perf(display): skip preview work nobody reads (lazy checksum, 1 Hz snapshot writer) (#717)

Mid-scroll, update_display() no longer checksums every frame: it asks is_currently_scrolling() once per frame and reuses the answer, hashes only when dirty tracking can skip a static frame, and the preview snapshot asks its policy first and hashes only when a write or touch could follow (decide() is monotone, pinned by a property test). With the preview open the snapshot is written at most once a second (VIEWER_INTERVAL 1.0 s, was 0.2 s; the SSE stream re-read it once a second, so four encodes in five went unread); the stream now polls its mtime every 0.25 s (VIEWER_POLL_INTERVAL), so the preview stays about as fresh. The PNG is written at compress_level=1. --preview soaks are not comparable across this change.

Merged with #716: _scan_segments takes the frame's one scrolling answer and #716's static-handover pass-through, as soaked on ledpi (A B B A, 20 min each: main 0.118% / 0.113% late, with #716 and this 0.107% / 0.104%).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -146,6 +146,38 @@ policies are unchanged.
   until the next minute, because the once-a-minute schedule check had
   already run that minute and the session had overridden its answer.
 
+### Web preview: less work per frame
+
+- Mid-scroll, `update_display()` no longer checksums every frame. The
+  checksum (`tobytes()` plus `adler32` over the whole framebuffer: ~0.17 ms a
+  frame at 256x64 on a Pi 4, so roughly twice that at 512x64 and well under
+  0.1 ms at 128x32) fed only the dirty-tracking skip, which never applies
+  while scrolling, and the preview snapshot's changed-frame check. The
+  snapshot now asks its policy first and hashes the frame only when a write
+  or touch could follow. That changes no snapshot decision:
+  `snapshot_policy.decide()` is monotone in `frame_changed`, and a test holds
+  it to that. Two small differences on the panel: the first static frame
+  after a scroll is pushed even when it matches the scroll's last frame (one
+  extra swap), and the frame on which a scroll that never said it stopped
+  times out is presented at a hold of 1 rather than the scroll's hold.
+- With the web preview open, the display writes the snapshot at most once a
+  second (`snapshot_policy.VIEWER_INTERVAL`, was 0.2 s). The preview already
+  showed at most one frame a second: its SSE stream re-read the file once a
+  second, so four PNG encodes in five were overwritten unread. The stream now
+  checks the file's mtime every 0.25 s (new `VIEWER_POLL_INTERVAL`) and sends
+  each frame soon after it is written, so the preview stays about as fresh;
+  it still touches the viewer marker once a second, and with no snapshot
+  file it still sends its placeholder once a second. A screen that animates
+  faster than once a second without marking itself as scrolling (a GIF, say)
+  was encoded on the render thread up to five times a second while the
+  preview was open, 12-14 ms each at 512x64 on a Pi 4; now at most once.
+- The snapshot PNG is written at `compress_level=1`. On a desktop that
+  encoded a text-dense 512x64 frame in about half Pillow's default time, into
+  a larger file (12 KB instead of 7 KB); sparser frames gain less.
+- `scripts/frame_soak.py --preview` soaks are not comparable across this
+  change: an open preview now costs at most one encode a second, not up to
+  five. Take both sides of an A/B pair on the same side of it.
+
 ### Scroller-to-static handovers
 
