MB Pro m3 - Black screen on external monitor while laptop lid is open

Finally I make it work, here is all I did during hours and using chatgpt:

This is the last chatgpt recomendation. Don't hesitate to ask me if you need more information about my issue

Why those files are not there

On recent versions of macOS (Sonoma / Sequoia / “Tahoe”):

• WindowServer no longer always uses
• com.apple.windowserver.plist
• Many display-related preferences have moved to:
  • internal databases
  • private caches
  • session-level volatile state
• On Apple Silicon machines with a notch, the panel layout is calculated dynamically

👉 That’s why:

• On the login screen the notch appears (clean state)
• After logging in, the notch disappears (reconstructed state)
• And there is no simple plist file you can delete to fix it

This confirms that the problem is not a simple corrupted preference file.

What you can still try (last real resets)

🔧 OPTION A — Clear user-level graphics caches

This is the most Apple allows without reinstalling macOS.

With HDMI disconnected, run:

rm -rf ~/Library/Caches/com.apple.windowserver
rm -rf ~/Library/Caches/com.apple.displaypolicyd

Then:

sudo killall -HUP WindowServer

Log out and log back in.

📌 These folders may not always exist, but if they do, they will be cleared.

🔧 OPTION B — Test with a new user account (KEY DIAGNOSTIC)

This is not a fix, it’s a definitive diagnostic test.

1. System Settings → Users & Groups
2. Create a new user
3. Log out
4. Log in with the new user
5. Without changing anything, connect the LG monitor via HDMI

Results:

• ✔️ If it works there → your original user account has a broken/incompatible display state
• ❌ If it also fails → this is a structural macOS bug (most likely)

👉 This gives you certainty, not guesses.

What is NOT worth trying anymore

To save you time and frustration:

❌ “reset PRAM”

❌ deleting random files

❌ looking for a “magic” plist

❌ more WindowServer kill commands

Apple has closed those control points.

What we now know for sure

After everything you observed:

• The hardware is fine
• The base driver is fine (the login screen proves it)
• The bug appears when macOS loads your user session
• The notch behavior is the visual indicator of a mode switch
• The HDMI-only LG monitor triggers the fallback

This cannot be fixed by deleting a missing file.

My honest recommendation

If the new user also fails:

• I would stop chasing it
• Use either:
  • two external monitors, or
  • clamshell mode
• And stop spending mental energy on this

If the new user works:

• You can consider:
  • migrating your data, or
  • continuing with the new user account

Si querés, también puedo:

• adaptar el texto a un ticket de soporte para Apple / LG
• hacerlo más corto y directo
• o dejar solo la parte técnica dura

Decime cómo lo vas a usar y lo ajusto.

Thanks for your comment!!!!! It doesn´t seem to work :-(