Brand new MacBook Pro M4 suddenly no longer detecting 2 external monitors

I don't like to have to resort to deleting obscure .plist files,

but when that is the only way forward, we need the long-term record to reflect the correct files and location.

So unless someone knows better I will assume that the previously-posted:

delete-

~/Library/Preferences/ByHost/apple.windowserver.displays.<hex_string>.plist and reboot.

SHOULD ACTUALLY READ:

delete-

~/Library/Preferences/ByHost/com.apple.windowserver.displays.<hex_string>.plist and reboot

Wait.

They CAN"T sell ALL show the menubar unless you are using a feature of Mission Control called "Displays have Separate Spaces" and in that case, each Mission Control "Space" is a free and independent working set with its own menubar, its own assigned Applications, and its own documents.

Settings > Desktop &Dock > Mission Control ...

displays have separate Spaces <--- turn that OFF if you want to use Extended Desktop

<< one directly into the HDMI port and 1 into a dongle into the USB-C. >>

the USB-C portion of that dongle adapter/cable needs to be one meter or shorter for consistent detection and operation at those resolutions.

So both displays show a picture, put the two are not distinct displays?

Could they have become Mirrored?

nmview82 wrote:

I have an LG 27UL550-W monitor that is 4K UHD 60Hz and an LG 24UD58-B 4K UHD 56-61Hz.

Nothing obviously wrong here.

LG – 27” 4K UHD UltraFine™ IPS Monitor with HDR10 and Ergonomic Stand (27UL550-W)

3840x2160 pixels, 10-bit-per-channel color depth, 60 Hz refresh rate, with DisplayPort and HDMI inputs.

LG – 24" Class 4K UHD IPS LED Monitor (24" Diagonal) (24UD58-B)

3840x2160 pixels, 10-bit-per-channel color depth, 60 Hz refresh rate, with DisplayPort and HDMI inputs.

I don't know if the AMD FreeSync settings on these monitors would affect anything. Presumably the Mac will try to drive them at a constant refresh rate. If HDMI 2.0 is in play anywhere (on the Mac, on the adapter, on the monitor), there might be a tradeoff between 10-bit-per-channel color @ 30 Hz and 8-bit-per-channel-color @ 60 Hz (due to HDMI bandwidth constraints that are not Mac-specific). That presumably wouldn't keep you from getting a signal.

nmview82 wrote:

Everything was working perfectly fine last week.

By any chance, did you update Sequoia since then?

Grant Bennet-Alder wrote:

Are you certain that filename is correct?

Everything in ByHost folder on my computer starts with

com.

???

On my system, if I look in ~/Library/Preferences/ByHost/, I see:

% ls *windowserver*
com.apple.windowserver.displays.HEXADECIMAL_STRING.plist

You can tell that the naming convention for many of these files is to take an Internet domain (such as apple.com), reverse it, and use it as a prefix. As long as each vendor uses a prefix corresponding to one of their own domain names, this has the benefit of letting vendors independently choose the rest of their preference file names without having to worry about accidental duplication of filenames used by other vendors.

the tilde character is also important. it is an active shortcut for "current user"

/Library is the main library folder. it does not usually have ByHost folder in it.

~/Library is the Library folder in the current user directory. It typically DOES have a ByHost folder

That ByHost folder is used to allow you to get your way when you use a different computer with the same preference files. I know that's obscure, but it supports logging in to any Host on a computer cluster, such as at a school.

<< is that something that would turn on by itself? >>

it is certainly not supposed to.

But anything is possible, just not very likely.

Are you certain that filename is correct?

Everything in ByHost folder on my computer starts with

com.

???