A regression in macOS on M4 and M5 Apple Silicon chips prevents full-resolution HiDPI modes on 4K external displays. While M2/M3 machines support 3840x2160 HiDPI (requiring a 7680x4320 backing store at 2x scale), M4/M5 caps the HiDPI backing store at ~1.75x native resolution (6720x3780), limiting users to 3360x1890 HiDPI. The limitation is not a hardware constraint — the M5 Max officially supports 8K at 60Hz — but a conservative framebuffer pre-allocation policy in the GPU driver (AppleDisplayCrossbar) that reserves headroom for hypothetical future display connections. Extensive troubleshooting confirmed no userspace workaround exists: display override plists, EDID patching, IOKit registry overrides, and private SkyLight APIs all fail. The fix requires Apple to implement smarter per-display framebuffer allocation or provide a user override option.
Table of contents
Environment and Test Setup #Diagnosis and Troubleshooting #Hypothesis: Where the Limit Is Applied #What Could Address This #Appendix: Diagnostic Commands and Output #References #Sort: