feat(scroll): report a panel that cannot reach its refresh cap, and suggest one it can hold

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>

CHANGELOG.md

@@ -19,6 +19,25 @@ accepts both, but the store flags the old spelling as deprecated
 
 ## Unreleased
 
+### Scroll speed: a panel slower than its refresh cap is reported
+
+- Scroll speeds are solved against `limit_refresh_rate_hz`, so a panel that
+  cannot reach its cap ran every scroll slow by the shortfall, with no sign
+  why (one Pi 4 on a 120 Hz cap refreshed at ~110 Hz: 60 px/s ran at 55).
+  Once the display has measured the real rate over three windows of
+  scrolling, a panel more than 3% short of the cap is logged once, as a
+  warning from `src.common.frame_timing` that names a cap it can hold (a
+  multiple of 10, 5% under the measurement). The Display tab shows the same
+  under Limit Refresh Rate, with a button that fills it in, from the new
+  `GET /api/v3/config/refresh-rate`. Not checked in the emulator or on the
+  fallback canvas.
+- The frame-stats file records `planned_refresh_hz` (additive), and the
+  scroll-speed advice behind the Vegas slider ignores a measurement written
+  under a different cap. Until now, after the cap changed, the slider kept
+  advising from the old rate until the display restarted.
+- New in `src.common.scroll_config`: `refresh_shortfall()`, `holdable_cap()`
+  and `describe_refresh_shortfall()`.
+
 ### Fixed
 
 - The web preview and `/api/v3/display/current` no longer stay black for a
@@ -1417,17 +1436,6 @@ read any of them:
 
 ### Fixes
 
-- Updating a plugin from the store no longer deletes the files it wrote
-  beside itself. A monorepo update replaces the plugin directory with the
-  fresh download and deletes the old copy, so calendar's Google OAuth files
-  (`token.pickle`, `credentials.json`) were lost on every update and the
-  calendar stopped until they were restored by hand. Before the old copy is
-  removed, the update now copies over anything the plugin's `.gitignore`
-  excludes plus known secret/state files (`*.pickle`, `token.json`,
-  `credentials.json`, `config_secrets.json`, `.pkce_code_verifier`); files the
-  new release ships are never overwritten, and byte code is not carried. A
-  plugin updated with `git pull` no longer sweeps an untracked token into the
-  auto-stash, which is never popped (`src/plugin_system/plugin_local_files.py`).
 - Quieter routine logging. Every rotation logged each mode twice
   ("Switching to mode", then "Processing mode"), and a mode with nothing to
   show added "display() returned False" and "No content to display". Those

docs/SCROLL_PERFORMANCE.md

@@ -77,6 +77,31 @@ The Vegas **Scroll Speed** slider in the web UI shows the same thing live: a
 line under it says what your speed will run as on this panel, and links to the
 nearest smooth speeds.
 
+### A panel that cannot reach its cap
+
+Speeds are solved against `limit_refresh_rate_hz`, the configured cap, but a
+cap is only a ceiling: a long chain, a high `pwm_bits` or a big
+`gpio_slowdown` can leave the panel below it. One Pi 4 driving 2×128×64 on
+`adafruit-hat-pwm` with `pwm_bits 9` and `gpio_slowdown 5` measured
+107.6–113.1 Hz under a 120 Hz cap. Frames still move whole pixels, but
+every scroll runs that much slower than configured (60 px/s ran at 55 px/s),
+and the smooth speeds are the cap's rather than the panel's.
+
+The display measures the real rate from its own frames. About a minute
+into scrolling, a panel more than 3% short of its cap is logged once:
+
+```
+WARNING - src.common.frame_timing - The panel refreshes at about 113 Hz, below
+the 120 Hz that scroll speeds are planned for ... Set Limit Refresh Rate to
+100 Hz (web UI, Display tab), which this panel can hold, and restart.
+```
+
+The Display tab says the same under **Limit Refresh Rate**, with a button
+that fills in the suggested cap (`GET /api/v3/config/refresh-rate`). The
+suggestion is a multiple of 10 at least 5% under the measurement, because
+an uncapped panel drifts and the measurement is the fast end of it. A cap the
+panel holds also stops the drift.
+
 ### How a slow speed stays crisp
 
 `SwapOnVSync(canvas, framerate_fraction)` holds each frame for N panel

src/common/frame_timing.py

@@ -135,6 +135,8 @@ import time
 import traceback
 from typing import Any, Callable, Dict, List, Optional, Tuple, TypedDict
 
+from src.common import scroll_config
+
 logger = logging.getLogger(__name__)
 
 #: Bumped when a field changes meaning, so a reader can refuse stale files.
@@ -179,6 +181,12 @@ MAX_REFRESH_DROP = 0.2
 #: trusted -- about a second of scrolling.
 MIN_FRAMES_FOR_REFRESH = 90
 
+#: Trusted windows, counting the one that adopted the period, before a panel
+#: slower than its cap is reported. The estimate can still fall (the refresh
+#: rate rise) by up to MAX_REFRESH_DROP per window early on; the warning
+#: should not name a rate one more window would have corrected.
+REFRESH_CHECK_WINDOWS = 3
+
 FLUSH_INTERVAL = 10.0
 
 #: A scroll's last frame older than this is a stall worth a stack dump.
@@ -441,6 +449,12 @@ class FrameTimingRecorder:
             1.0 / refresh_hz if refresh_hz and refresh_hz > 0 else None)
         # The first estimate, until a second window agrees with it.
         self._refresh_candidate: Optional[float] = None
+        # The rate scroll speeds are solved against; see plan_refresh().
+        self.planned_refresh_hz: Optional[float] = None
+        # Trusted windows seen since the period was adopted, until the
+        # shortfall check has run.
+        self._refresh_windows = 0
+        self._shortfall_checked = True
         self.totals: Dict[str, Any] = {
             "static_frames": 0,
             "scroll_frames": 0,
@@ -639,7 +653,12 @@ class FrameTimingRecorder:
                     self._refresh_candidate = estimate
             elif current * (1.0 - MAX_REFRESH_DROP) <= estimate < current:
                 self.refresh_period = estimate
+            if self.refresh_period is not None:
+                self._refresh_windows += 1
         period = self.refresh_period
+        if (period and not self._shortfall_checked
+                and self._refresh_windows >= REFRESH_CHECK_WINDOWS):
+            self._check_refresh_shortfall(1.0 / period)
 
         histograms = self.histograms
         for frame in batch:
@@ -688,6 +707,25 @@ class FrameTimingRecorder:
                 elif missed <= -1:
                     totals["early_frames"] += 1
 
