Scroll speeds are solved against display.hardware.limit_refresh_rate_hz,
which is only a ceiling. A panel that cannot reach it still moves whole
pixels per frame, but every scroll runs slow by the shortfall and the
"smooth" ladder is the cap's, not the panel's. A user rig (Pi 4, 2x128x64,
adafruit-hat-pwm, pwm_bits 9, gpio_slowdown 5) measured 107.6-113.1 Hz under
a 120 Hz cap: 60 px/s ran at 55, and nothing said why.
- scroll_config: refresh_shortfall() (more than 3% under the planned rate),
holdable_cap() (a multiple of 10, 5% under the measurement, since the
measurement is the fast end of an uncapped panel's drift), and
describe_refresh_shortfall().
- FrameTimingRecorder.plan_refresh(): once the measured period has held for
three trusted windows, a shortfall is logged once as a warning naming the
cap to use. DisplayManager calls it only for a real panel, not the
emulator or the fallback canvas. The stats file records
planned_refresh_hz (additive).
- GET /api/v3/config/refresh-rate, plus a hint under the Display tab's
Limit Refresh Rate field with a button that fills in the suggested cap.
- _panel_refresh_hz (behind the Vegas slider's advice) ignores a measurement
written under a different cap, so a changed cap stops being advised from
the old rate before the display restarts.
Verified on ledpi with a temporary 200 Hz cap: the warning logged about a
minute after the restart ("about 132 Hz ... Set Limit Refresh Rate to
120 Hz"), the endpoint returned the same shortfall, and the Display tab
showed the hint; its button filled in 120. ledpi was restored afterwards.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
