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I have an odd behavior on my home system that I was hoping someone could explain or at least provide more puzzle pieces to.

I have a 1440p thunderbolt monitor (Lenovo p27h-10) and a 4k DP monitor. In Macos, the 4k is the primary display. I run it at 3200x1800.

Every manual and tool reports the Lenovo p27h-10 as a 1440p display. But when I use this third-party "Display Menu" app to ask for mirrored displays, the Lenovo P27-h10 runs at 3360x1890. Even though even this app, even while mirroring and the resoluton being listed at 3360x1890 in the tool, the tool doesn't show this resolution in the dropdown as it normally does the "select" res along with other options.

Other than just being surprising, I am interested in what's going on here because it would be nice to run this thing at 3360x1890 when not mirroring, even if it turns out to be at 30hz.

Notes:

  • Text looks fine when it's surprisingly 3360x1890.
  • Doing the same mirroring through Settings puts both displays into 1440p
  • If the Lenovo is the primary display, putting it into mirroring from the third-party app results in both displays at 1440p

My tentative conclusion as a layman in this stuff is that this mode is somehow supported by the display but not advertised (in EDID?), and some bug in this Display Menu app causes it to try to switch to a mode that it shouldn't try.

From comments:

  • The OSD on the display still says 1440p@60, but "Display Menu" app says 3360x1890.
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  • Don't have an answer, but monitors are known to support multiple resolutions with different specs. For example, a higher resolution, but half the refreshrate. The physical pixels remain the same, but pixels are merged to show it, and the timing is correct, so the monitor doesn't care. You essentially lose information though. Another thing could be that its artificial, similarly to Nvidia's or AMD's super resolution.
    – LPChip
    Apr 3, 2022 at 19:03
  • Where are you getting the resolution from? Only the display itself can tell you what it receives. Since this is macOS we’re talking about, I highly doubt it’s really running at this resolution. Instead, the output is probably scaled.
    – Daniel B
    Apr 3, 2022 at 19:06
  • @DanielB Added info re: OSD showing 1440p. I think you're right, I've never used a system that either scaled this way much less so effectively. Very confusing that it's also not reachable in any first-class UI on the system and can only stumble into it with mirroring.
    – covener
    Apr 3, 2022 at 20:22
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    Maybe getting a big clue here: macworld.com/article/549493/… Which creates a dummy high res display just so you can mirror it and get Macos to scale it onto your physical cisplay.
    – covener
    Apr 3, 2022 at 20:26

1 Answer 1

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This seems to be something like a hidden scaling option used for screen-mirroring of 4k-or-greater displays to lower resolution displays.

Software like "BetterDummy" seems to exist largely to allow a fake high res display to be mirrored by a lower res display and then mirrored. I was able to create the same effect (albeit bad performance, mouse ghosting etc) by creating a dummy 3k display then asking for my Lenovo to mirror it.

Similarly I found there are dummy HDMI plugs that tell the host they are driving a 4k display for around ten dollars.

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