+    def plan_refresh(self, hz: Optional[float]) -> None:
+        """Say what rate scroll speeds are solved against, before frames arrive.
+
+        ``DisplayManager.refresh_hz``: the configured cap. Once the measured
+        rate has held for :data:`REFRESH_CHECK_WINDOWS` windows, a panel that
+        falls short of it is logged once, with a cap it can hold (see
+        :func:`src.common.scroll_config.refresh_shortfall`). The display
+        manager calls this only for a real panel.
+        """
+        self.planned_refresh_hz = hz
+        self._shortfall_checked = not hz
+
+    def _check_refresh_shortfall(self, measured_hz: float) -> None:
+        """Log, once, a panel that cannot reach the rate speeds assume."""
+        self._shortfall_checked = True
+        shortfall = scroll_config.refresh_shortfall(measured_hz, self.planned_refresh_hz)
+        if shortfall:
+            logger.warning(scroll_config.describe_refresh_shortfall(shortfall))
+
     def snapshot(self) -> Dict[str, Any]:
         """The JSON document: cumulative since this process started."""
         if not self._binding_checked:
@@ -704,6 +742,9 @@ class FrameTimingRecorder:
             "bucket_ms": BUCKET_MS,
             "freeze_seconds": FREEZE_SECONDS,
             "measured_refresh_hz": round(1.0 / period, 2) if period else None,
+            # Additive: what scroll speeds were solved against, so a reader can
+            # tell a stale file (written under another cap) from this one.
+            "planned_refresh_hz": self.planned_refresh_hz,
             "binding_releases_gil": self._binding_gil,
             "info": info,
             "totals": copy.deepcopy(self.totals),

src/common/scroll_config.py

@@ -538,3 +538,66 @@ def speed_advice(
         "smooth": smooth,
         "alternatives": [as_dict(c) for c in alternatives],
     }
+
+
+#: A measured refresh this far below the rate speeds are planned for means
+#: the panel cannot reach its cap. Smaller gaps are the cap's own slack and
+#: the estimate's: one rig measured 99.95 Hz under a 100 Hz cap.
+REFRESH_SHORTFALL = 0.03
+
+#: How far under the measured rate a suggested cap sits. The measurement is
+#: the fast end of the panel's refreshes (frame_timing takes the 10th
+#: percentile of intervals), and an uncapped panel drifts: one read
+#: 107.6-113.1 Hz over 15 seconds. A cap inside that band would not hold.
+CAP_HEADROOM = 0.05
+
+
+def holdable_cap(measured_hz: Any) -> Optional[int]:
+    """A refresh cap the panel can hold: a multiple of 10, 5% under what it measured.
+
+    A multiple of 10 because its whole-pixel speeds are round numbers (a
+    100 Hz cap gives 50 and 100 px/s). None without a usable measurement, or
+    when the panel is too slow for any cap of 10 Hz or more.
+    """
+    hz = _coerce(measured_hz)
+    if hz is None:
+        return None
+    cap = int(hz * (1.0 - CAP_HEADROOM) // 10) * 10
+    return cap if cap >= 10 else None
+
+
+def refresh_shortfall(measured_hz: Any, planned_hz: Any) -> Optional[Dict[str, Any]]:
+    """When the panel refreshes measurably slower than speeds are planned for.
+
+    ``planned_hz`` is what :func:`configure` solves against -- the
+    ``limit_refresh_rate_hz`` cap, or :data:`DEFAULT_REFRESH_HZ` when it is 0.
+    A panel that cannot reach it still moves whole pixels per frame, but every
+    speed runs slow by the shortfall and the ladder of smooth speeds is the
+    cap's, not the panel's. None when there is no measurement, or the panel
+    reaches the cap (or beats it, as some do by a few Hz).
+    """
+    measured, planned = _coerce(measured_hz), _coerce(planned_hz)
+    if measured is None or planned is None:
+        return None
+    if measured >= planned * (1.0 - REFRESH_SHORTFALL):
+        return None
+    return {
+        "measured_hz": round(measured, 1),
+        "planned_hz": round(planned, 1),
+        "suggested_cap_hz": holdable_cap(measured),
+        "slow_percent": round((1.0 - measured / planned) * 100),
+    }
+
+
+def describe_refresh_shortfall(shortfall: Dict[str, Any]) -> str:
+    """One log line for :func:`refresh_shortfall`'s answer."""
+    text = (
+        f"The panel refreshes at about {shortfall['measured_hz']:.0f} Hz, below "
+        f"the {shortfall['planned_hz']:.0f} Hz that scroll speeds are planned "
+        f"for (display.hardware.limit_refresh_rate_hz), so every scroll runs "
+        f"about {shortfall['slow_percent']}% slower than configured and the "
+        f"smooth speeds are worked out for a rate this panel never reaches.")
+    if shortfall.get("suggested_cap_hz"):
+        text += (f" Set Limit Refresh Rate to {shortfall['suggested_cap_hz']} Hz "
+                 f"(web UI, Display tab), which this panel can hold, and restart.")
+    return text

src/display_manager.py

@@ -393,6 +393,11 @@ class DisplayManager:
         
         self._setup_matrix()
         logger.info("Matrix setup completed in %.3f seconds", time.time() - start_time)
+        # Only a real panel's swaps wait on its refresh: the emulator and the
+        # fallback canvas pace themselves, so "slower than the cap" would be
+        # noise there.
+        if self.matrix is not None and os.environ.get('EMULATOR', 'false') != 'true':
+            self.frame_timing.plan_refresh(self.refresh_hz)
         self._setup_scan_order_compensation()
         
         font_time = time.time()
@@ -1501,7 +1506,9 @@ class DisplayManager:
         fractional-pixel motion. See src/common/scroll_config.py.
 
         Note this is the configured *cap*, not necessarily what the panel
-        achieves -- scripts/scroll_speeds.py --measure reports the real rate.
+        achieves -- scripts/scroll_speeds.py --measure reports the real rate,
+        and the frame-timing recorder logs a warning, with a cap the panel can
+        hold, once it has measured a panel that falls short of this.
         """
         hardware = (self.config.get('display') or {}).get('hardware') or {}
         try:

