MacBook Pro M3 Pro – External Monitor Only Shows Wallpaper

Question

MacBook Pro M3 Pro – External Monitor Only Shows Wallpaper

Hi,

I’m using a MacBook Pro M3 Pro with two external monitors (27" LG UltraGear and 24" Legion Y25-30). Both are connected via HDMI through a dock. This setup has worked fine before, but suddenly today I’m having problems.

The 27" monitor does turn on, but it only shows my wallpaper and cannot be used as an extended display.
If I connect ONLY the 27", it works normally. For example, I can open Chrome on it. But if I then plug in the 24", the 27" screen freezes (it keeps showing the Chrome window, but nothing updates or moves).
If I connect BOTH monitors from the start, only the 24" is recognised in System Settings → Displays (screenshot below).
I’ve tried different HDMI cables, bypassing the hub, updating macOS, and resetting display settings, but the issue persists.
macOS is up to date (Tahoe 26.0.1).

Has anyone else run into this issue? Is it a macOS bug, or something wrong with my display setup?

Thanks in advance!

[Edited by Moderator]

MacBook Pro 14″

Posted on Oct 11, 2025 8:19 PM

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Answer 1

Top-ranking reply

Servant of Cats

Level 7

33,203 points

Oct 11, 2025 9:38 PM in response to andrew_nguy3n

What dock are you using? Is it a plain USB-C dock, or a Thunderbolt one?

Macs do not support DisplayPort MST daisy-chaining or the equivalent. Macs that can drive multiple monitors over USB-C or Thunderbolt can drive

One monitor over a plain USB-C dock, or
Up to two monitors over a Thunderbolt dock

A typical symptom of using a hub/dock that drives some of its outputs using MST is that monitors appear to mirror each other, and the Mac only detects one monitor. This is because, in this situation, the dock is basically feeding a single video signal to two monitors.

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Answer 2

Oct 12, 2025 7:35 AM in response to andrew_nguy3n

That specific model Dock accepts USB-C signals form TWO adjacent ports on some MacBook Pro models, so that it can potentially support two displays using TWO USB-C connections.

the cables required for HDMI displays are very fussy.

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive.

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Answer 3

Oct 12, 2025 9:07 AM in response to andrew_nguy3n

andrew_nguy3n wrote:

I don’t think the problem is with the dock, because this setup has worked fine for about 6 months. Today it suddenly became faulty. I didn’t change anything (no updates or new software), I just opened my Mac and the issue appeared.

Here’s the dock I’m using: https://amzn.asia/d/4ny6166• . I don’t think it’s Thunderbolt.

It doesn't look like a Thunderbolt dock – but it has a rather unusual arrangement. As Mr. Bennett-Alder noted, it plugs into two USB-C ports on your Mac. It can grab one USB-C (DisplayPort Alt Mode) signal from each port – and therefore can support two displays while staying within the "one per plain USB-C connection" limit.

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Answer 4

Oct 12, 2025 12:32 AM in response to Servant of Cats

Hi Servant of Cats,

Thanks for your quick response.

I don’t think the problem is with the dock, because this setup has worked fine for about 6 months. Today it suddenly became faulty. I didn’t change anything (no updates or new software), I just opened my Mac and the issue appeared.

Here’s the dock I’m using: https://amzn.asia/d/4ny6166. I don’t think it’s Thunderbolt.

Also, the two monitors are not mirroring each other. What happens is:

If I connect only the 27", it works normally. For example, I can open Chrome on it.
But when I then plug in the 24", the 27" immediately freezes — it shows the Chrome window I had open, but nothing updates or moves.
If I connect both at startup, only the 24" is recognized in System Settings → Displays.

ChatGPT suggested that “the 27" monitor is detected but not configured.” Not sure if that’s exactly what’s happening, but it seems related.

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Answer 5

Oct 12, 2025 1:43 AM in response to andrew_nguy3n

Well it suddenly works now. I don't even know why man...

[Edited by Moderator]

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