perf(scroll): pace frames to the panel, not to a fixed sleep

Scrolling ran at 44-46 fps on a 2x128x64 chain and 14-17% of frames took
41-53ms, which reads as judder. Four independent causes, each measured on
the hardware; details and the diagnostic recipe are in
docs/SCROLL_PERFORMANCE.md.

The high-FPS loop slept a flat 8ms after every render. display() has
already blocked on the panel's vsync by then, so that sleep was added to a
wait that had happened: ~4ms of render plus 8ms put each iteration at ~12ms
against a 10ms refresh grid, so every swap missed a refresh and the loop
settled at 50fps while asking for 125 -- with no headroom, so a further
14% of frames slipped again. It now sleeps only the remainder, with a 1ms
floor so plugin threads still get the GIL.

ScrollHelper stepped position on a wall clock at 1/scroll_delay steps per
second. Plugins set scroll_delay to the frame period, so that comparison
sat exactly on its own threshold: a frame arriving a hair early moved zero
pixels and rendered an identical frame, dirty-tracking skipped the swap, it
returned in ~2ms, and the beat repeated. No scroll_delay value tunes that
out -- a shorter delay trades stalled frames for periodic double-steps.
Both modes now accumulate elapsed time at the same configured speed, so
position stays proportional to real time.

Sub-pixel blending goes back to off by default. It renders a half-step by
mixing two adjacent columns, which on a coarse panel showing pixel-font
text alternates crisp and smeared frames and reads as shimmer -- visibly
worse than integer stepping on the hardware. Vegas mode still opts in.

disk_cache uses orjson when importable, falling back to the stdlib. Encoding
a ~1MB record drops from 14.8ms to 5.4ms end-to-end, and that work holds the
GIL while a marquee is on screen. display_manager also checksummed the whole
framebuffer twice per frame (dirty tracking, then the preview snapshot); the
snapshot now takes the checksum the caller already computed.

New src/common/scroll_config.py resolves scroll settings in one place. Five
ticker plugins each hand-rolled this and disagreed: odds-ticker ranked the
deprecated scroll_pixels_per_second above the documented scroll_speed/delay
pair, and because that key carries a schema default the documented settings
were dead for every user (ChuckBuilds/ledmatrix-plugins#408), while
ledmatrix-leaderboard read the same key only as a fallback. The resolver also
warns when a speed will not advance a whole number of pixels per refresh,
which is the property that actually determines whether a scroll looks smooth.

scripts/build_rgbmatrix_nogil.sh rebuilds the rgbmatrix binding so it
releases the GIL. Upstream declares SwapOnVSync without nogil, unlike
SetPixel/Clear/Fill beside it, so the render thread held the GIL for the
whole vsync wait and starved background threads into long uninterruptible
bursts. The script patches, builds and self-verifies into a scratch tree;
--install backs up the original and rolls back if the service does not come
back healthy.

Measured after: 100 fps locked, no stalls observed, render thread down from
51% to 19% of one core.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/SCROLL_PERFORMANCE.md

@@ -0,0 +1,188 @@
+# Scroll Performance
+
+How scrolling is paced on this hardware, what was wrong with it, and how to
+configure a plugin so its marquee is smooth.
+
+Measured on a Raspberry Pi 4 driving a 2×128×64 chain (256×64 logical) at
+`limit_refresh_rate_hz: 100`. Numbers below come from that panel.
+
+| | before | after |
+|---|---|---|
+| scroll frame rate | 44–46 fps | **100 fps, locked** |
+| frames ≥ 45 ms | 14–17% | none observed |
+| dominant frame time | 20 ms | **10 ms** |
+| disk cache write (~1 MB) | 14.8 ms | **5.4 ms** |
+
+---
+
+## The one rule that matters
+
+**Motion is smooth when the strip advances a whole number of pixels per panel
+refresh.**
+
+On a 100 Hz panel the crisp speeds are 100 px/s, 200 px/s, 300 px/s. A speed
+that does not divide evenly has to do one of two bad things:
+
+- **blend** two adjacent columns to render a half-step — on pixel-font text
+  this alternates crisp and smeared frames and reads as shimmer, or as the
+  text jumping a pixel ahead of itself;
+- **repeat** a frame — the strip stands still, then jumps, which reads as
+  judder.
+
+Neither is tunable away. Pick a speed that divides evenly.
+
+`src.common.scroll_config.resolve()` warns when a configured speed will not,
+and names the nearest speed that will.
+
+## Configuring a plugin
+
+Use the shared resolver rather than reading config keys yourself:
+
+```python
+from src.common import scroll_config
+
+settings = scroll_config.configure(
+    self.scroll_helper,
+    plugin_config=self.config,
+    global_config=self.global_config,
+    refresh_hz=scroll_config.refresh_hz_from_config(self.global_config),
+    plugin_logger=self.logger,
+)
+```
+
+It resolves every config shape in one place, applies the speed, and returns
+what it did. Precedence, highest first:
+
+1. `display_options.scroll_speed` + `scroll_delay` — **the recommended form**
+2. `display.scroll_speed` + `scroll_delay` — deprecated shape
+3. `scroll_speed` + `scroll_delay` at the root — legacy flat
+4. `scroll_pixels_per_second` — deprecated
+5. the global `display` block
+6. the built-in default (100 px/s)
+
+`scroll_speed` is pixels per frame and `scroll_delay` is the frame period in
+seconds, so the pair means `scroll_speed / scroll_delay` px/s. The recommended
+config for a 100 Hz panel:
+
+```json
+"display_options": { "scroll_speed": 1.0, "scroll_delay": 0.01 }
+```
+
+### Why the deprecated key ranks below the explicit pair
+
+Because some plugins give `scroll_pixels_per_second` a **schema default**, and
+schema defaults are merged into plugin config. Ranking it above the pair means
+it is always present and always wins, so the documented settings become
+unreachable. That is a real, shipped bug — see
+[ledmatrix-plugins#408](https://github.com/ChuckBuilds/ledmatrix-plugins/issues/408).
+
+If you are writing a plugin: do not give a deprecated key a schema default.
+
+## What was actually wrong
+
+Four independent faults, each found by measurement.
+
+### 1. The frame loop slept on top of a wait it had already done
+
+`display_controller.py` ran the high-FPS loop as `render → SwapOnVSync (blocks
+to the panel's refresh) → time.sleep(0.008) → plugin ticks`. The sleep was
+unconditional and added to a wait that had already happened. Render work
+measured ~4 ms, so each iteration cost ~12 ms against a 10 ms refresh grid —
+every swap missed a refresh and landed on the next one. The loop settled at
+exactly 50 fps while asking for 125, with no headroom, so ~14% of frames
+slipped a further refresh.
+
+Now the loop sleeps only the remainder of the frame budget, with a 1 ms floor
+so plugin threads still get the GIL.
+
+### 2. `SwapOnVSync` held the GIL while blocking
+
+The rgbmatrix binding declares it without `nogil` (unlike `SetPixel`, `Clear`
+and `Fill` immediately above it in `cppinc.pxd`), so the render thread held the
+GIL for the entire vsync wait — most of every frame. Background threads were
+starved into long uninterruptible bursts; a 1.5 MB API response costs ~17 ms to
+parse and ~18 ms to re-encode for the cache, and `json.raw_decode` cannot be
+preempted mid-document. Those bursts are what the render loop then waited on.
+
+Fixed by rebuilding the binding: `scripts/build_rgbmatrix_nogil.sh`.
+
+### 3. Sub-pixel blending was wrong for this display
+
+Enabling it made things worse, not better — see the rule at the top. It is off
+by default and only Vegas mode opts in via `set_sub_pixel_scrolling(True)`.
+
+### 4. Frame-based stepping raced the vsync clock
+
+Frame-based mode gated motion on a wall clock at `1/scroll_delay` steps per
+second. Plugins set `scroll_delay` to the frame period, which puts that
+comparison exactly on its own threshold: a frame arriving a hair early moved
+zero pixels and rendered an identical frame, which dirty-tracking skipped, so
+it returned in ~2 ms and the beat repeated. No `scroll_delay` value tunes this
+out — a shorter delay just trades stalled frames for periodic double-steps.
+
+`ScrollHelper` now accumulates elapsed time in both modes at the same
+configured speed, so position stays proportional to real time.
+
+## Diagnosing a juddery scroller
+
+**`Avg FPS` will lie to you.** It is a 100-frame moving average, and a 2 ms
+duplicate frame plus a 21 ms double-wait average to exactly 10 ms. A ticker
+that is stalling on half its frames still reports a healthy `100.0`.
+
+Look at the **distribution** instead:
+
+```bash
+journalctl -u ledmatrix --since "-10min" --no-pager \
+  | grep -oE "Frame time: [0-9.]+ms" | awk '{print $3}' | sed 's/ms//' \
+  | awk '{printf "%.0f\n", $1}' | sort -n | uniq -c
+```
+
+Reading it, on a 100 Hz panel:
+
+| you see | it means |
+|---|---|
+| everything at 10 ms | healthy |
+| a mode at ~2 ms | **duplicate frames** — the swap was skipped because the image did not change. The scroller is advancing less than one pixel per frame. |
+| a mode at 20/30/50 ms | frames missing refreshes — per-frame work is overrunning, or a background thread is holding the GIL |
+| `Avg FPS` above 100 | duplicates present, unless the scroll cycle has completed and is idling |
+
+Then confirm what the plugin actually loaded — config edits do not always reach
+the running code:
+
+```bash
+journalctl -u ledmatrix --since "-5min" --no-pager | grep -iE "px/s|px/frame"
+```
+
+If a plugin logs its scroll config **twice** with different modes, the second
+line is what is running.
+
+## Rebuilding the binding
+
+```bash
+bash scripts/build_rgbmatrix_nogil.sh              # build into a scratch dir
+sudo bash scripts/build_rgbmatrix_nogil.sh --install
+sudo bash scripts/build_rgbmatrix_nogil.sh --rollback
+```
+
+The build never touches the installed module. `--install` backs up the original
+to `~/rgbmatrix-core.so.ORIGINAL` first, and rolls back automatically if the
+service does not come back healthy. Requires `build-essential`; Cython is
+installed into a cached venv under `~/.cache/ledmatrix-cython`.
+
+Re-run it after upgrading `rpi-rgb-led-matrix`, since a library upgrade
+replaces the patched binding.
+
+## Faster JSON
+
+`src/cache/disk_cache.py` uses `orjson` when it is importable and falls back to
+the stdlib otherwise, so it is optional:
+
+```bash
+sudo pip3 install --break-system-packages orjson
+```
+
+Encoding is where it pays — about 7× on this hardware. Decoding gains far less
+(~1.3× on large payloads) because the cost there is building Python objects,
+not scanning text. That is also why moving parsing to a subprocess does not
+help: `pickle.loads` of the same payload costs 8.1 ms against `json.loads` at
+10.9 ms, so the work just moves rather than disappearing.