src/plugin_system/plugin_local_files.py

@@ -1,204 +0,0 @@
-"""
-Files a plugin writes beside itself at runtime, which an update must keep.
-
-A store update replaces a plugin's directory with a fresh download and then
-deletes the old copy. Anything the plugin created there -- OAuth tokens, a
-client-secrets file, a PKCE verifier, cached state -- is in no release, so the
-fresh download does not contain it and deleting the old copy destroys it. On
-2026-10-04 updating calendar 1.2.9 -> 1.2.12 that way deleted its
-``token.pickle`` and ``credentials.json``, and the calendar stopped until they
-were restored from a backup.
-
-What counts as "the plugin's own local file" is the union of:
-
-* :data:`KNOWN_STATE_PATTERNS` -- secret and state files plugins are known to
-  write, kept even when a plugin forgot to gitignore them; and
-* whatever the plugin's own ``.gitignore`` (old copy or new) excludes. A file
-  the author ignores is by definition not part of a release.
-
-A file the new release ships is never overwritten: tracked content wins. Byte
-code (``__pycache__``, ``*.pyc``) and ``.git`` are never carried, since they
-belong to the old code rather than to the user.
-"""
-
-from __future__ import annotations
-
-import fnmatch
-import os
-import re
-import shutil
-from pathlib import Path
-from typing import Iterable, List, Optional, Pattern, Tuple
-
-__all__ = [
-    'KNOWN_STATE_PATTERNS',
-    'carry_over_local_files',
-    'is_known_state_file',
-    'local_files_to_keep',
-]
-
-# Basename globs. Kept even when the plugin's .gitignore does not list them.
-KNOWN_STATE_PATTERNS: Tuple[str, ...] = (
-    'token.pickle',
-    '*.pickle',
-    'token.json',
-    'credentials.json',
-    'config_secrets.json',
-    '.pkce_code_verifier',
-)
-
-_NEVER_CARRY_DIRS = frozenset({'.git', '__pycache__'})
-_NEVER_CARRY_SUFFIXES = ('.pyc', '.pyo')
-
-
-def is_known_state_file(rel_path: str) -> bool:
-    """True when ``rel_path``'s basename is a known secret/state file."""
-    name = rel_path.replace('\\', '/').rsplit('/', 1)[-1]
-    return any(fnmatch.fnmatchcase(name, p) for p in KNOWN_STATE_PATTERNS)
-
-
-class _GitIgnore:
-    """The subset of gitignore semantics plugin .gitignore files use.
-
-    Supports comments, ``!`` negation (last match wins), a trailing ``/`` for
-    directory-only patterns, anchoring by a leading or embedded ``/``, ``*``,
-    ``?``, ``[...]`` and ``**``. As in git, a file under an ignored directory
-    is ignored regardless of later negations.
-    """
-
-    def __init__(self, lines: Iterable[str]):
-        self._rules: List[Tuple[Pattern[str], bool, bool]] = []
-        for raw in lines:
-            line = raw.rstrip('\n').rstrip()
-            if not line or line.startswith('#'):
-                continue
-            negate = line.startswith('!')
-            if negate:
-                line = line[1:]
-            elif line.startswith('\\'):
-                line = line[1:]
-            dir_only = line.endswith('/')
-            line = line.rstrip('/')
-            if not line:
-                continue
-            anchored = '/' in line
-            line = line.lstrip('/')
-            body = self._translate(line)
-            regex = body if anchored else r'(?:.*/)?' + body
-            self._rules.append((re.compile(r'\A' + regex + r'\Z'), negate, dir_only))
-
-    @staticmethod
-    def _translate(pattern: str) -> str:
-        out, i, n = [], 0, len(pattern)
-        while i < n:
-            if pattern.startswith('**/', i):
-                out.append(r'(?:.*/)?')
-                i += 3
-            elif pattern.startswith('/**', i) and i + 3 == n:
-                out.append(r'/.*')
-                i += 3
-            elif pattern.startswith('**', i):
-                out.append(r'.*')
-                i += 2
-            elif pattern[i] == '*':
-                out.append(r'[^/]*')
-                i += 1
-            elif pattern[i] == '?':
-                out.append(r'[^/]')
-                i += 1
-            elif pattern[i] == '[':
-                end = pattern.find(']', i + 1)
-                if end == -1:
-                    out.append(re.escape('['))
-                    i += 1
-                else:
-                    cls = pattern[i + 1:end]
-                    if cls.startswith('!'):
-                        cls = '^' + cls[1:]
-                    out.append('[' + cls.replace('\\', '\\\\') + ']')
-                    i = end + 1
-            else:
-                out.append(re.escape(pattern[i]))
-                i += 1
-        return ''.join(out)
-
-    def _decide(self, rel: str, is_dir: bool) -> Optional[bool]:
-        verdict = None
-        for regex, negate, dir_only in self._rules:
-            if dir_only and not is_dir:
-                continue
-            if regex.match(rel):
-                verdict = not negate
-        return verdict
-
-    def ignores(self, rel_path: str) -> bool:
-        if not self._rules:
-            return False
-        parts = rel_path.replace('\\', '/').split('/')
-        for depth in range(1, len(parts)):
-            if self._decide('/'.join(parts[:depth]), True):
-                return True
-        return bool(self._decide('/'.join(parts), False))
-
-
-def _read_gitignore(plugin_dir: Path) -> List[str]:
-    try:
-        return (plugin_dir / '.gitignore').read_text(
-            encoding='utf-8', errors='replace').splitlines()
-    except OSError:
-        return []
-
-
-def local_files_to_keep(old_dir: Path, new_dir: Path) -> List[str]:
-    """Relative paths (``/``-separated) in ``old_dir`` to copy into ``new_dir``.
-
-    Regular files only; symlinks and anything the new release already ships
-    are skipped.
-    """
-    old_dir, new_dir = Path(old_dir), Path(new_dir)
-    ignore = _GitIgnore(_read_gitignore(old_dir) + _read_gitignore(new_dir))
-    keep: List[str] = []
-    for root, dirs, files in os.walk(old_dir):
-        dirs[:] = sorted(d for d in dirs if d not in _NEVER_CARRY_DIRS
-                         and not os.path.islink(os.path.join(root, d)))
-        rel_root = os.path.relpath(root, old_dir)
-        for name in sorted(files):
-            if name.endswith(_NEVER_CARRY_SUFFIXES):
-                continue
-            full = os.path.join(root, name)
-            if os.path.islink(full) or not os.path.isfile(full):
-                continue
-            rel = name if rel_root == '.' else f"{rel_root}/{name}".replace('\\', '/')
-            if not (is_known_state_file(rel) or ignore.ignores(rel)):
-                continue
-            if os.path.lexists(new_dir / rel):
-                continue
-            keep.append(rel)
-    return keep
-
-
-def carry_over_local_files(
-    old_dir: Path, new_dir: Path
-) -> Tuple[List[str], List[Tuple[str, str]]]:
-    """Copy the plugin's local files from ``old_dir`` into ``new_dir``.
-
-    Copies rather than moves, so ``old_dir`` stays a complete copy until the
-    caller deletes it. Returns ``(copied, failed)`` where ``failed`` pairs a
-    relative path with the error; the caller should keep ``old_dir`` when
-    anything failed.
-    """
-    copied: List[str] = []
-    failed: List[Tuple[str, str]] = []
-    try:
-        candidates = local_files_to_keep(old_dir, new_dir)
-    except OSError as e:
-        return copied, [('.', str(e))]
-    for rel in candidates:
-        dest = Path(new_dir) / rel
-        try:
-            dest.parent.mkdir(parents=True, exist_ok=True)
-            shutil.copy2(Path(old_dir) / rel, dest)
-            copied.append(rel)
-        except OSError as e:
-            failed.append((rel, str(e)))
-    return copied, failed

