale the appropriate parts of the system and application UI in a consistent manner.

That's what it does. OS X will by default render at the panel's native resolution. But it scales everything to a lower effective resolution. Say you're running a 5K monitor at "2560x1440" (as in the iMac 5K). And the app displays a 32x32 icon at (0,0). The OS will see that you're running in 2x mode (real resolution = 2x effective resolution). So it'll pull a 64x64 version of the icon from the app's art assets. It'll take up 32x32 on the virtual 2560x1440 display, but a 64x64 image will be what's copied to the frame buffer. Text glyphs, and images, likewise, will be rendered at full resolution.

When you bypass the default setup and tell the OS to run at 3840x2160, you're not telling it to render at that resolution. You're telling it to size things as if you're running on a standard definition monitor of that size. So when the app displays a 32x32 icon, the OS will see you're running at 1x mode, and pull a 32x32 icon from the art assets. The result will be a tiny icon.