requirements.txt

@@ -55,6 +55,14 @@ packaging>=23.0,<27.0
 #                     range as a hard dependency — keep the two in sync.
 #   pip install 'psutil>=6.0.0,<7.0.0'
 #
+# orjson            — faster JSON for the disk cache
+#                     (src/cache/disk_cache.py). Encoding a ~1MB cache
+#                     record drops from ~12ms to ~1.6ms on a Pi 4, which
+#                     matters because that work holds the GIL and stalls
+#                     the render thread mid-scroll. Falls back to the
+#                     stdlib json when missing — see docs/SCROLL_PERFORMANCE.md.
+#   pip install 'orjson>=3.9,<4.0'
+#
 # Flask-Limiter     — request rate limiting in web_interface/app.py
 #                     (accidental-abuse protection, not security). The
 #                     web interface starts without rate limiting when

scripts/build_rgbmatrix_nogil.sh

@@ -0,0 +1,242 @@
+#!/usr/bin/env bash
+#
+# Rebuild the rgbmatrix Python binding so it releases the GIL.
+#
+# WHY THIS EXISTS
+# ---------------
+# The upstream binding declares FrameCanvas::SwapOnVSync WITHOUT `nogil`
+# (cppinc.pxd), unlike SetPixel/Clear/Fill on the lines just above it.
+# SwapOnVSync blocks until the panel's next vertical sync -- up to a full
+# refresh period on every frame -- so the render thread was holding the GIL
+# for most of every frame. Background threads (API fetches, JSON parsing,
+# image decode) were starved into long uninterruptible bursts, which in turn
+# made the render loop miss refreshes.
+#
+# Measured on a Pi 4 driving a 2x128x64 chain at limit_refresh_rate_hz=100:
+#
+#     before   ~44 fps average, 14-17% of frames 41-53ms
+#     after    100 fps locked,  no stalls observed
+#
+# This script also releases the GIL across the per-pixel blit
+# (SetPixelsPillow) and walks the Pillow buffer row-major instead of
+# column-major so each row is contiguous.
+#
+# SAFETY
+# ------
+# Builds into a scratch directory; touches the installed module only in the
+# --install step, and backs up the original first. Roll back at any time with:
+#
+#     sudo bash scripts/build_rgbmatrix_nogil.sh --rollback
+#
+# USAGE
+#     bash scripts/build_rgbmatrix_nogil.sh              # build only
+#     sudo bash scripts/build_rgbmatrix_nogil.sh --install
+#     sudo bash scripts/build_rgbmatrix_nogil.sh --rollback
+#
+set -uo pipefail
+
+SRC_TREE="${RGB_SRC_TREE:-$HOME/LEDMatrix/rpi-rgb-led-matrix-master}"
+BUILD_DIR="${RGB_BUILD_DIR:-$HOME/rgbmatrix-nogil-build}"
+VENV="${RGB_CYTHON_VENV:-$HOME/.cache/ledmatrix-cython}"
+BACKUP="${RGB_BACKUP:-$HOME/rgbmatrix-core.so.ORIGINAL}"
+
+die() { echo "FATAL: $*" >&2; exit 1; }
+
+py_site() {
+    python3 -c 'import rgbmatrix, os; print(os.path.dirname(rgbmatrix.__file__))' 2>/dev/null
+}
+
+abi_so() {
+    ls "$BUILD_DIR"/bindings/python/rgbmatrix/core.cpython-*.so 2>/dev/null | head -1
+}
+
+do_rollback() {
+    local dst; dst="$(py_site)" || die "rgbmatrix not importable"
+    [ -n "$dst" ] || die "could not locate the installed rgbmatrix package"
+    [ -f "$BACKUP" ] || die "no backup at $BACKUP"
+    systemctl stop ledmatrix 2>/dev/null
+    cp -a "$BACKUP" "$dst/core.so" || die "restore failed"
+    find "$dst" -name __pycache__ -type d -exec rm -rf {} + 2>/dev/null
+    systemctl start ledmatrix 2>/dev/null
+    echo "rolled back to the original core.so"
+    exit 0
+}
+
+do_install() {
+    local so dst
+    so="$(abi_so)"; [ -n "$so" ] || die "no built module found - run the build first"
+    dst="$(py_site)"; [ -n "$dst" ] || die "could not locate the installed rgbmatrix package"
+
+    if [ ! -f "$BACKUP" ]; then
+        cp -a "$dst/core.so" "$BACKUP" || die "could not back up the original"
+        echo "backed up original core.so -> $BACKUP"
+    else
+        echo "backup already present at $BACKUP (keeping the true original)"
+    fi
+
+    systemctl stop ledmatrix 2>/dev/null
+    cp "$so" "$dst/core.so" || die "install failed"
+    find "$dst" -name __pycache__ -type d -exec rm -rf {} + 2>/dev/null
+    systemctl start ledmatrix 2>/dev/null
+
+    echo "waiting 25s for the display to come back..."
+    sleep 25
+    local healthy=1
+    systemctl is-active --quiet ledmatrix || healthy=0
+    if journalctl -u ledmatrix --since "40 sec ago" --no-pager \
+         | grep -qiE "Traceback|ImportError|Segmentation fault|undefined symbol"; then
+        healthy=0
+    fi
+    if [ "$healthy" = "1" ]; then
+        echo "SUCCESS - running on the rebuilt binding"
+    else
+        echo "UNHEALTHY - rolling back"
+        cp -a "$BACKUP" "$dst/core.so"
+        systemctl restart ledmatrix
+        journalctl -u ledmatrix --since "90 sec ago" --no-pager | tail -25
+        exit 1
+    fi
+    exit 0
+}
+
+case "${1:-}" in
+    --rollback) do_rollback ;;
+    --install)  do_install ;;
+    "" )        ;;
+    *)          die "unknown option: $1" ;;
+esac
+
+# ---------------------------------------------------------------- build ----
+[ -d "$SRC_TREE" ] || die "matrix source tree not found at $SRC_TREE (set RGB_SRC_TREE)"
+command -v g++ >/dev/null || die "g++ not installed (apt install build-essential)"
+
+echo "==> staging a scratch copy at $BUILD_DIR"
+rm -rf "$BUILD_DIR"
+cp -r "$SRC_TREE" "$BUILD_DIR" || die "copy failed"
+
+echo "==> patching the bindings to release the GIL"