 - A static plugin screen that follows a scroller no longer starts with the

docs/ARCHITECTURE.md

@@ -51,8 +51,8 @@ each other. They share three things:
 | Fetch statistics (requests per plugin and host) | cache `fetch_stats_snapshot` | display: `FetchStatsPublisher` ([`src/common/fetch_service.py`](../src/common/fetch_service.py)), at most once a minute on change | web: `read_fetch_stats()` for `/api/v3/plugins/fetch-stats` |
 | Plugin health | cache `plugin_health:<id>` | display (web writes on reset) | web: `/api/v3/plugins/health` |
 | Plugin runtime (loaded, state, last error, version) | cache `plugin_runtime_snapshot` | display: `PluginRuntimePublisher` ([`src/plugin_system/plugin_runtime.py`](../src/plugin_system/plugin_runtime.py)) | web: `read_plugin_runtime()` for `/api/v3/plugins/installed`, `/plugins/state`, reconciliation |
-| Preview frame | `/tmp/led_matrix_preview.png` | display: `DisplayManager`, gated by [`snapshot_policy`](../src/common/snapshot_policy.py) | web: display SSE stream, `/api/v3/health` (file age) |
+| Preview frame | `/tmp/led_matrix_preview.png` | display: `DisplayManager`, gated by [`snapshot_policy`](../src/common/snapshot_policy.py): a changed frame at most once a second with a viewer, every 30 s without | web: display SSE stream (checks the mtime every 0.25 s), `/api/v3/health` (file age) |
-| Preview viewer marker | `/tmp/led_matrix_preview_viewer` | web, while a preview is open | display: writes full-rate snapshots only while it is fresh |
+| Preview viewer marker | `/tmp/led_matrix_preview_viewer` | web, about once a second while a preview is open | display: writes viewer-rate snapshots only while it is fresh (5 s) |
 | Hardware init status | `/tmp/led_matrix_hw_status.json` | display | web: `/api/v3/hardware/status` |
 | Render-loop heartbeat | `/run/ledmatrix/display-heartbeat.json` (tmpfs) | display: the render thread, via [`display_watchdog`](../src/display_watchdog.py) | web: `/api/v3/health` (`checks.display_loop`); the update health check |
 

docs/SCROLL_PERFORMANCE.md

@@ -335,8 +335,12 @@ python3 scripts/frame_soak.py --json a.json   # keep the report to compare later
 It runs as any user next to the display service and stops nothing. It needs
 something to *scroll* during the run: a live game holding a static scoreboard
 on screen gives no verdict. `--preview` keeps the web preview's viewer marker
-fresh, which puts the preview's PNG encoding at full rate -- run it as the web
+fresh, which puts the preview's PNG encoding at the viewer rate, as an open
-service's user.
+preview does -- run it as the web service's user. That rate is at most one
+frame a second. Through 3.8.0 it was up to five, so a `--preview` soak taken
+before that change is not comparable with one taken after it (the hdpi
+results below are from before it): take both sides of an A/B pair on
+the same side of it.
 
 | line | what it tells you |
 |---|---|

scripts/frame_soak.py

@@ -9,7 +9,10 @@ reports the difference. Nothing is stopped, restarted or drawn.
     python3 scripts/frame_soak.py
 
     # the same with the web preview open (the preview's PNG encodes are one of
-    # the things that used to make the render loop miss refreshes)
+    # the things that used to make the render loop miss refreshes). An open
+    # preview is encoded at most once a second; through 3.8.0 it was up to
+    # five times, so a --preview run from before that change is not comparable
+    # with one from after it
     python3 scripts/frame_soak.py --preview
 
     # quick look at the totals since the service started
@@ -65,7 +68,8 @@ from src.common.frame_timing import (  # noqa: E402
 )
 
 #: Touched by the web UI while someone has the preview open; a fresh marker
-#: puts the display service's snapshot writer at full rate. Same path as
+#: puts the display service's snapshot writer at the viewer rate
+#: (snapshot_policy.VIEWER_INTERVAL). Same path as
 #: DisplayManager._viewer_marker_path.
 VIEWER_MARKER = "/tmp/led_matrix_preview_viewer"  # nosec B108 - fixed path shared with the service
 

src/common/README.md

@@ -231,7 +231,8 @@ rather than the `set_*` methods. Vegas mode reads a plugin's
 [`snapshot_policy.py`](snapshot_policy.py). Core-internal. `decide()`
 tells `DisplayManager` whether to write `/tmp/led_matrix_preview.png`, only
 touch its mtime, or skip, based on whether a browser is watching the preview.
-The web health check reads the file's age.
+The web health check reads the file's age, and the web preview stream checks
+its mtime every `VIEWER_POLL_INTERVAL`.
 
 ### sports_card
 

src/common/frame_timing.py

@@ -19,8 +19,9 @@ Only intervals between two consecutive *scrolling* frames count: a static
 screen that changes once a second has no timing to get wrong, and the first
 frame of a scroll has no predecessor worth measuring against.
 
