P2421DC, minified UI-Elements on macOS

@lazercaveman  The MacBook's built-in display has a pixel density of about 220 ppi, which is within Apple's range of Retina displays and therefore triggers the use of "@2x" art assets in macOS so that things are incredibly sharp and an appropriate size on the display rather than incredibly tiny.  The P2421DC has a pixel density of 108 ppi, which is within Apple's range of non-Retina displays and therefore triggers the use of standard, non-Retina art assets so that things are not too huge.  I'm pretty sure macOS can be configured to use different scaling settings for each display, since running a laptop Retina display with a standard external display is a pretty common setup.  But if that's somehow not the case, then there isn't going to be a single scale factor that will make sense for both of these displays.  There wouldn't be on Windows either, but Windows supports specifying different scale factors for each attached display (although you do take an image quality hit on all non-primary displays if you do this, so whenever possible it's preferable to make sure that all active displays can use the same scale factor).

Okay, so if mac os hasn't any such feature like windows provides it, is there maybe an app to handle the problem anyways?

@lazercaveman  Ok, blur is unavoidable in this situation, and if it's any consolation, that's also true on Windows.  The reason is that while both macOS and Windows support using different scale factors for different displays, both of them as of this writing still only render at a single scale factor.  For Windows, it's the scale factor of the display that was primary when you logged in.  On macOS, it's the one you choose to optimize for.  For any OTHER displays, the OS takes that rendered output and then applies post-render GPU scaling to upsize or downsize the result as needed for any other displays that are using a different scale factor from the native rendering scale factor.  As you might expect, that does not look as good as native rendering for each display's scale factor.  The difference is especially noticeable on text.  But at least for now, neither OS supports rendering at multiple native scale factors simultaneously to accommodate these types of setups involving displays with very different pixel densities.

So if you want to avoid this, you'll need to get a display that macOS would treat as a Retina display just like your system's built-in display, such as a 24" 4K display or a 27" 5K display.  And actually a 24" QHD display like you have now really isn't an ideal choice for a Mac at all, even if you were using it on its own.  That results in a pixel density of 122 ppi.  That falls between the two pixel density ranges that Apple actually optimizes macOS for, namely standard (100-110 ppi) and Retina (200-220 ppi).  A 27" 4K display is even worse at 163 ppi.

@jphughan thanks for your advice - I think these kind of tools won't do the job, since these just change the resolution, which leads to bluring ...
I could also change the resolution within mac os natively (even if there are less options) whcih leads to the same blury result.

Thats not true - I do use windows 10 on dual boot and the ui items dimensions (elements, texts, aso) do have the same dimensions and can be changed through zoom factor, which does not lead to scaling and does not change the displays resolution! instead it changes the elements size ... it works perfectly fine and everything has the same size on both displays (the mac books and the external dell monitor).

This is btw. also possible in mac os, as long as I use the scaled option in resolution settings and choose the max of "more space" ... even if the displays resolution differs. Changing from scaled to standard will only change ui-elements dimensions on the mac book - not on the external display - other then in windows, wehere the external displays ui-elements dimensions would change as well.

"And actually a 24" QHD display like you have now really isn't an ideal choice for a Mac" ... I would say that this depends on the context and cannot be said as a general rule.

@lazercaveman  Several years ago I remember using a utility called SwitchResX on macOS in order to get access to more display setup options than macOS natively offered.  But that was a while ago, so I have no idea if it's still usable today.  But if not, Googling "SwitchResX replacement" might turn up some forum threads that suggest alternatives.