+python3 - "$BUILD_DIR" <<'PYEOF' || die "patch failed"
+import io, sys
+base = sys.argv[1] + "/bindings/python/rgbmatrix/"
+
+p = base + "cppinc.pxd"
+s = io.open(p, encoding="utf-8").read()
+old = "        FrameCanvas *SwapOnVSync(FrameCanvas*, uint8_t)\n"
+new = "        FrameCanvas *SwapOnVSync(FrameCanvas*, uint8_t) nogil\n"
+if old in s:
+    io.open(p, "w", encoding="utf-8", newline="\n").write(s.replace(old, new, 1))
+    print("   cppinc.pxd: SwapOnVSync declared nogil")
+elif new in s:
+    print("   cppinc.pxd: already nogil")
+else:
+    sys.exit("could not find the SwapOnVSync declaration")
+
+p = base + "core.pyx"
+s = io.open(p, encoding="utf-8").read()
+
+old = """    def SwapOnVSync(self, FrameCanvas newFrame, uint8_t framerate_fraction = 1):
+        return __createFrameCanvas(self.__matrix.SwapOnVSync(newFrame.__canvas, framerate_fraction))
+"""
+new = """    def SwapOnVSync(self, FrameCanvas newFrame, uint8_t framerate_fraction = 1):
+        # Blocks until the panel's next vertical sync. Holding the GIL across
+        # that wait starves every other Python thread for most of each frame.
+        cdef cppinc.RGBMatrix* matrix = self.__matrix
+        cdef cppinc.FrameCanvas* frame = newFrame.__canvas
+        cdef uint8_t fraction = framerate_fraction
+        cdef cppinc.FrameCanvas* swapped
+        with nogil:
+            swapped = matrix.SwapOnVSync(frame, fraction)
+        return __createFrameCanvas(swapped)
+"""
+if old in s:
+    s = s.replace(old, new, 1)
+    print("   core.pyx: SwapOnVSync releases the GIL")
+elif "with nogil:\n            swapped = matrix.SwapOnVSync" in s:
+    print("   core.pyx: SwapOnVSync already patched")
+else:
+    sys.exit("could not find the SwapOnVSync body")
+
+old = """        buffer = get_pillow_buffer(image_capsule)
+
+        for col in range(max(0, -xstart), min(width, frame_width - xstart)):
+            for row in range(max(0, -ystart), min(height, frame_height - ystart)):
+                pixel = buffer[row][col]
+                r = (pixel ) & 0xFF
+                g = (pixel >> 8) & 0xFF
+                b = (pixel >> 16) & 0xFF
+                my_canvas.SetPixel(xstart+col, ystart+row, r, g, b)
+"""
+new = """        buffer = get_pillow_buffer(image_capsule)
+
+        # Bounds hoisted so the blit needs no Python state and can run without
+        # the GIL: it touches only a C buffer and a C++ canvas. Row-major order
+        # walks each row contiguously; col-outer re-strided the whole buffer.
+        cdef int col_start = max(0, -xstart)
+        cdef int col_end = min(width, frame_width - xstart)
+        cdef int row_start = max(0, -ystart)
+        cdef int row_end = min(height, frame_height - ystart)
+
+        with nogil:
+            for row in range(row_start, row_end):
+                for col in range(col_start, col_end):
+                    pixel = buffer[row][col]
+                    r = (pixel ) & 0xFF
+                    g = (pixel >> 8) & 0xFF
+                    b = (pixel >> 16) & 0xFF
+                    my_canvas.SetPixel(xstart+col, ystart+row, r, g, b)
+"""
+if old in s:
+    s = s.replace(old, new, 1)
+    print("   core.pyx: pixel blit releases the GIL, row-major")
+elif "with nogil:\n            for row in range(row_start, row_end):" in s:
+    print("   core.pyx: blit already patched")
+else:
+    sys.exit("could not find the SetPixelsPillow loop")
+
+io.open(p, "w", encoding="utf-8", newline="\n").write(s)
+PYEOF
+
+echo "==> building librgbmatrix.a (this takes a few minutes)"
+nice -n 10 make -C "$BUILD_DIR/lib" -j2 >/dev/null 2>&1 \
+    || die "library build failed - rerun 'make -C $BUILD_DIR/lib' to see why"
+[ -f "$BUILD_DIR/lib/librgbmatrix.a" ] || die "librgbmatrix.a was not produced"
+
+echo "==> preparing Cython"
+[ -d "$VENV" ] || python3 -m venv --system-site-packages "$VENV" || die "venv failed"
+"$VENV/bin/pip" install --quiet cython || die "cython install failed"
+
+cat > "$BUILD_DIR/bindings/python/setup.py" <<'EOF'
+from setuptools import setup, Extension
+from Cython.Build import cythonize
+
+core = Extension(
+    "rgbmatrix.core",
+    sources=["rgbmatrix/core.pyx", "rgbmatrix/shims/pillow.c"],
+    include_dirs=["../../include", "rgbmatrix/shims"],
+    extra_objects=["../../lib/librgbmatrix.a"],
+    language="c++",
+    extra_compile_args=["-O3", "-Wall", "-fno-exceptions", "-std=c++11"],
+    extra_link_args=["-lrt", "-lm", "-lpthread"],
+)
+
+setup(name="rgbmatrix",
+      ext_modules=cythonize([core], language_level="3str",
+                            compiler_directives={"binding": False}))
+EOF
+
+echo "==> compiling the extension"
+( cd "$BUILD_DIR/bindings/python" && "$VENV/bin/python" setup.py build_ext --inplace ) \
+    >/dev/null 2>&1 || die "extension build failed"
+
+SO="$(abi_so)"; [ -n "$SO" ] || die "no .so produced"
+
+# Verify the GIL really is released before anyone installs this.
+PAIRS=$(grep -c "PyEval_SaveThread\|Py_UNBLOCK_THREADS" "$BUILD_DIR/bindings/python/rgbmatrix/core.cpp")
+[ "$PAIRS" -ge 2 ] || die "generated C++ has only $PAIRS GIL releases, expected >= 2"
+
+echo
+echo "BUILT: $SO"
+echo "       ($PAIRS GIL-release sites in the generated C++)"
+echo
+echo "Install with:  sudo bash $0 --install"
+echo "Roll back with: sudo bash $0 --rollback"