-"Scrolling" is DisplayManager's scroll state when the frame is presented, and
+"Scrolling" is the scroll state ``DisplayManager.update_display`` acted on for
-that state can go missing in the middle of a scroll. It expires after 2s
+the frame, sampled once before the blit and swap, and that state can go missing
+in the middle of a scroll. It expires after 2s
 without scroll activity, which a long enough stall outlasts, and any thread can
 clear it: plugins call ``set_scrolling_state(False)`` from their own
 ``display()``, and Vegas captures some of those on the render thread between
@@ -393,7 +394,9 @@ class FrameTimingRecorder:
         :param blit: seconds spent copying the frame into the canvas.
         :param wait: seconds SwapOnVSync blocked.
         :param hold: the refreshes this frame was held for.
-        :param scrolling: whether a scroll was running when it was presented.
+        :param scrolling: whether a scroll was running for this frame: the
+            scroll state ``update_display`` acted on, sampled once before the
+            blit and swap.
         :param presented_at: ``time.perf_counter()`` when the swap returned.
         """
         previous = self._previous

src/common/snapshot_policy.py

@@ -26,14 +26,33 @@ Policy:
 - Unchanged frames are never re-encoded; the mtime is touched every
   TOUCH_INTERVAL so the health check (60s threshold) never degrades.
 
+The writer and the SSE reader (web_interface/app.py) have two periods, not
+one shared value. The reader sends each write it sees, so the preview shows
+at most one frame per VIEWER_INTERVAL. (That was 0.2 s while the reader
+slept 1 s between reads, so four encodes in five were overwritten unread.)
+The reader checks the file's mtime every VIEWER_POLL_INTERVAL, which is only
+a stat, and so sends each write within that long of it landing. Equal
+periods would alias: two unsynchronised 1 s clocks leave the preview up to a
+second stale, and now and then 2 s between frames.
+
+decide() is monotone in frame_changed: SKIP for a changed frame means SKIP
+for an unchanged one. DisplayManager relies on that to skip hashing the
+frame when even a changed one would be skipped; test_snapshot_policy.py
+checks it.
+
 If any constant here changes, re-check the health threshold in
 api_v3/misc.py (get_hardware_status) — TOUCH_INTERVAL must stay well under it.
 """
 
 from enum import Enum
 
-# Snapshot cadence with a browser preview open (seconds).
+# Snapshot cadence with a browser preview open (seconds): the shortest gap
-VIEWER_INTERVAL = 0.2
+# between two preview frames.
+VIEWER_INTERVAL = 1.0
+# How often the web SSE reader checks the snapshot's mtime (seconds). Must
+# stay well under VIEWER_INTERVAL -- half of it at most -- or the two clocks
+# alias (see above).
+VIEWER_POLL_INTERVAL = 0.25
 # Snapshot cadence with no viewers — cheap freshness for page-open (seconds).
 IDLE_INTERVAL = 30.0
 # Max age of the last write/touch before bumping mtime for the health

src/display_manager.py

@@ -80,6 +80,15 @@ _CALENDAR_FONT_PX = 7
 #: frame, so a fault that persists would otherwise log ~100 lines a second.
 _UPDATE_ERROR_LOG_INTERVAL = 60.0
 
+#: zlib level for the preview snapshot PNG. The fastest level: each file is
+#: read by the web UI and soon replaced by the next, so encode time (paid on
+#: the render thread for a static screen) matters more than its size.
+#: Lossless at any level. Against Pillow's default (6), on a desktop with
+#: Pillow 12.3, a text-dense 512x64 frame encoded in about half the time,
+#: into 12 KB instead of 7 KB; sparser frames saved less time (10-20%) and
+#: stayed under 2 KB.
+_SNAPSHOT_PNG_COMPRESS_LEVEL = 1
+
 
