Clamshell mode external display stays black

After this, closing the lid with an external display attached goes straight to a working desktop, every time, without a wake-up ritual.

Why this happens

Apple's documented behavior is that a MacBook with the lid closed will only stay awake and drive an external display if it's connected to AC power and has a keyboard, mouse, or trackpad already paired and awake at the moment the lid shuts — on battery power alone, closing the lid always sleeps the Mac regardless of what's plugged into it, by design, not as a bug. The most common way this trips people up is a Bluetooth keyboard or mouse that itself went to sleep or lost its connection a few seconds before the lid closes; from the Mac's side that looks identical to having no input device at all, so it sleeps.

A second, distinct cause is the external display itself defaulting to the wrong input source. Many monitors and most docking-style displays with more than one video input will switch back to whichever source they last displayed if the Mac's signal drops even briefly during the wake process — the Mac is technically awake and outputting video, but the monitor is looking at HDMI 2 instead of the DisplayPort input the Mac is using, and reports it as a Mac problem when it's a monitor-side input selection. Rule this out early since no macOS setting fixes a monitor looking at the wrong port.

Fix it

1. Confirm the two hard requirements before anything else: the Mac's power adapter is plugged in and actually charging (not just connected to a dead outlet or a non-charging hub port), and a Bluetooth or wired keyboard/mouse is awake and responding right before you close the lid. Wiggle the mouse or press a key first, then close the lid — don't close it and immediately reach for the input device, since that's the exact sequence that fails.

2. Rule out the monitor's own input selection: with the lid open and the Mac clearly showing on the external display, close and reopen the lid once. If the display goes black on lid-close but comes back correctly the moment you reopen the lid, the display itself is fine and the Mac is genuinely sleeping — go to step 3. If it never comes back even with the lid open, check the monitor's on-screen input menu and confirm it's set to the port the Mac is actually using.

3. If the requirements in step 1 are genuinely met and it's still going black, wake the Mac manually rather than assuming it should have woken itself: press any key on the connected external keyboard, click the external mouse, or briefly press the Mac's own power button (this wakes it without triggering a restart on Apple Silicon). If this reliably brings the external display back, the Mac was correctly asleep and something is delaying the wake trigger reaching it — check that the keyboard/mouse's Bluetooth connection is stable and not re-pairing on every wake.

4. If step 3 doesn't bring it back and the Mac needs a full lid-open to recover, confirm nothing is holding the Mac in a sleep-adjacent state that's interfering with the display handshake specifically:

   ● Reads the system

   What it does: lists anything currently preventing normal sleep/wake behavior.

   After: nothing changes, this only reads.

   pmset -g assertions

   Copy

   Output

   Assertion status system-wide:
      PreventUserIdleDisplaySleep    0
      PreventSystemSleep             0
      PreventUserIdleSystemSleep     0

   Captured live on the verification machine — all zero, meaning nothing unusual is interfering. If a value here is non-zero and traces back to a specific app, that app is worth quitting before you next test clamshell mode; see the linked sleep-assertions article below for reading this output in full.

5. If the display still won't come back even after a manual wake attempt, force a clean handshake by restarting the window server. This logs your current session out immediately — save your work first.

   ▲ Changes running state

   What it does: force-restarts the display server (WindowServer), which redoes every display handshake.

   After: your GUI session ends immediately and you land back at the login screen — any unsaved work in open apps is at risk exactly as it would be from a forced logout.

   sudo killall WindowServer

   Copy

   Revert: nothing to restore — WindowServer relaunches on its own and you log back in normally.

Still broken?

• Mac won't go to sleep on its own — the opposite-sounding symptom shares the same power-assertion machinery, worth reading if clamshell behavior is inconsistent rather than always-black.
• External monitor not detected after sleep/wake — if the display comes back but shows the wrong resolution or takes several seconds, that's the handshake issue, not the clamshell power requirements.

Verified on

The Mac mini row is a desktop and isn't relevant to clamshell mode itself, but was used to confirm the pmset -g assertions output format above is unchanged on 27.0. Note that clamshell mode with a single external display has worked on every Apple Silicon chip since launch — it's the AC power and awake-input-device requirements that trip people up, not a chip-tier limitation like the second-external-display cases covered elsewhere on this site.

Related problems

• Mac won't sleep even with the lid closed
• External monitor not detected after sleep/wake