src/cache/disk_cache.py

@@ -14,6 +14,11 @@ import zlib
 from typing import Dict, Any, Optional, Protocol
 from datetime import datetime
 
+try:  # optional: large speedup on the cache write path, see _dumps below
+    import orjson
+except ImportError:  # pragma: no cover - exercised on hosts without the wheel
+    orjson = None
+
 # How old an abandoned write's temp file must be before the sweep removes it.
 # A real write holds its temp file for milliseconds, so an hour is far beyond
 # any in-flight write while still clearing the same day's debris. Deliberately
@@ -40,13 +45,52 @@ class CacheStrategyProtocol(Protocol):
 
 
 class DateTimeEncoder(json.JSONEncoder):
-    """JSON encoder that handles datetime objects."""
+    """JSON encoder that handles datetime objects.
+
+    Retained for the stdlib fallback path and for any caller importing it.
+    """
     def default(self, obj: Any) -> Any:
         if isinstance(obj, datetime):
             return obj.isoformat()
         return super().default(obj)
 
 
+def _datetime_default(obj: Any) -> Any:
+    """Serialise datetimes exactly as DateTimeEncoder did."""
+    if isinstance(obj, datetime):
+        return obj.isoformat()
+    raise TypeError(f"Object of type {type(obj).__name__} is not JSON serializable")
+
+
+if orjson is not None:
+    # Encoding the cache record dominated the background fetch worker: on a
+    # Pi 4, stdlib json.dumps runs ~12ms per MB and holds the GIL for all of
+    # it, which stalls the render thread mid-scroll. orjson measures ~7x
+    # faster on the same payloads (11.9ms -> 1.6ms for 985KB). Decoding gains
+    # far less (~1.3x on large payloads) because the cost there is building
+    # the Python objects, not scanning the text, but it is still free to take.
+    #
+    # OPT_NON_STR_KEYS: stdlib json coerces int/float dict keys to strings;
+    # orjson raises without this, and cache records do carry numeric keys.
+    # OPT_PASSTHROUGH_DATETIME: orjson would otherwise emit its own RFC 3339
+    # form for datetimes instead of calling default(). Routing them through
+    # _datetime_default keeps byte-for-byte parity with the records already
+    # on disk.
+    _DUMPS_OPTS = orjson.OPT_NON_STR_KEYS | orjson.OPT_PASSTHROUGH_DATETIME
+
+    def _dumps(data: Any) -> bytes:
+        return orjson.dumps(data, default=_datetime_default, option=_DUMPS_OPTS)
+
+    def _loads(raw: bytes) -> Any:
+        return orjson.loads(raw)
+else:
+    def _dumps(data: Any) -> bytes:
+        return json.dumps(data, cls=DateTimeEncoder).encode("utf-8")
+
+    def _loads(raw: bytes) -> Any:
+        return json.loads(raw)
+
+
 class DiskCache:
     """Manages persistent disk-based cache."""
     
@@ -99,8 +143,8 @@ class DiskCache:
         
         try:
             with self._lock:
-                with open(cache_path, 'r', encoding='utf-8') as f:
-                    record = json.load(f)
+                with open(cache_path, 'rb') as f:
+                    record = _loads(f.read())
             
             # Determine record timestamp (prefer embedded, else file mtime)
             record_ts = None
@@ -189,12 +233,12 @@ class DiskCache:
         # write path below, and cache files are machine-read only — indenting
         # them just multiplied the bytes written to the SD card.
         try:
-            payload = json.dumps(data, cls=DateTimeEncoder)
+            payload = _dumps(data)
         except (TypeError, ValueError) as e:
             self.logger.warning("Cache data for key '%s' not serializable: %s", key, e)
             return
 