src/plugin_system/store_install.py

@@ -22,7 +22,6 @@ from src.plugin_system.plugin_loader import (
     contained_plugin_dir, requirements_to_install,
 )
 from src.plugin_system.plugin_dirs import BACKUP_MARKER
-from src.plugin_system.plugin_local_files import carry_over_local_files
 from src.plugin_system.repo_urls import (
     USER_AGENT, github_api_headers, github_owner_repo, normalize_repo_url,
 )
@@ -93,9 +92,7 @@ class _InstallMixin:
                 raise
 
             if installed:
-                self._discard_backup(
-                    plugin_id, backup_path, "install",
-                    new_path=self._existing_install(plugin_id) or plugin_path)
+                self._discard_backup(plugin_id, backup_path, "install")
                 return True
 
             self._restore_backup(plugin_id, plugin_path, backup_path, "Install")
@@ -136,33 +133,8 @@ class _InstallMixin:
             return f"could not set aside {plugin_path}: {e}"
         return None
 
-    def _discard_backup(
-        self, plugin_id: str, backup_path: Path, action: str,
-        new_path: Optional[Path] = None,
-    ) -> None:
-        """Remove the set-aside copy after a successful (re)install.
-
-        With ``new_path`` (where the new copy landed), first carries the
-        plugin's own runtime files -- OAuth tokens, client secrets, anything
-        its .gitignore excludes -- from the old copy into the new one: no
-        release contains them, so deleting the old copy would destroy them.
-        See src/plugin_system/plugin_local_files.py. If any could not be
-        copied the old copy is kept, so nothing is lost.
-        """
-        if new_path is not None and new_path.is_dir():
-            copied, failed = carry_over_local_files(backup_path, new_path)
-            if copied:
-                self.logger.info(
-                    "Kept %d local file(s) of %s across the %s: %s",
-                    len(copied), plugin_id, action, ", ".join(copied))
-            if failed:
-                self.logger.error(
-                    "Could not carry %s's local files into the new copy (%s); "
-                    "the previous copy is kept at %s -- copy them back by hand",
-                    plugin_id,
-                    "; ".join(f"{rel}: {err}" for rel, err in failed),
-                    backup_path)
-                return
+    def _discard_backup(self, plugin_id: str, backup_path: Path, action: str) -> None:
+        """Remove the set-aside copy after a successful (re)install."""
         if not self._safe_remove_directory(backup_path):
             self.logger.warning(
                 "%s of %s succeeded but the previous copy at %s could not be "
@@ -570,8 +542,7 @@ class _InstallMixin:
                     raise
                 temp_dir = None  # Prevent cleanup since we moved it
                 if backup_path is not None:
-                    self._discard_backup(
-                        plugin_id, backup_path, "install", new_path=final_path)
+                    self._discard_backup(plugin_id, backup_path, "install")
 
             # Install dependencies
             self._install_dependencies(final_path)

src/plugin_system/store_update.py

@@ -10,9 +10,6 @@ import subprocess  # nosec B404 - list-form argv only, no shell  # nosemgrep
 from pathlib import Path
 from typing import Dict, Optional, Tuple
 from src.plugin_system.plugin_dirs import BACKUP_MARKER
-from src.plugin_system.plugin_local_files import (
-    KNOWN_STATE_PATTERNS, is_known_state_file,
-)
 from src.plugin_system.repo_urls import same_repo
 
 
@@ -305,11 +302,7 @@ class _UpdateMixin:
                 installed = False
 
             if installed:
-                # install_plugin may land the new copy under the manifest id
-                # rather than the old directory name.
-                self._discard_backup(
-                    plugin_id, backup_path, "update",
-                    new_path=self._existing_install(plugin_id) or plugin_path)
+                self._discard_backup(plugin_id, backup_path, "update")
                 return True
 
             # Bad network, registry error...: the user keeps a working plugin.
@@ -516,12 +509,8 @@ class _UpdateMixin:
                             for line in untracked_result.stdout.strip().split('\n'):
                                 if line.startswith('??'):
                                     # Untracked file
-                                    file_path = line[3:].strip().strip('"')
-                                    # Tokens and secrets stay out of the
-                                    # stash (see below), so they alone are
-                                    # not a reason to stash.
-                                    if not is_known_state_file(file_path):
-                                        untracked_files.append(file_path)
+                                    file_path = line[3:].strip()
+                                    untracked_files.append(file_path)
                         
                         # Check for tracked file changes
                         status_result = subprocess.run(
@@ -548,17 +537,9 @@ class _UpdateMixin:
                     if has_changes:
                         self.logger.info(f"Stashing local changes in {plugin_id} before update")
                         try:
-                            # Use -u to include untracked files in stash --
-                            # except the plugin's tokens and secrets, which a
-                            # repo may have forgotten to gitignore. The stash
-                            # is never popped, so a stashed token.pickle would
-                            # vanish from the plugin and break it.
-                            stash_cmd = (
-                                ['git', '-C', str(plugin_path), 'stash', 'push', '-u',
-                                 '-m', f'LEDMatrix auto-stash before update {plugin_id}', '--', '.']
-                                + [f':(exclude,glob)**/{p}' for p in KNOWN_STATE_PATTERNS])
+                            # Use -u to include untracked files in stash
                             stash_result = subprocess.run(
-                                stash_cmd,
+                                ['git', '-C', str(plugin_path), 'stash', 'push', '-u', '-m', f'LEDMatrix auto-stash before update {plugin_id}'],
                                 capture_output=True,
                                 text=True,
                                 timeout=30,

test/fixtures/api_v3_url_map.json

@@ -175,6 +175,15 @@
       "POST"
     ]
   ],
+  [
+    "/api/v3/config/refresh-rate",
+    "api_v3.get_refresh_rate",
+    [
+      "GET",
+      "HEAD",
+      "OPTIONS"
+    ]
+  ],
   [
     "/api/v3/config/schedule",
     "api_v3.get_schedule_config",