 def _bdf_native_size(face) -> int:
     """The pixel height a BDF Face declares, or 0 if it does not say.
@@ -296,8 +305,8 @@ class DisplayManager:
         self._TEXT_WIDTH_CACHE_MAX = 1024
         # Snapshot mirror for web preview + health check (service writes, web
         # reads). Cadence/skip decisions live in src/common/snapshot_policy.py:
-        # full rate only while the web SSE broadcaster keeps the viewer marker
+        # the viewer rate only while the web SSE broadcaster keeps the viewer
-        # fresh; unchanged frames are never re-encoded, only mtime-touched.
+        # marker fresh; unchanged frames are never re-encoded, only mtime-touched.
         self._snapshot_path = "/tmp/led_matrix_preview.png"  # nosec B108 - fixed path intentional; web UI reads same path
         self._viewer_marker_path = "/tmp/led_matrix_preview_viewer"  # nosec B108 - touched by web SSE broadcaster
         self._last_snapshot_ts = 0.0
@@ -946,16 +955,32 @@ class DisplayManager:
                     self._write_snapshot_if_due()
                     return
 
+                # Asked once per frame and the answer reused below: the call
+                # has side effects (it expires a stale scroll and drops its
+                # frame hold), so asking again further down could disagree
+                # with what this frame was already treated as. Asked first,
+                # so the dirty check, the scan-order segments, the pacing gate,
+                # the swaps and frame timing all see one answer and the frame
+                # hold it leaves.
+                scrolling = self.is_currently_scrolling()
                 digest = None
                 frame_checksum = None
-                if self._dirty_tracking_enabled:
+                # No digest mid-scroll. The skip it feeds is never taken while
+                # scrolling (see below), so all it bought there was the
+                # snapshot's changed-frame check -- a tobytes() plus adler32
+                # over the whole framebuffer every frame (~0.17ms at 256x64
+                # on a Pi 4, twice that at 512x64) for a decision acted on at
+                # most once a second. _write_snapshot_if_due hashes for
+                # itself when a write or touch is actually due. The cost: the
+                # first static frame after a scroll is always pushed, once.
+                if self._dirty_tracking_enabled and not scrolling:
                     try:
                         brightness = getattr(self.matrix, 'brightness', None)
                     except AttributeError:
                         brightness = None
                     frame_checksum = zlib.adler32(self.image.tobytes())
                     digest = (frame_checksum, brightness)
-                    if digest == self._last_pushed_digest and not self.is_currently_scrolling():
+                    if digest == self._last_pushed_digest:
                         # Nothing changed since the last push — the panel is
                         # already showing exactly this frame.
                         #
@@ -982,7 +1007,7 @@ class DisplayManager:
                 if self._double_sided is not None:
                     segments = [(self._composite_double_sided(), self._frame_hold)]
                 else:
-                    segments = self._scan_segments(self.image)
+                    segments = self._scan_segments(self.image, scrolling)
                 gate = self.render_gate
                 blit_time = swap_time = 0.0
                 # Usually one segment: the frame, held for _frame_hold
@@ -1008,7 +1033,7 @@ class DisplayManager:
                 self._last_blit_seconds = blit_time / len(segments)
                 self.frame_timing.record(
                     blit_time, swap_time,
-                    self._frame_hold, self.is_currently_scrolling(), presented_at)
+                    self._frame_hold, scrolling, presented_at)
 
                 self._last_pushed_digest = digest
 
@@ -1055,7 +1080,9 @@ class DisplayManager:
             ", ".join(f"rows {top}-{bottom - 1} show {lag} refresh(es) behind"
                       for top, bottom, lag in bands))
 
-    def _scan_segments(self, image: Image.Image) -> List[Tuple[Image.Image, int]]:
+    def _scan_segments(self, image: Image.Image,
+                       scrolling: Optional[bool] = None
+                       ) -> List[Tuple[Image.Image, int]]:
         """What to present for this frame: ``[(image, refreshes), ...]``.
 
