LockDown Browser External Monitor on Mac (Detection + Workarounds 2026)
How the detection works
LDB calls CGDisplayActiveDisplayCount at launch. This is the same Core Graphics API that System Information uses to enumerate displays. It returns the count of currently-active displays in the macOS display list.
Returns ≥2 → LDB shows "Multiple displays detected" error (default Dashboard config) or "Multiple displays detected - close them and continue" warning (softer config).
What counts as an "external display"
| Setup | Detected? |
|---|---|
| HDMI cable to external monitor | ✓ Detected |
| DisplayPort cable to external monitor | ✓ Detected |
| USB-C / Thunderbolt cable to display | ✓ Detected |
| USB-C dock with display passthrough | ✓ Detected |
| Apple Studio Display, Pro Display XDR | ✓ Detected |
| Sidecar (iPad as second display) | ✓ Detected - see Sidecar page |
| AirPlay to a TV / Apple TV | ✓ Detected |
| Closed-Lid Mode (lid closed, only external) | ✗ Not detected (single-display) |
| iPad next to laptop NOT in Sidecar | ✗ Not detected (separate device) |
| External display attached but DISABLED in System Settings | ✗ Not detected (inactive) |
Why instructors enable the block
A second monitor allows test-takers to display reference material outside the camera's view. Monitor records what the camera sees, which is the laptop screen + your face/torso, but not adjacent monitors. The hard block sidesteps this by requiring single-display.
The reliable fix: disconnect before launch
- Quit any open LDB session.
- Disconnect physical cables: unplug HDMI, DisplayPort, USB-C-to-display from your laptop.
- Disable Sidecar: System Settings → Displays → click the iPad → Disconnect.
- Disable AirPlay: Control Center → Screen Mirroring → off.
- Verify in System Settings → Displays that only your built-in display is listed.
- Wait 10 seconds for macOS to fully release the additional display.
- Launch LDB from your LMS.
Re-enable peripherals after the exam.
Soft-block configuration (warning instead of error)
Some institutions configure the Dashboard to warn but not block on multi-display. Symptom: LDB launches with a "Multiple displays detected" warning that you click through. Recommendation: still disconnect external displays even when warned-only - the warning is logged in the Monitor recording timeline and visible to the instructor reviewing.
Workaround: closed-lid mode
If you have a desk setup where you want to use only the external display (with the MacBook lid closed), this works:
- Plug your laptop into power (closed-lid mode requires AC power).
- Connect external display + USB keyboard + USB mouse.
- Sign in to your laptop normally with lid open.
- Close the laptop lid. After ~10 seconds, the external display becomes the only active display.
- Launch LDB. CGDisplayActiveDisplayCount returns 1; no block.
This is sometimes preferable to disconnecting the external entirely if your camera + microphone are USB-attached to the dock.
What about projector / classroom display?
Projectors connected via HDMI / VGA / wireless count as external displays. If you're taking the exam in a lab + the room has a projector on, disconnect or disable it before launch.
Frequently asked questions
My external monitor is plugged in but I'm not using it. Does LDB still detect it?
Yes - physical connection + active in macOS's display list = detected. Even an unused external display is "active" if the cable is plugged in. Unplug.
Can I use the laptop screen as the only display while the external is plugged in?
Mirroring (same content on both) sometimes shows as 1 display, sometimes 2, depending on macOS version. Safest: disconnect the external entirely.
Will LDB detect the second display if it's on a different desk?
Physical location doesn't matter - only whether macOS reports the display as active via CGDisplayActiveDisplayCount. If the cable is plugged in and macOS lists the display, LDB sees it.