test/test_frame_timing.py

@@ -807,3 +807,54 @@ def test_a_process_with_the_gc_monitor_exits_cleanly():
     assert proc.returncode == 0, proc.stderr
     assert "Exception ignored" not in proc.stderr
     assert "installed at exit: False" in proc.stdout
+
+
+SLOW = 1 / 110.0  # a panel that cannot reach a 120 Hz cap
+
+
+def _windows(recorder, n, interval, start=0.0):
+    for i in range(n):
+        _feed(recorder, [interval] * 200, start=start + 50.0 * i)
+        _aggregate(recorder)
+
+
+def _shortfall_warnings(caplog):
+    return [r for r in caplog.records
+            if r.name == "src.common.frame_timing" and "Limit Refresh Rate" in r.getMessage()]
+
+
+def test_a_panel_slower_than_its_cap_is_reported_once(tmp_path, caplog):
+    r = _recorder(tmp_path)
+    r.plan_refresh(120.0)
+    caplog.set_level("WARNING")
+    _windows(r, 3, SLOW)          # adopted on the 2nd window, checked on the 4th
+    assert _shortfall_warnings(caplog) == []
+    _windows(r, 3, SLOW, start=1000.0)
+    warnings = _shortfall_warnings(caplog)
+    assert len(warnings) == 1
+    assert "about 110 Hz" in warnings[0].getMessage()
+    assert "to 100 Hz" in warnings[0].getMessage()
+
+
+def test_a_panel_that_reaches_its_cap_is_not_reported(tmp_path, caplog):
+    r = _recorder(tmp_path)
+    r.plan_refresh(100.0)
+    caplog.set_level("WARNING")
+    _windows(r, 6, PERIOD)
+    assert _shortfall_warnings(caplog) == []
+
+
+def test_without_a_planned_rate_nothing_is_checked(tmp_path, caplog):
+    # The emulator and the fallback canvas: DisplayManager never calls
+    # plan_refresh(), since their frames are not paced by a panel.
+    r = _recorder(tmp_path)
+    caplog.set_level("WARNING")
+    _windows(r, 6, SLOW)
+    assert _shortfall_warnings(caplog) == []
+
+
+def test_the_snapshot_records_the_planned_rate(tmp_path):
+    r = _recorder(tmp_path)
+    assert r.snapshot()["planned_refresh_hz"] is None
+    r.plan_refresh(120.0)
+    assert r.snapshot()["planned_refresh_hz"] == 120.0

test/test_on_demand_live_and_restore.py

@@ -21,18 +21,6 @@ SPORTS_MODES = ['nfl_live', 'nfl_recent', 'nfl_upcoming',
                 'ncaa_fb_live', 'ncaa_fb_recent', 'ncaa_fb_upcoming']
 
 
-def _last_write(cache_manager, key):
-    """The last ``cache_manager.set(key, ...)`` call.
-
-    Not simply the last ``set`` call: the controller's font-usage publisher
-    thread writes ``font_usage_snapshot`` to the same cache manager whenever
-    it wakes, so on a slow runner it can land after the write under test.
-    """
-    writes = [c for c in cache_manager.set.call_args_list if c.args and c.args[0] == key]
-    assert writes, f"nothing was written to {key!r}"
-    return writes[-1]
-
-
 def _sports_plugin(has_live_content=False):
     plugin = MagicMock(spec=['display', 'has_live_content', 'has_live_priority',
                              'get_live_modes'])
@@ -99,7 +87,8 @@ class TestANamedLiveModeIsShown:
     def test_the_named_mode_survives_a_restart(self, football):
         football._activate_on_demand({'plugin_id': 'football-scoreboard',
                                       'mode': 'ncaa_fb_live'})
-        saved = _last_write(football.cache_manager, 'display_on_demand_config')
+        saved = football.cache_manager.set.call_args_list[-1]
+        assert saved.args[0] == 'display_on_demand_config'
         config = saved.args[1]
         assert config['named_mode'] == 'ncaa_fb_live'
 
@@ -131,7 +120,10 @@ class TestARestoreWithNothingToResume:
     def test_it_is_reported_as_an_error(self, restored):
         assert restored.on_demand_status == 'error'
         assert restored.on_demand_last_error == 'restore-failed'
-        published = _last_write(restored.cache_manager, 'display_on_demand_state')
+        # The last on-demand state write, not the last write of any key: the
+        # font-usage publisher thread writes its own key at its own pace.
+        published = [c for c in restored.cache_manager.set.call_args_list
+                     if c.args and c.args[0] == 'display_on_demand_state'][-1]
         assert published.args[1]['status'] == 'error'
         assert published.args[1]['error'] == 'restore-failed'
 

test/test_scroll_config.py

@@ -21,6 +21,7 @@ from src.common.scroll_config import (  # noqa: E402
     refresh_hz_from_config,
     resolve,
 )
+from src.common import scroll_config  # noqa: E402
 
 
 class FakeHelper:
@@ -504,3 +505,46 @@ class TestSpeedAdvice:
         got = solve_crisp(50, 125.74)
         assert got.steppiness == "smooth"
         assert got.pixels_per_frame == 1
+
+
+class TestRefreshShortfall:
+    """A panel that cannot reach its cap runs every scroll slow."""
+
+    def test_the_ledmatrix_rig_is_told_to_cap_at_100(self):
+        # Pi 4, 2x128x64 on adafruit-hat-pwm under a 120 Hz cap: measured
+        # 107.6-113.1 Hz, and frame_timing reports the fast end.
+        s = scroll_config.refresh_shortfall(113.1, 120)
+        assert s == {"measured_hz": 113.1, "planned_hz": 120.0,
+                     "suggested_cap_hz": 100, "slow_percent": 6}
+
+    def test_a_panel_that_holds_its_cap_is_fine(self):
+        assert scroll_config.refresh_shortfall(99.95, 100) is None
+        assert scroll_config.refresh_shortfall(97.5, 100) is None
+
+    def test_a_panel_that_beats_its_cap_is_fine(self):
+        assert scroll_config.refresh_shortfall(125.7, 120) is None
+
+    def test_nothing_measured_says_nothing(self):
+        assert scroll_config.refresh_shortfall(None, 120) is None
+        assert scroll_config.refresh_shortfall(0, 120) is None
+        assert scroll_config.refresh_shortfall("fast", 120) is None
+
+    def test_the_suggestion_leaves_headroom_under_the_measurement(self):
+        assert scroll_config.holdable_cap(113.1) == 100
+        assert scroll_config.holdable_cap(95.0) == 90
+        # 5% under 105 is 99.75: 100 would sit inside the panel's drift.
+        assert scroll_config.holdable_cap(105.0) == 90
+        assert scroll_config.holdable_cap(9.0) is None
+        assert scroll_config.holdable_cap(None) is None
+
+    def test_the_log_line_names_the_cap_to_use(self):
+        text = scroll_config.describe_refresh_shortfall(
+            scroll_config.refresh_shortfall(113.1, 120))
+        assert "about 113 Hz" in text and "120 Hz" in text
+        assert "6% slower" in text
+        assert "Set Limit Refresh Rate to 100 Hz" in text
+
+    def test_no_suggestion_for_a_panel_too_slow_for_any_cap(self):
+        text = scroll_config.describe_refresh_shortfall(
+            scroll_config.refresh_shortfall(9.0, 100))
+        assert "Set Limit Refresh Rate" not in text