         Mid-scroll with compensation on, lagging rows are taken from earlier
@@ -1068,10 +1095,17 @@ class DisplayManager:
         after a scroll, while the scroll state is still set (see
         end_scroll_for_static_screen): one segment, held for the scroll's
         hold, with no rows from the scroller's frames.
+
+        ``scrolling`` is the caller's is_currently_scrolling() answer for this
+        frame. update_display() asks once, before calling this, so the frame
+        hold read here is the one that answer left (an expired scroll's hold
+        is already dropped). None asks here.
         """
         hold = self._frame_hold
         bands = getattr(self, '_scan_lag_bands', None)
-        if (not bands or not self.is_currently_scrolling()
+        if (not bands
+                or not (scrolling if scrolling is not None
+                        else self.is_currently_scrolling())
                 or self._static_handover):
             if bands:
                 self._scan_history.clear()
@@ -1563,9 +1597,10 @@ class DisplayManager:
         frame -- the bottom half of the old ticker under the new screen on a
         96x48 panel. At hold 1 that frame stays up for a whole second; at a
         longer hold its first refresh flashes the old rows. For a second frame
-        in the same call, the rows would come from the first. Dirty tracking
+        in the same call, the rows would come from the first, shown for as long
-        compares frames as drawn, so once the scroll is over it skips every
+        as the first's would be. Dirty tracking does not keep such a frame up
-        identical 1 Hz redraw of such a frame, and nothing would replace it.
+        past the screen's next redraw: a frame pushed while the scroll state
+        is set leaves it no digest to match, so that redraw is pushed.
 
         The rest of that scroll is left on purpose, until the controller ends
         it:
@@ -1729,11 +1764,14 @@ class DisplayManager:
 
         Args:
             frame_checksum: adler32 of the current frame, when the caller has
-                already computed one. Dirty tracking checksums every frame a
+                already computed one. Dirty tracking checksums every static
-                few lines above the call site, and re-deriving it here meant a
+                frame a few lines above the call site, and re-deriving it here
-                second tobytes() plus a second pass over the whole framebuffer
+                meant a second tobytes() plus a second pass over the whole
-                on every single frame — ~0.17ms per frame of the two combined
+                framebuffer on every single frame — ~0.17ms per frame of the
-                at 256x64, paid 100 times a second to reach the same number.
+                two combined at 256x64, paid 100 times a second to reach the
+                same number. None (mid-scroll, dirty tracking off, no
+                hardware): the frame is hashed here, and only when the policy
+                could act on it.
         """
         try:
             now = time.time()
@@ -1744,11 +1782,29 @@ class DisplayManager:
                 self._last_snapshot_ts = 0.0
             self._viewer_was_fresh = viewer_fresh
 
-            digest = (frame_checksum if frame_checksum is not None
+            if frame_checksum is not None:
-                      else zlib.adler32(self.image.tobytes()))
+                digest = frame_checksum
-            action = snapshot_policy.decide(
+                action = snapshot_policy.decide(
-                now, self._last_snapshot_ts, self._last_snapshot_touch_ts,
+                    now, self._last_snapshot_ts, self._last_snapshot_touch_ts,
-                viewer_fresh, digest != self._last_snapshot_digest)
+                    viewer_fresh, digest != self._last_snapshot_digest)
+            else:
+                # Ask as if the frame had changed before paying to find out.
+                # decide() is monotone in frame_changed -- a SKIP for a
+                # changed frame is a SKIP for an unchanged one too (its touch
+                # branch ignores frame_changed) -- so returning here gives the
+                # same answer the hash would have, and on most frames the hash
+                # is never taken. test_snapshot_policy.py holds decide() to it.
+                action = snapshot_policy.decide(
+                    now, self._last_snapshot_ts, self._last_snapshot_touch_ts,
+                    viewer_fresh, True)
+                if action is snapshot_policy.SnapshotAction.SKIP:
+                    return
+                digest = zlib.adler32(self.image.tobytes())
+                if digest == self._last_snapshot_digest:
+                    # Unchanged after all: the decision an unchanged frame gets.
+                    action = snapshot_policy.decide(
+                        now, self._last_snapshot_ts,
+                        self._last_snapshot_touch_ts, viewer_fresh, False)
             if action is snapshot_policy.SnapshotAction.SKIP:
                 return
             if (action is snapshot_policy.SnapshotAction.TOUCH
@@ -1826,7 +1882,8 @@ class DisplayManager:
             prefix=f".{snapshot_path_obj.name}.", suffix=".tmp")
         try:
             with os.fdopen(_fd, "wb") as _f:
-                image.save(_f, format='PNG')
+                image.save(_f, format='PNG',
+                           compress_level=_SNAPSHOT_PNG_COMPRESS_LEVEL)
             os.chmod(tmp_path, 0o644)
             os.replace(tmp_path, self._snapshot_path)
         except Exception:
@@ -1837,7 +1894,8 @@ class DisplayManager:
             except OSError:
                 pass
             # Fallback to direct save if replace not supported
-            image.save(self._snapshot_path, format='PNG')
+            image.save(self._snapshot_path, format='PNG',
+                       compress_level=_SNAPSHOT_PNG_COMPRESS_LEVEL)
         # Set proper file permissions after saving
         try:
             ensure_file_permissions(snapshot_path_obj, get_assets_file_mode())