-        digest = zlib.adler32(payload.encode('utf-8'))
+        digest = zlib.adler32(payload)
 
         try:
             # Atomic write to avoid partial/corrupt files
@@ -242,7 +286,7 @@ class DiskCache:
                         # wear source (dozens of fsyncs/min on API-heavy
                         # installs) for data that can be re-downloaded.
                         try:
-                            with os.fdopen(fd, 'w', encoding='utf-8') as tmp_file:
+                            with os.fdopen(fd, 'wb') as tmp_file:
                                 tmp_file.write(payload)
                             os.replace(tmp_path, cache_path)
                             self._write_digests[key] = digest
@@ -260,7 +304,7 @@ class DiskCache:
                     else:
                         # Fallback: direct write (not atomic, but better than failing)
                         try:
-                            with open(cache_path, 'w', encoding='utf-8') as cache_file:
+                            with open(cache_path, 'wb') as cache_file:
                                 cache_file.write(payload)
                             self._write_digests[key] = digest
                             # Set proper permissions: 660 (rw-rw----) for group-readable cache files
@@ -290,7 +334,7 @@ class DiskCache:
                             # is a different path, so future sets must keep
                             # retrying the primary location.
                             fallback_path = os.path.join(fallback_dir, os.path.basename(cache_path))
-                            with open(fallback_path, 'w', encoding='utf-8') as tmp_file:
+                            with open(fallback_path, 'wb') as tmp_file:
                                 tmp_file.write(payload)
                             # Set proper permissions: 660 (rw-rw----) for group-readable cache files
                             try:

src/common/__init__.py

@@ -23,6 +23,13 @@ from src.common.error_handler import (
 )
 from src.common.api_helper import APIHelper
 from src.common.scroll_helper import ScrollHelper
+from src.common import scroll_config
+from src.common.scroll_config import (
+    ScrollSettings,
+    configure as configure_scroll,
+    resolve as resolve_scroll_settings,
+    refresh_hz_from_config,
+)
 from src.common.logo_helper import LogoHelper
 from src.common.text_helper import TextHelper
 
@@ -60,6 +67,11 @@ __all__ = [
     'log_and_raise',
     'APIHelper',
     'ScrollHelper',
+    'scroll_config',
+    'ScrollSettings',
+    'configure_scroll',
+    'resolve_scroll_settings',
+    'refresh_hz_from_config',
     'LogoHelper',
     'TextHelper',
     # adaptive layout & images

src/common/scroll_config.py

@@ -0,0 +1,224 @@
+"""One place that turns plugin config into a configured ScrollHelper.
+
+Five ticker plugins each hand-rolled this resolution (odds-ticker, news and
+ledmatrix-leaderboard reference the deprecated ``scroll_pixels_per_second``
+key 16-18 times apiece), and they disagreed in ways that were invisible until
+someone watched the panel:
+
+* odds-ticker read ``scroll_pixels_per_second`` on the *recommended* config
+  path and let it override ``scroll_speed``/``scroll_delay``. Because that key
+  carries a schema default, the documented settings were dead for every user
+  -- see ChuckBuilds/ledmatrix-plugins#408.
+* ledmatrix-leaderboard read the same key only as a fallback, so identical
+  config produced different speeds in the two plugins.
+* stock-news derived px/frame from it via its own arithmetic.
+
+What matters on the hardware
+----------------------------
+Motion is smooth when the strip advances a **whole number of pixels per panel
+refresh**. On a 100Hz panel that means 100 px/s, 200 px/s, and so on. Anything
+else has to either blend adjacent columns (which on pixel-font text reads as
+shimmer) or repeat frames (which reads as judder). :func:`resolve` warns when
+the requested speed will not divide evenly, because that is a real display
+artefact and not a rounding detail.
+
+Speed is always expressed to the helper as pixels per second and applied in
+time-based mode. Frame-based stepping gates motion on a wall clock at
+``1/scroll_delay`` steps per second; plugins set ``scroll_delay`` to the frame
+period, which puts that comparison exactly on its own threshold and makes the
+step count flip on sub-millisecond jitter. Accumulating elapsed time keeps
+position proportional to real time instead.
+"""
+
+from __future__ import annotations
+
+import logging
+from dataclasses import dataclass
+from typing import Any, Dict, Optional
+
+logger = logging.getLogger(__name__)
+
+#: Speed used when a plugin supplies nothing usable. One pixel per refresh on a
+#: 100Hz panel, which is the slowest crisp scroll that hardware can show.
+DEFAULT_PIXELS_PER_SECOND = 100.0
+
+#: Bounds accepted from config. Below the floor a marquee appears frozen;
+#: above the ceiling it outruns any panel's refresh and tears.
+MIN_PIXELS_PER_SECOND = 1.0
+MAX_PIXELS_PER_SECOND = 500.0
+
+#: Assumed refresh when the caller does not say. Matches the usual
+#: ``display.hardware.limit_refresh_rate_hz``.
+DEFAULT_REFRESH_HZ = 100.0
+
+#: How far px/s may sit from a whole number of pixels per refresh before it is
+#: worth warning about. 0.05px per frame is invisible; a third of a pixel is not.
+_WHOLE_PIXEL_TOLERANCE = 0.05
+
+
+@dataclass(frozen=True)
+class ScrollSettings:
+    """The resolved outcome, and which config key produced it."""
+
+    pixels_per_second: float
+    source: str
+    target_fps: Optional[float] = None
+    pixels_per_frame: Optional[float] = None
+    warning: Optional[str] = None
+
+    def describe(self) -> str:
+        text = f"{self.pixels_per_second:.1f} px/s (from {self.source})"
+        if self.pixels_per_frame is not None:
+            text += f" = {self.pixels_per_frame:.2f} px/frame"
+        if self.target_fps:
+            text += f" at {self.target_fps:.0f} fps"
+        return text
+
+
+def _coerce(value: Any) -> Optional[float]:
+    """A positive float, or None. Config reaches us with nulls and strings."""
+    if value is None or isinstance(value, bool):
+        return None
+    try:
+        number = float(value)
+    except (TypeError, ValueError):
+        return None
+    return number if number > 0 else None
+
+
+def _from_speed_and_delay(block: Any) -> Optional[float]:
+    """px/s from a ``scroll_speed`` (px/frame) + ``scroll_delay`` (s) pair."""
+    if not isinstance(block, dict):
+        return None
+    speed = _coerce(block.get("scroll_speed"))
+    delay = _coerce(block.get("scroll_delay"))
+    if speed is None or delay is None:
+        return None
+    return speed / delay
+
+
+def resolve(
+    plugin_config: Optional[Dict[str, Any]] = None,
+    global_config: Optional[Dict[str, Any]] = None,
+    default_pixels_per_second: float = DEFAULT_PIXELS_PER_SECOND,
+    refresh_hz: Optional[float] = None,
+) -> ScrollSettings:
+    """Resolve one scroll speed from the several shapes plugins accept.
+
+    Precedence, highest first. The deprecated flat key sits *below* the
+    explicit pairs deliberately: it carries schema defaults in some plugins, so
+    ranking it above them silently disables the documented settings.
+
+    1. ``display_options.scroll_speed`` + ``scroll_delay``  (current)
+    2. ``display.scroll_speed`` + ``scroll_delay``          (deprecated shape)
+    3. ``scroll_speed`` + ``scroll_delay`` at the root      (legacy flat)
+    4. ``scroll_pixels_per_second``, nested or flat         (deprecated)
+    5. the global ``display`` block
+    6. ``default_pixels_per_second``
+
+    :param refresh_hz: panel refresh, used only to check whether the resolved
+        speed lands on whole pixels per frame and to fill in ``target_fps``.
+    """
+    plugin_config = plugin_config or {}
+    global_config = global_config or {}
+    refresh = _coerce(refresh_hz) or DEFAULT_REFRESH_HZ
+
+    display_options = plugin_config.get("display_options")
+    display_block = plugin_config.get("display")
+
+    candidates = [
+        (_from_speed_and_delay(display_options), "display_options.scroll_speed/delay"),
+        (_from_speed_and_delay(display_block), "display.scroll_speed/delay"),
+        (_from_speed_and_delay(plugin_config), "scroll_speed/delay (root)"),
+    ]
+    for block, label in (
+        (display_options, "display_options.scroll_pixels_per_second"),
+        (display_block, "display.scroll_pixels_per_second"),
+        (plugin_config, "scroll_pixels_per_second"),
+    ):
+        if isinstance(block, dict):
+            candidates.append((_coerce(block.get("scroll_pixels_per_second")), label))
+
+    global_display = global_config.get("display")
+    candidates.append((_from_speed_and_delay(global_display), "global display.scroll_speed/delay"))
+
+    pixels_per_second = None
+    source = "default"
+    for value, label in candidates:
+        if value is not None:
+            pixels_per_second, source = value, label
+            break
+    if pixels_per_second is None:
+        pixels_per_second = default_pixels_per_second
+
+    clamped = max(MIN_PIXELS_PER_SECOND, min(MAX_PIXELS_PER_SECOND, pixels_per_second))
+    warning = None
+    if clamped != pixels_per_second:
+        warning = (
+            f"scroll speed {pixels_per_second:.1f} px/s out of range, "
+            f"clamped to {clamped:.1f}"
+        )
+        pixels_per_second = clamped
+
+    pixels_per_frame = pixels_per_second / refresh if refresh > 0 else None
+    if warning is None and pixels_per_frame is not None:
+        offset = abs(pixels_per_frame - round(pixels_per_frame))
+        if pixels_per_frame < 1.0 - _WHOLE_PIXEL_TOLERANCE or offset > _WHOLE_PIXEL_TOLERANCE:
+            suggestion = max(1.0, round(pixels_per_frame)) * refresh
+            warning = (
+                f"{pixels_per_second:.1f} px/s is {pixels_per_frame:.2f} px per "
+                f"refresh at {refresh:.0f}Hz, so some frames repeat and the "
+                f"scroll will judder; {suggestion:.0f} px/s divides evenly"
+            )
+
+    return ScrollSettings(
+        pixels_per_second=pixels_per_second,
+        source=source,
+        target_fps=refresh,
+        pixels_per_frame=pixels_per_frame,
+        warning=warning,
+    )
+
+
+def configure(
+    scroll_helper: Any,
+    plugin_config: Optional[Dict[str, Any]] = None,
+    global_config: Optional[Dict[str, Any]] = None,
+    default_pixels_per_second: float = DEFAULT_PIXELS_PER_SECOND,
+    refresh_hz: Optional[float] = None,
+    plugin_logger: Optional[logging.Logger] = None,
+) -> ScrollSettings:
+    """Resolve the config and apply it to ``scroll_helper``.
+
+    Applied in time-based mode: see the module docstring for why frame-based
+    stepping is not used. ``hasattr`` guards keep this usable against older
+    ScrollHelper builds that a plugin may be running on.
+
+    :returns: the settings applied, so the caller can log or assert on them.
+    """
+    log = plugin_logger or logger
+    settings = resolve(
+        plugin_config,
+        global_config,
+        default_pixels_per_second=default_pixels_per_second,
+        refresh_hz=refresh_hz,
+    )
+
+    if hasattr(scroll_helper, "set_frame_based_scrolling"):
+        scroll_helper.set_frame_based_scrolling(False)
+    scroll_helper.set_scroll_speed(settings.pixels_per_second)
+    if settings.target_fps and hasattr(scroll_helper, "set_target_fps"):
+        scroll_helper.set_target_fps(settings.target_fps)
+
+    log.info("Scroll configured: %s", settings.describe())
+    if settings.warning:
+        log.warning("Scroll speed: %s", settings.warning)
+    return settings
+
+
+def refresh_hz_from_config(global_config: Optional[Dict[str, Any]]) -> float:
+    """The panel's refresh cap from the global config, or the default."""
+    if not isinstance(global_config, dict):
+        return DEFAULT_REFRESH_HZ
+    hardware = (global_config.get("display") or {}).get("hardware") or {}
+    return _coerce(hardware.get("limit_refresh_rate_hz")) or DEFAULT_REFRESH_HZ