test/test_store_update_keeps_local_files.py

@@ -1,244 +0,0 @@
-"""A plugin update must keep the files the plugin wrote beside itself.
-
-Field incident, 2026-10-04: updating calendar 1.2.9 -> 1.2.12 from the web UI
-replaced plugin-repos/calendar/ with the fresh download and deleted the old
-copy -- and with it token.pickle and credentials.json, the plugin's Google
-OAuth files. No release contains them (the repo gitignores them), so the hot
-reload logged "Credentials file not found" and the calendar stayed broken
-until the files were restored by hand.
-
-Both update routes are covered: a monorepo plugin (registry ``plugin_path``),
-which is reinstalled into a fresh directory, and a plugin installed from its
-own git repository, which is updated with ``git pull`` after an auto-stash.
-"""
-
-import json
-import shutil
-import subprocess
-
-import pytest
-
-from src.plugin_system.plugin_local_files import (
-    is_known_state_file, local_files_to_keep,
-)
-from src.plugin_system.store_manager import PluginStoreManager
-
-PLUGIN_ID = "calendar"
-
-
-def _manifest(version):
-    return {"id": PLUGIN_ID, "name": "Calendar", "class_name": "CalendarPlugin",
-            "display_modes": ["calendar"], "version": version}
-
-
-def _write_release(target, version):
-    """What a download of ``version`` puts on disk."""
-    target.mkdir(parents=True, exist_ok=True)
-    (target / "manifest.json").write_text(json.dumps(_manifest(version)))
-    (target / "manager.py").write_text(f"VERSION = {version!r}\n")
-    (target / ".gitignore").write_text("credentials.json\ntoken.pickle\ncache/\n")
-
-
-def _drop_local_files(plugin_dir):
-    """What the plugin writes at runtime: OAuth files plus cached state."""
-    (plugin_dir / "token.pickle").write_bytes(b"\x80\x04oauth-token")
-    (plugin_dir / "credentials.json").write_text('{"installed": {}}')
-    (plugin_dir / "cache").mkdir()
-    (plugin_dir / "cache" / "events.json").write_text("[]")
-
-
-def _assert_local_files_kept(plugin_dir):
-    assert (plugin_dir / "token.pickle").read_bytes() == b"\x80\x04oauth-token"
-    assert (plugin_dir / "credentials.json").read_text() == '{"installed": {}}'
-    assert (plugin_dir / "cache" / "events.json").read_text() == "[]"
-
-
-def _leftover_backups(plugins_dir):
-    return [p.name for p in plugins_dir.iterdir() if "standalone-backup" in p.name]
-
-
-@pytest.fixture
-def store(tmp_path, monkeypatch):
-    mgr = PluginStoreManager(
-        plugins_dir=str(tmp_path / "plugin-repos"),
-        uninstalled_registry_path=str(tmp_path / "uninstalled.json"))
-    mgr.plugins_dir.mkdir(parents=True, exist_ok=True)
-    monkeypatch.setattr(mgr, "_install_dependencies", lambda *a, **k: True)
-    monkeypatch.setattr(mgr, "fetch_registry", lambda *a, **k: {"plugins": []})
-    return mgr
-
-
-class TestMonorepoUpdate:
-    @pytest.fixture
-    def installed(self, store, monkeypatch):
-        registry_entry = {
-            "id": PLUGIN_ID, "repo": "https://github.com/ChuckBuilds/ledmatrix-plugins",
-            "plugin_path": "plugins/calendar", "branch": "main",
-            "latest_version": "1.2.9",
-        }
-        monkeypatch.setattr(store, "get_plugin_info", lambda *a, **k: registry_entry)
-        release = {"version": "1.2.9"}
-
-        def fake_monorepo_download(download_url, plugin_subpath, target):
-            assert plugin_subpath == "plugins/calendar"
-            _write_release(target, release["version"])
-            return True
-
-        monkeypatch.setattr(store, "_install_from_monorepo", fake_monorepo_download)
-        assert store.install_plugin(PLUGIN_ID) is True
-
-        def publish(version):
-            registry_entry["latest_version"] = release["version"] = version
-        return store, store.plugins_dir / PLUGIN_ID, publish
-
-    def test_update_keeps_token_and_gitignored_files(self, installed):
-        store, plugin_dir, publish = installed
-        _drop_local_files(plugin_dir)
-
-        publish("1.2.12")
-        assert store.update_plugin(PLUGIN_ID) is True
-
-        assert json.loads((plugin_dir / "manifest.json").read_text())["version"] == "1.2.12"
-        _assert_local_files_kept(plugin_dir)
-        assert _leftover_backups(store.plugins_dir) == []
-
-    def test_token_is_kept_even_when_the_release_does_not_gitignore_it(self, installed):
-        store, plugin_dir, publish = installed
-        (plugin_dir / ".gitignore").unlink()
-        (plugin_dir / "token.pickle").write_bytes(b"tok")
-        (plugin_dir / "config_secrets.json").write_text("{}")
-
-        publish("1.2.12")
-        assert store.update_plugin(PLUGIN_ID) is True
-
-        assert (plugin_dir / "token.pickle").read_bytes() == b"tok"
-        assert (plugin_dir / "config_secrets.json").read_text() == "{}"
-
-    def test_release_content_wins_and_old_code_is_not_carried(self, installed):
-        store, plugin_dir, publish = installed
-        # A file the old copy had that the new release dropped, byte code, and
-        # an old copy of a file the new release also ships.
-        (plugin_dir / "removed_module.py").write_text("OLD = True\n")
-        (plugin_dir / "__pycache__").mkdir()
-        (plugin_dir / "__pycache__" / "manager.cpython-313.pyc").write_bytes(b"pyc")
-
-        publish("1.2.12")
-        assert store.update_plugin(PLUGIN_ID) is True
-
-        assert not (plugin_dir / "removed_module.py").exists()
-        assert not (plugin_dir / "__pycache__").exists()
-        assert "1.2.12" in (plugin_dir / "manager.py").read_text()
-
-    def test_reinstall_over_an_existing_copy_keeps_them_too(self, installed):
-        store, plugin_dir, publish = installed
-        _drop_local_files(plugin_dir)
-
-        assert store.install_plugin(PLUGIN_ID) is True
-
-        _assert_local_files_kept(plugin_dir)
-        assert _leftover_backups(store.plugins_dir) == []
-
-
-class TestInstallFromUrlReplace:
-    def test_replacing_an_installed_copy_keeps_the_token(self, store, monkeypatch):
-        plugin_dir = store.plugins_dir / PLUGIN_ID
-        _write_release(plugin_dir, "1.0.0")
-        _drop_local_files(plugin_dir)
-
-        def fake_clone(repo_url, target, branches):
-            _write_release(target, "2.0.0")
-            return "main"
-
-        monkeypatch.setattr(store, "_install_via_git", fake_clone)
-        result = store.install_from_url(
-            "https://github.com/example/ledmatrix-calendar", plugin_id=PLUGIN_ID)
-
-        assert result["success"] is True
-        assert json.loads((plugin_dir / "manifest.json").read_text())["version"] == "2.0.0"
-        _assert_local_files_kept(plugin_dir)
-
-
-def _git(*args, cwd):
-    subprocess.run(["git", "-c", "user.email=t@example.com", "-c", "user.name=t",
-                    "-c", "core.autocrlf=false", *args],
-                   cwd=cwd, check=True, capture_output=True)
-
-
-@pytest.mark.skipif(shutil.which("git") is None, reason="git not installed")
-class TestGitRepoUpdate:
-    @pytest.fixture
-    def cloned(self, store, tmp_path, monkeypatch):
-        monkeypatch.setattr(store, "get_plugin_info", lambda *a, **k: None)
-        upstream = tmp_path / "upstream"
-        _write_release(upstream, "1.0.0")
-        # This repo does NOT gitignore the token: an untracked, non-ignored
-        # file is exactly what `git stash push -u` used to sweep away.
-        (upstream / ".gitignore").write_text("cache/\n")
-        _git("init", "-q", "-b", "main", cwd=upstream)
-        _git("add", ".", cwd=upstream)
-        _git("commit", "-qm", "1.0.0", cwd=upstream)
-
-        plugin_dir = store.plugins_dir / PLUGIN_ID
-        _git("clone", "-q", str(upstream), str(plugin_dir), cwd=tmp_path)
-
-        def publish(version):
-            (upstream / "manifest.json").write_text(json.dumps(_manifest(version)))
-            _git("commit", "-qam", version, cwd=upstream)
-        return store, plugin_dir, publish
-
-    def test_pull_update_keeps_untracked_token(self, cloned):
-        store, plugin_dir, publish = cloned
-        _drop_local_files(plugin_dir)
-        # An unrelated untracked file, so the update really does stash.
-        (plugin_dir / "notes.txt").write_text("scratch")
-
-        publish("1.1.0")
-        assert store.update_plugin(PLUGIN_ID) is True
-
-        assert json.loads((plugin_dir / "manifest.json").read_text())["version"] == "1.1.0"
-        _assert_local_files_kept(plugin_dir)
-
-    def test_token_alone_does_not_trigger_a_stash(self, cloned):
-        store, plugin_dir, publish = cloned
-        (plugin_dir / "token.pickle").write_bytes(b"tok")
-
-        publish("1.1.0")
-        assert store.update_plugin(PLUGIN_ID) is True
-
-        assert (plugin_dir / "token.pickle").read_bytes() == b"tok"
-        stashes = subprocess.run(["git", "-C", str(plugin_dir), "stash", "list"],
-                                 capture_output=True, text=True, check=True)
-        assert stashes.stdout.strip() == ""
-
-
-class TestWhatIsKept:
-    @pytest.mark.parametrize("path,expected", [
-        ("token.pickle", True),
-        ("data/session.pickle", True),
-        ("credentials.json", True),
-        ("token.json", True),
-        ("config_secrets.json", True),
-        (".pkce_code_verifier", True),
-        ("manager.py", False),
-        ("config.json", False),
-    ])
-    def test_known_state_files(self, path, expected):
-        assert is_known_state_file(path) is expected
-
-    def test_gitignore_rules(self, tmp_path):
-        old, new = tmp_path / "old", tmp_path / "new"
-        new.mkdir()
-        for rel in ["a.log", "logs/x.txt", "sub/deep/b.log", "keep.log",
-                    "anchored.txt", "sub/anchored.txt", "assets/x/y_backup/z.png",
-                    "manager.py", "shipped.log"]:
-            (old / rel).parent.mkdir(parents=True, exist_ok=True)
-            (old / rel).write_text("x")
-        (new / "shipped.log").write_text("new")
-        (old / ".gitignore").write_text(
-            "# comment\n*.log\n!keep.log\nlogs/\n/anchored.txt\n"
-            "assets/**/*_backup/\n")
-
-        assert local_files_to_keep(old, new) == [
-            "a.log", "anchored.txt", "assets/x/y_backup/z.png",
-            "logs/x.txt", "sub/deep/b.log",
-        ]

test/web_interface/test_api_v3_refresh_rate.py

@@ -0,0 +1,63 @@
+"""GET /api/v3/config/refresh-rate: the cap, the measured rate, a cap to hold."""
+import json
+from unittest.mock import MagicMock
+
+import pytest
+from flask import Flask
+
+from web_interface.blueprints.api_v3 import api_v3
+
+
+@pytest.fixture
+def client(monkeypatch, tmp_path):
+    stats = tmp_path / "stats.json"
+    monkeypatch.setattr("src.common.frame_timing.default_stats_path", lambda: str(stats))
+    manager = MagicMock()
+    manager.load_config.return_value = {
+        "display": {"hardware": {"limit_refresh_rate_hz": 120}}}
+    monkeypatch.setattr(api_v3, "config_manager", manager, raising=False)
+    app = Flask(__name__)
+    app.register_blueprint(api_v3, url_prefix="/api/v3")
+    c = app.test_client()
+    c.stats_path = stats
+    return c
+
+
+def _get(client):
+    body = client.get("/api/v3/config/refresh-rate").get_json()
+    assert body["status"] == "success"
+    return body["data"]
+
+
+def test_nothing_measured_yet(client):
+    data = _get(client)
+    assert data == {"planned_hz": 120.0, "measured_hz": None, "shortfall": None}
+
+
+def test_a_panel_short_of_its_cap_gets_a_cap_it_can_hold(client):
+    client.stats_path.write_text(json.dumps(
+        {"measured_refresh_hz": 110.4, "planned_refresh_hz": 120.0}))
+    data = _get(client)
+    assert data["measured_hz"] == 110.4
+    assert data["shortfall"]["suggested_cap_hz"] == 100
+    assert data["shortfall"]["slow_percent"] == 8
+
+
+def test_a_panel_at_its_cap_has_no_shortfall(client):
+    client.stats_path.write_text(json.dumps(
+        {"measured_refresh_hz": 121.3, "planned_refresh_hz": 120.0}))
+    assert _get(client)["shortfall"] is None
+
+
+def test_a_file_written_under_another_cap_is_stale(client):
+    # The cap was changed to 120 but the display still runs under 100 Hz.
+    client.stats_path.write_text(json.dumps(
+        {"measured_refresh_hz": 99.9, "planned_refresh_hz": 100.0}))
+    data = _get(client)
+    assert data["measured_hz"] is None
+    assert data["shortfall"] is None
+
+
+def test_a_file_from_a_display_too_old_to_record_its_cap_still_counts(client):
+    client.stats_path.write_text(json.dumps({"measured_refresh_hz": 110.4}))
+    assert _get(client)["shortfall"]["suggested_cap_hz"] == 100

