MacBook Pro M3: External display grays out with two USB-C monitors
Summary: With two external displays connected simultaneously (one via native HDMI, one via USB-C/Thunderbolt DisplayPort Alt-Mode), the USB-C-connected display intermittently goes blank/black/gray while macOS continues to report it as active and connected. Root cause traced to a failing HDCP re-authentication cycle on the DP Alt-Mode link. Single-display configurations (either port alone) work perfectly.
System:
- MacBook Pro 14", Nov 2023, Apple M3 (base, non-Pro/Max), 8GB unified memory
- macOS Tahoe 26.5.1
- Ports used: native HDMI + 1x USB-C/Thunderbolt (direct or via powered USB-C hub)
- Monitor A: MSI MAG272, HDMI, own PSU
- Monitor B: USB-C monitor, DisplayPort Alt-Mode, bus-powered
Symptom:
- Reproducible on both wake-from-sleep and full reboot
- Only occurs with 2 external displays connected — a single display on either port is fully stable
- The blanking display is always whichever one is on USB-C/Thunderbolt DP Alt-Mode, never the one on native HDMI, regardless of which physical monitor is in that role
- macOS never fires a display-disconnect event during the failure — System Information, displayplacer list, and WindowServer logs all continue to show the display as active/connected the entire time
Root cause (from log show, kernel + AppleGraphicsControl subsystem):
- WindowServer enumerates and configures both displays correctly at connect time — the failure isn't a hotplug/dropout at that layer, which is why it still reports "connected" throughout.
- AppleDCPDPTXHDCP2Controller periodically re-authenticates HDCP on the link, roughly every 15–20 seconds.
- That re-authentication intermittently fails with abortHDCPNegotiation status=0xe00002eb. Instead of falling back cleanly to HDCP1, the entire AV link (video and audio) tears down via stopLinkGated — the panel goes dark, but macOS's display registry never re-fires a hotplug/disconnect event, so it keeps reporting the display as active.
- Each failed cycle is preceded by a AppleUSBHostPort::cableChangeOccurred: powering on → powering off phantom blip, with no preceding software trigger found in kernel/powerd/assertion logs in the 2-second window before it fires. This blip only appears when 2 external displays are active — never with one.
Tests performed (all ruled out):
TestResultBoth displays forced to 60Hz (bandwidth theory)No changeSwapped which monitor connects firstBlanking follows connection order, not monitor identityStaggered connection (~15s gap)No changeSwapped Mac USB-C port for the USB-C monitorNo changeSwapped USB-C monitor's cableNo changeCharged via MagSafe instead of USB-C hub (rules out PD power-budget contention)No changeCleared cached WindowServer display prefs (com.apple.windowserver*.plist) + rebootNo changeUSB-C monitor via USB-C hub's HDMI passthrough insteadStill blankSwapped which monitor uses native HDMI vs. USB-CWhichever monitor is on native HDMI is stable; whichever is on USB-C/Thunderbolt DP Alt-Mode blanks — confirms this is specific to the USB-C Alt-Mode HDCP path, not a monitor or cable defect
Current workaround: Keep the higher-priority display on the native HDMI port. Whatever's on USB-C/Thunderbolt DP Alt-Mode will intermittently blank during use, sleep/wake, and especially reboot. No known way to stabilize two simultaneous USB-C DP Alt-Mode HDCP links on this machine/OS combination.
[Re-Titled by Moderator]
Original Title: MacBook Pro 14" M3 (2023) — External display blanks/grays out only when 2 displays connected via USB-C DP Alt-Mode (macOS Tahoe 26.5.1) — HDCP re-auth abort confirmed in logs
MacBook Pro 14″, macOS 26.5