src/common/scroll_helper.py

@@ -75,8 +75,19 @@ class ScrollHelper:
         # Pre-allocated buffer for output frame (reused to avoid allocations)
         self._frame_buffer: Optional[np.ndarray] = None
         
-        # Sub-pixel scrolling settings (disabled - using high FPS integer scrolling instead)
-        self.sub_pixel_scrolling = False  # Disabled - use high frame rate for smoothness
+        # Sub-pixel scrolling: OFF by default, and that is deliberate.
+        # Blending renders a half-step by mixing two adjacent columns 50/50.
+        # On a high-resolution screen that reads as smooth motion; on a coarse
+        # LED matrix showing pixel-font text it does not. A one-pixel stroke
+        # becomes two half-brightness pixels, so frames alternate between crisp
+        # and smeared and the text appears to shimmer and jump a pixel ahead --
+        # tested on a 2x128x64 panel and clearly worse than integer stepping.
+        #
+        # The rule this display obeys: motion is smooth when it advances a
+        # whole number of pixels per refresh. Anything slower must either
+        # blend (blur) or repeat frames (judder); blending is the worse of the
+        # two here. Vegas mode still opts in via set_sub_pixel_scrolling().
+        self.sub_pixel_scrolling = False
         self._last_integer_position = 0  # Cache for integer position to avoid repeated calculations
         
         # Frame-based scrolling settings
@@ -244,19 +255,31 @@ class ScrollHelper:
             if self.last_step_time == 0.0:
                 self.last_step_time = current_time
             
-            # Check if scroll_delay has passed
-            time_since_last_step = current_time - self.last_step_time
-            if time_since_last_step >= self.scroll_delay:
-                # Move pixels (can move multiple steps if lag occurred, but cap to prevent huge jumps)
-                steps = int(time_since_last_step / self.scroll_delay)
-                # Cap at reasonable number to prevent huge jumps from lag
-                max_steps = max(1, int(0.04 / self.scroll_delay))  # Limit to 0.04s (2 steps at 50 FPS) for smoother scrolling
-                steps = min(steps, max_steps)
-                pixels_to_move = self.scroll_speed * steps
-                # Update last_step_time, preserving fractional delay for smooth timing
-                self.last_step_time = current_time - (time_since_last_step % self.scroll_delay)
+            # Frame-based mode advances by elapsed time, exactly like the
+            # time-based branch below, at the same configured speed
+            # (scroll_speed px per scroll_delay seconds).
+            #
+            # It used to step discretely: 0, 1 or 2 whole pixels depending on
+            # whether a wall clock had passed scroll_delay. Plugins set
+            # scroll_delay to the target frame period, so that comparison sits
+            # exactly on its own threshold and the decision flips on sub-
+            # millisecond jitter -- a frame a hair early moved nothing and
+            # rendered an identical frame, a frame a hair late moved two
+            # pixels. Rounding the step count fixed the stalls but still
+            # discarded the remainder, so the error never corrected.
+            #
+            # Accumulating elapsed time keeps position exactly proportional to
+            # real time: jitter shifts a pixel boundary by a fraction of a
+            # frame instead of flipping a whole step, and nothing is lost or
+            # gained. This is what the one visibly smooth scroller on the
+            # hardware (the stock ticker) was already doing by virtue of never
+            # enabling frame-based mode.
+            if self.scroll_delay > 0:
+                pixels_per_second = self.scroll_speed / self.scroll_delay
             else:
-                pixels_to_move = 0.0
+                pixels_per_second = self.scroll_speed * 100.0
+            pixels_to_move = pixels_per_second * delta_time
+            self.last_step_time = current_time
         else:
             # Time-based: move based on time delta (correct speed over time)
             # scroll_speed is pixels per second

src/display_controller.py

@@ -2405,6 +2405,7 @@ class DisplayController:
                             )
 