test/web_interface/test_starlark_pixlet_routes.py

@@ -1131,17 +1131,9 @@ class TestPixletEditorHostDefaultsButDoesNotOverride:
         class FakeProcess:
             pid = 424242
 
-        real_popen = mod.subprocess.Popen
-
         def fake_popen(cmd, *args, env=None, **kwargs):
-            # Only the editor launch is faked. Patching subprocess.Popen
-            # patches it for the whole request, and the captive-portal
-            # before_request hook runs `systemctl is-active hostapd` through
-            # subprocess.run whenever its 30s cache has expired -- which
-            # needs a real process (run() uses it as a context manager).
-            if str(script) not in cmd:
-                return real_popen(cmd, *args, env=env, **kwargs)
-            captured['env'] = env
+            if env is not None:
+                captured['env'] = env
             return FakeProcess()
 
         with patch.object(mod, '_validate_starlark_app_path',

web_interface/blueprints/api_v3/config.py

@@ -158,11 +158,16 @@ def _panel_refresh_hz(config):
     cap = scroll_config.refresh_hz_from_config(config)
     try:
         with open(frame_timing.default_stats_path(), encoding='utf-8') as fh:
-            measured = float(json.load(fh).get('measured_refresh_hz') or 0)
+            stats = json.load(fh)
+        measured = float(stats.get('measured_refresh_hz') or 0)
+        planned = float(stats.get('planned_refresh_hz') or 0)
     except (OSError, ValueError, TypeError, AttributeError):
+        measured = planned = 0.0
+    # Reject a stale file from a previous hardware config: one written under
+    # another cap (the display has not restarted since it changed), or, from
+    # a display too old to record its cap, a measurement far off this one.
+    if planned and abs(planned - cap) > 0.5:
         measured = 0.0
-    # Reject a stale file from a previous hardware config: a measurement far
-    # off the cap says the config changed since it was written.
     if measured > 0 and 0.5 * cap <= measured <= 1.5 * cap:
         return measured, 'measured'
     return cap, 'configured'
@@ -189,6 +194,29 @@ def get_scroll_speed_advice():
     return jsonify({'status': 'success', 'data': advice})
 
 
+@api_v3.route('/config/refresh-rate', methods=['GET'])
+def get_refresh_rate():
+    """The refresh cap, what the panel measured, and a cap it can hold.
+
+    Backs the hint under the Display tab's Limit Refresh Rate field. Scroll
+    speeds are solved against the cap, so a panel that cannot reach it runs
+    every scroll slow; ``shortfall`` (None when the panel keeps up, or nothing
+    has been measured yet) says by how much and suggests a cap.
+    """
+    from src.common import scroll_config
+    if not api_v3.config_manager:
+        return jsonify({'status': 'error', 'message': 'Config manager not initialized'}), 500
+    config = api_v3.config_manager.load_config()
+    planned = scroll_config.refresh_hz_from_config(config)
+    hz, source = _panel_refresh_hz(config)
+    measured = hz if source == 'measured' else None
+    return jsonify({'status': 'success', 'data': {
+        'planned_hz': planned,
+        'measured_hz': round(measured, 1) if measured else None,
+        'shortfall': scroll_config.refresh_shortfall(measured, planned),
+    }})
+
+
 @api_v3.route('/config/schedule', methods=['GET'])
 def get_schedule_config():
     """Get current schedule configuration"""

web_interface/templates/v3/partials/display.html

@@ -318,6 +318,7 @@
                            min="0"
                            max="1000"
                            class="form-control">
+                    <p id="limit_refresh_rate_hz_hint" class="mt-1 text-xs text-amber-700" aria-live="polite"></p>
                 </div>
             </div>
 
@@ -915,6 +916,41 @@ document.getElementById('brightness').addEventListener('input', function() {
         });
     }
 
+    // Say so when the panel cannot reach its refresh cap. Scroll speeds are
+    // worked out against the cap, so every scroll then runs slow, and the
+    // display has measured a cap the panel can hold.
+    (function refreshRateHint() {
+        const hint = document.getElementById('limit_refresh_rate_hz_hint');
+        const input = document.getElementById('limit_refresh_rate_hz');
+        if (!hint || !input) return;
+        fetch('/api/v3/config/refresh-rate')
+            .then(function(r) { return r.json(); })
+            .then(function(body) {
+                const s = body.status === 'success' && body.data.shortfall;
+                hint.textContent = '';
+                if (!s) return;
+                hint.appendChild(document.createTextNode(
+                    'This panel refreshes at about ' + Math.round(s.measured_hz) +
+                    ' Hz, below this ' + Math.round(s.planned_hz) + ' Hz cap, so scrolls run about ' +
+                    s.slow_percent + '% slower than set.' + (s.suggested_cap_hz ? ' ' : '')));
+                if (!s.suggested_cap_hz) return;
+                const btn = document.createElement('button');
+                btn.type = 'button';
+                btn.className = 'underline font-medium';
+                btn.textContent = 'Use ' + s.suggested_cap_hz + ' Hz';
+                btn.addEventListener('click', function() {
+                    input.value = s.suggested_cap_hz;
+                    input.dispatchEvent(new Event('input', {bubbles: true}));
+                    input.dispatchEvent(new Event('change', {bubbles: true}));
+                    hint.textContent = 'Save, then restart the display, to apply ' +
+                        s.suggested_cap_hz + ' Hz.';
+                });
+                hint.appendChild(btn);
+                hint.appendChild(document.createTextNode(', a cap it can hold.'));
+            })
+            .catch(function() { hint.textContent = ''; });
+    })();
+
     // Declared before first use: let is not hoisted usably.
     let scrollHintTimer = null;
     let scrollHintSeq = 0;