USB C external monitors 26.6.2 problem after macOS upgrade.

I can add another data point here, might help narrow down what's actually going on.

My setup: MacBook Pro 14 (M4 Pro), connected to a Samsung Odyssey S49CG950EN (49" ultrawide) via USB-C/DisplayPort. Same story as everyone else in this thread: worked fine for months, then broke right after updating to 26.6.2.

In my case it wasn't flickering, it was total non-detection: with the monitor in its normal single-input mode, the Mac just stops seeing it entirely. Doesn't even show up in System Information > Graphics/Displays.

After a lot of testing (safe mode, deleting the windowserver plists like hohaocheng mentioned above, trying every port, a second cable, and comparing against an identical MacBook Pro still on 26.5.2 where it works perfectly with zero changes), I found two things that reliably bring it back:

• Setting the monitor's DisplayPort mode down from 1.4 to 1.1 in its own on-screen menu
• Or forcing the monitor into PBP/PIP mode, which switches it to a multi-stream signal instead of single-stream

Both point to something breaking specifically in the DisplayPort 1.4 single-stream negotiation (likely DSC related) between macOS and these newer high-bandwidth Samsung panels, not a cable or hardware fault.

One thing worth flagging for tjpatter and anyone else waiting on macOS 27: it did not fix this for me. I'm on "Golden Gate" 27.0 now and get the exact same failure at DP 1.4, same workaround needed. So it looks like it may have fixed it for some docks/configurations but not this one.

I filed a formal bug report with Apple with the full environment details and repro steps. If several of us with different Samsung monitor models are hitting the same DP 1.4 negotiation failure right after 26.6.2, it's probably one shared regression rather than a bunch of unrelated ones. Happy to share the exact repro steps if it helps anyone else file their own report.

Since both displays started doing this immediately after the macOS update, I’d check the display settings before changing cables or adapters.

For the Samsung, go to System Settings > Displays, select it, and set Refresh Rate to a fixed value instead of Variable/Adaptive if that option is available. Apple notes that Adaptive Sync can cause flickering or stuttering on some external displays.

I’d also test each display separately, connected directly to one of the rear Thunderbolt/USB-C ports on the Mac mini, with no hub or dock in between. Apple specifically recommends a direct connection when troubleshooting an external display.

If both displays are stable individually but start flashing again when both are connected, that would help narrow this down to a macOS/display configuration issue rather than two displays failing at the same time.

It seems to be a recurring issue for me after updating to 26.6.2 (25G83).

I have a similar issue where my displays connected by 2 hdmi cables no longer gets detected, and i can temporarily resolve it by deleting the following files:

/Library/Preferences/com.apple.windowserver.plist

And remove every file named:

/Library/Preferences/ByHost/com.apple.windowserver.*.plist

After that i restart my macbook, and it works like a charm for a few days before i have to remove the files again.

Both monitors are Samsung S24R35xFZ set to 75 Hertz refresh rate using the same brand of cables.

Following up... macOS 27 fixed the issue!!! Working through my docking station once again like I had it prior to the 26.6.2 update.

that Samsung S49CG950EN appears to be like a 5120 by 2160 5k display with the bottom half missing missing.

the display supports up to 10 bits/color and refresh rates 48 to 240 Hz.

Interfaces include:

2x HDMI 2.1

1x DisplayPort 1.4

For Macs with Thunderbolt-3 or -4 and USB-3 ports:

adapting from a USB-3.2 port is limited to the maximum speed of USB-3.2, which is 20 G bits/sec

for DisplayPort over USB-C, the highest standard speed that can be supported by the USB-3.2 protocol (when starting with USB-3.2) is 17.28 G Bits/sec HBR2, equivalent to DisplayPort 1.2

This is not an Apple issue. Most adapters are USB 3.2 adapters, and are Speed Limited because of the maximum speed of USB 3.2 ports.

That allows a 10 bits/color 4K HDR display to run directly at up to 60 Hz

or in your case 5120 by 1440 can run at about 70 Hz before Display Stream Compression.

to run faster, you would need a Mac with USB-4 ports (which you have) and one of the absolute latest adapters that support USB-4 speeds, and claim to support higher DisplayPort data rates, but only when used on a USB-4 port. CableMatters sells such an adapter that claims support for DisplayPort 2.1

that S34C50 display appears to be a 3440 by 1440 display (like 4K with the bottom missing). It features 10 bits/color (implemented with FRC dithering). Refresh rates to 100 Hz

interfaces include:

• 2x HDMI 2.0, which is very troubling because that will NOT support the top display specs over one cable. (fails just short of 100 Hz at 10 bits/color)

• DisplayPort 1.2 which supports HBR2, which supports up to the top specs of the display, even when derived from USB-C

I'm having the exact same issue as reported here. 26.6.2 update, connects and disconnects repeatedly. I'm also using a UGREEN 25054A Thunderbolt docking station with my setup. I also tried it with my SonnetTech docking station and separate monitors I have for my work MacBook. Both behave in the same way... Definitely points to something in macOS itself, as this is a completely different set of cables, etc.

Truly hoping macOS 27 fixes this! Will report back...