                             while True:
+                                _frame_start = time.perf_counter()
                                 try:
                                     with self._display_lock_or_skip(plugin_id) as can_display:
                                         if can_display:
@@ -2425,11 +2426,26 @@ class DisplayController:
                                 # Multi-display sync: send follower frame after each render
                                 self._send_follower_frame(manager_to_display)
 
-                                time.sleep(display_interval)
                                 self._tick_plugin_updates()
                                 self._poll_on_demand_requests()
                                 self._check_on_demand_expiration()
 
+                                # Pace to the frame deadline rather than sleeping a flat
+                                # interval on top of the work. display() has already
+                                # blocked on the panel's vsync by this point, so an
+                                # unconditional sleep is added to a wait that already
+                                # happened. Measured on a 2x128x64 chain at
+                                # limit_refresh_rate_hz=100: ~4ms of render plus a flat
+                                # 8ms put each iteration at ~12ms against a 10ms refresh
+                                # grid, so every swap missed a refresh and the loop
+                                # settled at 50fps where display_interval asks for 125 --
+                                # and with zero headroom, ~14% of frames slipped a
+                                # further refresh, which is what reads as scroll stutter.
+                                _remaining = display_interval - (time.perf_counter() - _frame_start)
+                                # Yield even when the frame overran its budget, so plugin
+                                # update threads and the web UI are not starved of the GIL.
+                                time.sleep(_remaining if _remaining > 0 else 0.001)
+
                                 if self.current_display_mode != active_mode:
                                     logger.debug("Mode changed during high-FPS loop, breaking early")
                                     break

src/display_manager.py

@@ -768,16 +768,18 @@ class DisplayManager:
                     return  # Skip hardware write — content is being captured off-screen
 
                 digest = None
+                frame_checksum = None
                 if self._dirty_tracking_enabled:
                     try:
                         brightness = getattr(self.matrix, 'brightness', None)
                     except AttributeError:
                         brightness = None
-                    digest = (zlib.adler32(self.image.tobytes()), brightness)
+                    frame_checksum = zlib.adler32(self.image.tobytes())
+                    digest = (frame_checksum, brightness)
                     if digest == self._last_pushed_digest:
                         # Nothing changed since the last push — the panel is
                         # already showing exactly this frame.
-                        self._write_snapshot_if_due()
+                        self._write_snapshot_if_due(frame_checksum)
                         return
 
                 # Copy the current image to the offscreen canvas. In double-sided
@@ -796,7 +798,7 @@ class DisplayManager:
                 self._last_pushed_digest = digest
 
                 # Write a snapshot for the web preview (throttled)
-                self._write_snapshot_if_due()
+                self._write_snapshot_if_due(frame_checksum)
         except Exception as e:
             logger.error(f"Error updating display: {e}")
 
@@ -1410,11 +1412,20 @@ class DisplayManager:
                 self._viewer_fresh = False
         return self._viewer_fresh
 
-    def _write_snapshot_if_due(self) -> None:
+    def _write_snapshot_if_due(self, frame_checksum: Optional[int] = None) -> None:
         """Mirror the current frame to the preview snapshot when the policy
         says it's worth it — see src/common/snapshot_policy.py. Unchanged
         frames are never re-encoded; without viewers the cadence drops to
-        the idle keepalive."""
+        the idle keepalive.
+
+        Args:
+            frame_checksum: adler32 of the current frame, when the caller has
+                already computed one. Dirty tracking checksums every frame a
+                few lines above the call site, and re-deriving it here meant a
+                second tobytes() plus a second pass over the whole framebuffer
+                on every single frame — ~0.17ms per frame of the two combined
+                at 256x64, paid 100 times a second to reach the same number.
+        """
         try:
             now = time.time()
             viewer_fresh = self._viewer_is_fresh(now)
@@ -1424,7 +1435,8 @@ class DisplayManager:
                 self._last_snapshot_ts = 0.0
             self._viewer_was_fresh = viewer_fresh
 
-            digest = zlib.adler32(self.image.tobytes())
+            digest = (frame_checksum if frame_checksum is not None
+                      else zlib.adler32(self.image.tobytes()))
             action = snapshot_policy.decide(
                 now, self._last_snapshot_ts, self._last_snapshot_touch_ts,
                 viewer_fresh, digest != self._last_snapshot_digest)