@lazercaveman  I'm referring to the Windows Display Settings option called "Change the size of text, apps, and other items", which has a variety of percentage options.  That is a scale factor.  If you're using some other setting, then there was some confusion there.  But what I said above is applicable to Windows systems when using multiple different scale factors across multiple simultaneously active displays.  If you only have a single scale factor in play for all active displays, e.g. all of them at 125%, then you won't have any post-render GPU scaling.  The degree to which you have scaling when using any non-100% scaling setting in Windows depends largely on the application, because Windows has implemented a wide range of scaling mechanisms over the years, so different apps will behave differently.  But if you for example have one display set at 125% and another display set at 200%, then you will have post-render GPU scaling on whichever display is not using the scale factor of the display that was primary when you first logged in.  This can become especially annoying for people who might log into Windows while undocked and using the built-in display set to 200% scaling, then they dock their system and use ONLY external displays that use a scale factor of 100%.  In that case, even though there's only one active scale factor (100%), Windows will still be rendering internally at 200% and applying GPU scaling for those displays, since as I said the scale factor Windows uses is the scale factor of the display that was primary when the user logged in.  If you want Windows to optimize for your new setup, you'd have to log off and back on.  If you're thinking this would be annoying for people who use laptops and dock and undock several times during the day, you'd be absolutely right.

As for 24" QHD on a Mac, I stand by what I said.  Apple designed macOS to support standard displays with a reference pixel density of 100-110 ppi, and Retina displays through the use of "@2x" art assets targeting displays with a pixel density of 200-220 ppi.  If you use any scaled option in between, macOS will be using one set of art or the other and will apply post-render GPU scaling, even if you only have one active display running in a scaled mode.  This design of only having "true" support for two pixel density ranges is what made it so easy for Apple and their developers to implement support for Retina displays, since they just had to create "@2x" versions of their art assets.  And that's why Macs with Retina displays have been a great experience for years now.  But it also means that if you use a display that is outside of what Apple is explicitly designing for, your experience won't be great.  Admittedly, 25" QHD's pixel density isn't all that far outside of Apple's standard range.  A 27" 4K display with 163 ppi is the worst because it falls right between Apple's ranges and is therefore a bad fit for both of them.

By comparison, Windows only defines a reference pixel density of 96 ppi, meaning if you use a display with that density at 100% scaling, then something that's supposed to be an inch wide (like a one-inch gap on the Microsoft Word ruler) will be a real-world inch wide on the display.  But Microsoft never created a special category for a Retina equivalent.  They just have all those different scaling percentages, because unlike Apple that controls the software and hardware (at least the hardware they care about), Microsoft has to support a broad range of hardware.  That makes Windows a bit more flexible, but it's also why scaled Windows today still doesn't work as well as macOS Retina has for several years now.

Anyhow, good luck with your setup.  Hopefully you can find something you're happy with.

I don't agree - anyways, everyone can have his own opinion ...

I could not find any satisfying solution for mac os yet. If this changes I'll leave a post for others within this post...

@lazercaveman  If you can comfortably use a 25” QHD display without any scaling enabled, then yes that setup can be completely fine even on a Mac. Things will be smaller than reference, e.g. that one-inch gap on your Microsoft Word ruler would be smaller than an inch, but if you don’t care about that and your eyesight is good enough to use it comfortably, then more power to you. But many people cannot comfortably use a 25” QHD display with standard rendering. They have to enable scaling, and any scaling other than standard and Retina on macOS introduces blur, which hopefully we can agree is not ideal. And regardless of whether you agree, it is a fact that Apple designed their OS around desktop displays with a pixel density of 100-110 ppi. That’s why ALL of their own own non-Retina displays have fallen within that range, including the old 23” and 30” Studio displays and the 27” Thunderbolt display, as well as their old non-Retina laptop displays. And they designed Retina displays to have double the pixel density, which is why all of Apple’s own Retina displays fall within that 200-220 ppi range, including the 23” 4K iMac display, 27” 5K iMac display, 32” 6K Pro XDR display, and all of their Retina laptop displays. But if Apple’s design that was optimized for a certain type of display works for you on a different kind of display, then fantastic.

I'm not sure I get everything on this topic, but for me it worked this project 

https://github.com/waydabber/BetterDummy

Dummy Display for Apple Silicon Macs to Have Custom HiDPI Resolutions - an app from one of the makers of MonitorControl.

Found following this thread https://forums.macrumors.com/threads/solution-quadhd-monitor-with-hidpi-and-mac-mini-m1.2303291/