test/test_display_dirty_tracking.py

@@ -68,6 +68,7 @@ class _SwapSpy:
         self.matrix = matrix
         self.count = 0
         self.last_frame_hold = None
+        self.holds = []
         self._orig = matrix.SwapOnVSync
 
     def __enter__(self):
@@ -77,6 +78,7 @@ class _SwapSpy:
             # the real binding or the spy hides a TypeError as a failed push.
             self.count += 1
             self.last_frame_hold = args[0] if args else 1
+            self.holds.append(self.last_frame_hold)
             return self._orig(canvas, *args)
         self.matrix.SwapOnVSync = counting
         return self
@@ -501,3 +503,393 @@ class TestSnapshotOffRenderThread:
         dm.set_scrolling_state(False)
         dm.update_display()
         assert threads == [threading.current_thread().name]
+
+
+class TestLazyDigest:
+    """Mid-scroll the frame is hashed only when the preview snapshot is due.
+
+    The digest feeds two things: the dirty-tracking skip, which is never
+    taken while scrolling, and the snapshot's changed-frame check, which acts
+    at most once a second. Hashing every scrolled frame for them cost a
+    tobytes() plus adler32 over the whole framebuffer on every frame (~0.3ms
+    at 512x64 on a Pi 4). update_display() now leaves the digest out while
+    scrolling, and _write_snapshot_if_due() hashes only when the policy could
+    act on the answer.
+    """
+
+    @pytest.fixture
+    def hashes(self, dm, tmp_path, monkeypatch):
+        """Every adler32 display_manager takes, while no viewer is watching."""
+        import types
+        import zlib
+        import src.display_manager as display_manager_module
+        calls = []
+
+        def counting(data, *args):
+            calls.append(len(data))
+            return zlib.adler32(data, *args)
+
+        monkeypatch.setattr(display_manager_module, "zlib",
+                            types.SimpleNamespace(adler32=counting))
+        # The fixture's marker is the real /tmp one, which a preview open on
+        # this machine would keep fresh. Point it somewhere nobody touches.
+        monkeypatch.setattr(dm, "_viewer_marker_path", str(tmp_path / "no-viewer"))
+        monkeypatch.setattr(dm, "_snapshot_path", str(tmp_path / "snap.png"))
+        dm._viewer_check_ts = 0.0
+        dm._viewer_fresh = False
+        dm._viewer_was_fresh = False
+        # TestSnapshotOffRenderThread's cleanup test stops this shared
+        # manager's snapshot writer for good; queued writes here need one.
+        with dm._snapshot_cond:
+            dm._snapshot_stop = False
+            dm._snapshot_pending = None
+        yield calls
+        dm.set_scrolling_state(False)
+        dm._viewer_check_ts = 0.0
+        dm._viewer_fresh = False
+        dm._viewer_was_fresh = False
+
+    def _scroll(self, dm, frames):
+        """Present ``frames`` scrolled frames, each different from the last."""
+        for shade in range(frames):
+            dm.set_scrolling_state(True)       # as a scroller does every frame
+            dm.draw.rectangle([0, 0, 10, 10], fill=(shade * 10 % 256, 40, 0))
+            dm.update_display()
+
+    def test_scrolled_frames_are_not_hashed_when_no_snapshot_is_due(
+            self, dm, hashes):
+        now = time.time()
+        dm._last_snapshot_ts = now             # written a moment ago
+        dm._last_snapshot_touch_ts = now
+        with _SwapSpy(dm.matrix) as spy:
+            self._scroll(dm, 20)
+        assert spy.count == 20, "every scrolled frame must still be pushed"
+        assert hashes == []
+
+    def test_a_due_snapshot_hashes_the_frame_once(