test/test_scroll_config.py

@@ -0,0 +1,213 @@
+"""Tests for the shared scroll configuration resolver."""
+
+import logging
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from src.common.scroll_config import (  # noqa: E402
+    DEFAULT_PIXELS_PER_SECOND,
+    MAX_PIXELS_PER_SECOND,
+    MIN_PIXELS_PER_SECOND,
+    ScrollSettings,
+    configure,
+    refresh_hz_from_config,
+    resolve,
+)
+
+
+class FakeHelper:
+    """Records what configure() applied."""
+
+    def __init__(self, with_optional=True):
+        self.speed = None
+        self.frame_based = None
+        self.target_fps = None
+        if not with_optional:
+            del FakeHelper.set_frame_based_scrolling
+            del FakeHelper.set_target_fps
+
+    def set_scroll_speed(self, speed):
+        self.speed = speed
+
+    def set_frame_based_scrolling(self, enabled):
+        self.frame_based = enabled
+
+    def set_target_fps(self, fps):
+        self.target_fps = fps
+
+
+class MinimalHelper:
+    """An older helper exposing only set_scroll_speed."""
+
+    def __init__(self):
+        self.speed = None
+
+    def set_scroll_speed(self, speed):
+        self.speed = speed
+
+
+class TestPrecedence:
+    def test_display_options_pair_wins(self):
+        s = resolve({"display_options": {"scroll_speed": 1.0, "scroll_delay": 0.01}})
+        assert s.pixels_per_second == 100.0
+        assert s.source == "display_options.scroll_speed/delay"
+
+    def test_display_block_used_when_options_absent(self):
+        s = resolve({"display": {"scroll_speed": 2.0, "scroll_delay": 0.01}})
+        assert s.pixels_per_second == 200.0
+        assert s.source == "display.scroll_speed/delay"
+
+    def test_root_pair_used_when_both_blocks_absent(self):
+        s = resolve({"scroll_speed": 1.0, "scroll_delay": 0.02})
+        assert s.pixels_per_second == 50.0
+
+    def test_pixels_per_second_used_when_no_pair_given(self):
+        s = resolve({"display_options": {"scroll_pixels_per_second": 120.0}})
+        assert s.pixels_per_second == 120.0
+        assert s.source == "display_options.scroll_pixels_per_second"
+
+    def test_global_display_is_the_last_resort_before_default(self):
+        s = resolve({}, {"display": {"scroll_speed": 1.0, "scroll_delay": 0.005}})
+        assert s.pixels_per_second == 200.0
+
+    def test_default_when_nothing_configured(self):
+        s = resolve({}, {})
+        assert s.pixels_per_second == DEFAULT_PIXELS_PER_SECOND
+        assert s.source == "default"
+
+
+class TestDeprecatedKeyCannotOverrideExplicitPair:
+    """Regression for ChuckBuilds/ledmatrix-plugins#408.
+
+    odds-ticker ranked scroll_pixels_per_second above the documented
+    scroll_speed/scroll_delay pair. Because that key carries a schema default,
+    the documented settings became unreachable for every user and the ticker
+    silently ran at the default speed. The pair must win.
+    """
+
+    def test_pair_beats_pixels_per_second_in_the_same_block(self):
+        s = resolve(
+            {
+                "display_options": {
+                    "scroll_speed": 1.0,
+                    "scroll_delay": 0.01,
+                    "scroll_pixels_per_second": 50.0,  # schema default
+                }
+            }
+        )
+        assert s.pixels_per_second == 100.0
+        assert "scroll_speed/delay" in s.source
+
+    def test_pair_beats_pixels_per_second_in_an_outer_block(self):
+        s = resolve(
+            {
+                "display_options": {"scroll_speed": 1.0, "scroll_delay": 0.01},
+                "scroll_pixels_per_second": 50.0,
+            }
+        )
+        assert s.pixels_per_second == 100.0
+
+
+class TestMalformedValues:
+    @pytest.mark.parametrize("bad", [None, "fast", "", {}, [], float("nan")])
+    def test_unusable_speed_falls_through(self, bad):
+        s = resolve({"display_options": {"scroll_speed": bad, "scroll_delay": 0.01}})
+        assert s.pixels_per_second == DEFAULT_PIXELS_PER_SECOND
+
+    @pytest.mark.parametrize("bad", [0, -5, 0.0])
+    def test_non_positive_values_fall_through(self, bad):
+        s = resolve({"display_options": {"scroll_pixels_per_second": bad}})
+        assert s.pixels_per_second == DEFAULT_PIXELS_PER_SECOND
+
+    def test_booleans_are_not_treated_as_numbers(self):
+        s = resolve({"display_options": {"scroll_pixels_per_second": True}})
+        assert s.pixels_per_second == DEFAULT_PIXELS_PER_SECOND
+
+    def test_zero_delay_does_not_divide_by_zero(self):
+        s = resolve({"display_options": {"scroll_speed": 1.0, "scroll_delay": 0}})
+        assert s.pixels_per_second == DEFAULT_PIXELS_PER_SECOND
+
+    def test_non_dict_blocks_are_ignored(self):
+        s = resolve({"display_options": "nonsense", "display": 5})
+        assert s.pixels_per_second == DEFAULT_PIXELS_PER_SECOND
+
+
+class TestClamping:
+    def test_absurdly_fast_is_clamped(self):
+        s = resolve({"display_options": {"scroll_pixels_per_second": 100000.0}})
+        assert s.pixels_per_second == MAX_PIXELS_PER_SECOND
+        assert s.warning and "clamped" in s.warning
+
+    def test_absurdly_slow_is_clamped(self):
+        s = resolve({"display_options": {"scroll_pixels_per_second": 0.01}})
+        assert s.pixels_per_second == MIN_PIXELS_PER_SECOND
+
+
+class TestWholePixelWarning:
+    """The display rule: whole pixels per refresh, or it judders."""
+
+    def test_no_warning_when_speed_divides_evenly(self):
+        s = resolve({"display_options": {"scroll_pixels_per_second": 100.0}}, refresh_hz=100)
+        assert s.warning is None
+        assert s.pixels_per_frame == pytest.approx(1.0)
+
+    def test_no_warning_at_an_integer_multiple(self):
+        s = resolve({"display_options": {"scroll_pixels_per_second": 200.0}}, refresh_hz=100)
+        assert s.warning is None
+
+    def test_warns_on_a_half_pixel_per_frame(self):
+        s = resolve({"display_options": {"scroll_pixels_per_second": 50.0}}, refresh_hz=100)
+        assert s.warning is not None
+        assert "judder" in s.warning
+        assert "100 px/s" in s.warning
+
+    def test_warns_on_a_fractional_multiple(self):
+        s = resolve({"display_options": {"scroll_pixels_per_second": 130.0}}, refresh_hz=100)
+        assert s.warning is not None
+
+    def test_respects_a_non_default_refresh_rate(self):
+        s = resolve({"display_options": {"scroll_pixels_per_second": 150.0}}, refresh_hz=150)
+        assert s.warning is None
+        assert s.pixels_per_frame == pytest.approx(1.0)
+
+
+class TestConfigure:
+    def test_applies_time_based_mode_and_speed(self):
+        helper = FakeHelper()
+        s = configure(helper, {"display_options": {"scroll_speed": 1.0, "scroll_delay": 0.01}})
+        assert helper.speed == 100.0
+        assert helper.frame_based is False, "must not use the wall-clock step gate"
+        assert helper.target_fps == 100.0
+        assert s.pixels_per_second == 100.0
+
+    def test_works_against_a_helper_without_optional_methods(self):
+        helper = MinimalHelper()
+        configure(helper, {"display_options": {"scroll_pixels_per_second": 100.0}})
+        assert helper.speed == 100.0
+
+    def test_logs_the_warning_when_speed_will_judder(self, caplog):
+        helper = FakeHelper()
+        with caplog.at_level(logging.WARNING):
+            configure(helper, {"display_options": {"scroll_pixels_per_second": 50.0}})
+        assert any("judder" in r.getMessage() for r in caplog.records)
+
+    def test_returns_settings_describing_the_source(self):
+        s = configure(FakeHelper(), {"display_options": {"scroll_speed": 2.0, "scroll_delay": 0.01}})
+        assert isinstance(s, ScrollSettings)
+        assert "200.0 px/s" in s.describe()
+
+
+class TestRefreshFromConfig:
+    def test_reads_the_hardware_limit(self):
+        assert refresh_hz_from_config(
+            {"display": {"hardware": {"limit_refresh_rate_hz": 150}}}
+        ) == 150.0
+
+    @pytest.mark.parametrize("cfg", [None, {}, {"display": {}}, {"display": {"hardware": {}}},
+                                     "nonsense", {"display": {"hardware": {"limit_refresh_rate_hz": None}}}])
+    def test_falls_back_to_the_default(self, cfg):
+        assert refresh_hz_from_config(cfg) == 100.0