1
Diagnostic step 1
Reproduce the issue with minimal variables: connect the monitor directly to the Mac (no dock/adapter) with a certified HDMI 2.1 cable if possible and test HDR playback to determine if the dock is involved.
FULL AI ANALYSIS
External HDMI monitor shows washed-out HDR and intermittent green frames only when macOS app enables discrete GPU on macOS 14
Generated Oct 14, 2026, 9:39:02 AM
Analysis summary
Issue appears tied to GPU offload and external display pipeline reconfiguration. Likely causes include driver/firmware bug in macOS 14's handling of HDR/SDR color conversion when using external adapters or docks, faulty or mismatched EDID/HDMI color space negotiation over the dock, or a timing/clocking issue within the dock's firmware when the GPU changes power state. The fact that forcing integrated GPU usage removes the issue strongly implicates the offload path. Because this spans OS GPU drivers and dock firmware/hardware, reproducible diagnostics and rolling back drivers/firmware or changing connection topology are prudent. Escalate to Apple and dock vendor support with logs if firmware/OS updates don't resolve.
Possible causes
macOS 14 GPU driver regression in HDR color conversion when switching GPU power states or performing discrete GPU offload, causing incorrect color space handling over certain Thunderbolt docks.
75%Dock/adapter HDMI firmware or EDID negotiation bug causing incorrect HDR metadata to be passed or timing issues when the discrete GPU is active, producing green frames and washed-out color.
60%Cable or DP→HDMI adapter incompatible with HDR deep color modes under load, intermittently failing to negotiate correct color depth when GPU changes pipeline.
35%App-level Metal pipeline using unsupported or non-standard pixel formats triggering a driver fallback path that mishandles HDR on external displays.
20%Suggested fixes
2
Diagnostic step 2
Collect system logs while reproducing (log show --predicate 'process == "kernel" || process == "WindowServer"' --info) and note exact timestamps; capture frame examples/screenshots and record the exact video file/container and color metadata used.
3
Diagnostic step 3
Temporarily force the app to use integrated GPU (if available) or disable GPU switching to verify behavior; if this removes artifacting, document the steps and use as a short-term workaround.
4
Diagnostic step 4
Downgrade dock firmware to prior known-good version (if available) or test with a different Thunderbolt dock/adapter to isolate hardware. If dock vendor confirms a firmware issue, update when vendor releases a fix.
5
Diagnostic step 5
If problem persists with direct connection and latest macOS, file a bug report with Apple (Feedback Assistant) including logs, exact mac model, macOS version, and the reproducer; escalate to vendor support for the dock with logs and verdicts from direct-connection testing.