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I am trying to connect my 2020 MacBook Pro (M1) to an external display via a thunderbolt 3 dock.

The dock is the Kensington SD5200T which supports M1 Macs with 1 external display.

The screen I am trying to connect to is the ASUS PG349Q via DisplayPort (1.2).

The MacBook is running the latest version of macOS (Big Sur 11.2.3).

When I connect the MacBook to the dock using the included Thunderbolt 3 cable the internal display flickers (resolution changes, windows flash on and off the display) until it eventually connects to the external display (this can take upwards of 1 minute).

I made a video demonstrating the problem, it can be found here.

I have tried the dock and MacBook with an LG 5k thunderbolt display which works fine and I'm using the DisplayPort cable from my PC so I know the cable isn't the issue.

Does anyone know how to fix this?

Update:

I downloaded SwitchresX but unfortunately, it did not resolve my issue.

Then I tried a DisplayPort to USB-C cable connected directly to the MacBook but this also didn't work (no video output).

However, I tried connecting the dock's DisplayPort to the monitor's HDMI (1.4) port using an adapter and it works! I only get 50Hz instead of the monitors 120hz but at this point, I'll take this as a win.

Thank you to everyone who commented, I really appreciate the help.

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  • I've not seen anything like that before, very strange. It is an ultra wide screen but within the supported resolution of the Mac. There is a utility (with a free trial, I believe) called SwitchresX which gives you some very detailed control over display settings, internal and external. You might want to try that and see what results that gives you and add those to your question. Apr 13 '21 at 13:10
  • Talk to Apple. On other sites I have seen issues with ultra wide screen monitors and macOS and M1. Apple has accepted there is an issue. Q: Does it work when connected directly to the MBP? I suspect not, but that would take the dock out of the issue.
    – Gilby
    Apr 13 '21 at 22:35
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Numerous people report issues with ultra wide screens (width > 2*height) and M1 Macs. By my count, there have been three questions here in the past week. There are 12 pages in this macrumors thread.

I suggest a web search for "Mac m1 ultrawide" to get a feel for the number of problems.

Apple have acknowledged the issue, but are yet to provide a fix. For a report on this 9to5Mac.

To reduce the number of uncertainties, try and get the monitor to work without any dock. When that fails, contact Apple.

I am aware that this "answer" does nothing to resolve problems with utrawides, it is little more than a pointer to it being a widespread issue.

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