MacBook Pro 16" Radeon + external display mirroring issue

[vldmr1]

Setup

Computer: MacBook Pro (16-inch, 2019)
OS: macOS Monterey Version 12.2
Display: Philips Brilliance 258B
Connection: HDMI cable through Satechi Type-C Aluminum Multi-Port Adapter

BetterDummy settings

Problem

When trying to select a HiDPI resolution through BetterDummy menu, it never actually sets to that resolution.

Instead, it just drops to its low resolutions counterpart.

The only HiDPI resolution that actually works is 1280x720, which is unusable to me.

Is there anything I can do to make this work?

[waydabber]

Does this resolution override happen only when doing mirroring? If so, then does it work when you try to mirror to the internal display?

To be honest, BetterDummy was not really tested in depth on Intel+Radeon configs (these have a totally different GPU drivers). If your goal is to have custom resolutions on these units, you might be better off by trying RDM) (free) or SwitchResX (paid).

[vldmr1]

Yes, if I set Dummy 16:9 as Main display and my Philips 258B as Extended display, I can select any HiDPI resolution and it won't be overriden.

If both the internal display (Built-in Retina Display) and Philips 258B are set to mirror Dummy 16:9 the HiDPI resolution can also be selected and it sticks. But that introduces a black border around content on the Philips.

[waydabber]

This is probably because the interplay of Intel and AMD GPU drivers are not well implemented - due to the multiple different GPUs the system cannot use a common framebuffer for mirroring (can't use hardware mirroring) + mirroring a virtual display to different GPUs at the same time is not implemented properly. Virtual display mirroring has its own issues on M1 as well so it is obviously not on the priority list for Apple and probably there is no in-house testing for these scenarios. You can try whether the same happens if you try to mirror an AirPlay or Sidecar display as these use the same APIs as BetterDummy - you'll probably experience something similar.

[waydabber]

But what I just wrote might not make sense after all. I don't know how the discrete and built-in GPUs are utilized in such a mirroring scenario (virtual display + external GPU + internal GPU). The virtual display might be software accellerated only on Intel but since some shader operations are needed this is probably not the case (as on M1) but is accellerated by the Intel GPU and then when mirroring is happening, the contents is simply copied to the AMD GPU's framebuffer. This might have some constraints as it is a costly operation, this might be the reason for why some resolutions don't work. Or there might be something else altogether. I am just theorizing. :)