+            self, dm, hashes, monkeypatch):
+        import threading
+        wrote = threading.Event()
+        monkeypatch.setattr(dm, "_save_snapshot", lambda image: wrote.set())
+        dm._last_snapshot_ts = 0.0             # a write is due (no viewer:
+        dm._last_snapshot_touch_ts = 0.0       # the idle keepalive)
+        dm._last_snapshot_digest = None
+        self._scroll(dm, 20)
+        assert wrote.wait(5), "the due snapshot was never written"
+        assert len(hashes) == 1
+
+    def test_with_a_viewer_a_scroll_is_hashed_once_per_write(
+            self, dm, hashes, tmp_path, monkeypatch):
+        import threading
+        from src.common import snapshot_policy
+        marker = tmp_path / "viewer"
+        marker.touch()
+        monkeypatch.setattr(dm, "_viewer_marker_path", str(marker))
+        writes = []
+        wrote = threading.Event()
+
+        def save(image):
+            writes.append(image)
+            wrote.set()
+
+        monkeypatch.setattr(dm, "_save_snapshot", save)
+        dm._last_snapshot_ts = time.time()     # the viewer opening resets it
+        dm._last_snapshot_touch_ts = time.time()
+        dm._last_snapshot_digest = None        # every frame here is new
+        started = time.time()
+        self._scroll(dm, 30)
+        elapsed = time.time() - started
+        assert wrote.wait(5)
+        # One write as the viewer appears, then at most one per interval.
+        assert 1 <= len(hashes) <= 1 + int(elapsed / snapshot_policy.VIEWER_INTERVAL)
+        assert len(hashes) < 30
+
+    def test_an_unchanged_scrolled_frame_is_not_re_encoded(
+            self, dm, hashes, monkeypatch):
+        # A write would be due for a changed frame, but this one is already
+        # on disk: the lazy hash must re-ask the policy for an unchanged frame
+        # (SKIP here) rather than act on the changed-frame answer (WRITE).
+        import zlib
+        saved, touched = [], []
+        monkeypatch.setattr(dm, "_save_snapshot", lambda image: saved.append(image))
+        monkeypatch.setattr(os, "utime", lambda *a, **k: touched.append(a))
+        dm.set_scrolling_state(True)
+        dm.draw.rectangle([0, 0, 10, 10], fill=(3, 3, 3))
+        digest = zlib.adler32(dm.image.tobytes())
+        dm._last_snapshot_digest = digest
+        dm._saved_snapshot_digest = digest
+        last_write = time.time() - 1000             # a changed frame would write
+        dm._last_snapshot_ts = last_write
+        dm._last_snapshot_touch_ts = time.time()    # no touch due
+        dm.update_display()
+        # A write, even one only queued, would have moved the bookkeeping.
+        assert dm._last_snapshot_ts == last_write
+        assert dm._snapshot_pending is None
+        assert saved == [] and touched == []
+        assert len(hashes) == 1
+
+    def test_an_unchanged_scrolled_frame_is_touched_only_once_on_disk(
+            self, dm, hashes, monkeypatch):
+        # A touch vouches for the file on disk, so through the lazy hash too it
+        # must only happen once the frame has actually been saved.
+        import threading
+        import zlib
+        from src.common import snapshot_policy
+        saved, touched = [], []
+        wrote = threading.Event()
+
+        def save(image):
+            saved.append(image)
+            wrote.set()
+
+        monkeypatch.setattr(dm, "_save_snapshot", save)
+        monkeypatch.setattr(os, "utime", lambda *a, **k: touched.append(a))
+        dm.set_scrolling_state(True)
+        dm.draw.rectangle([0, 0, 10, 10], fill=(5, 5, 5))
+        digest = zlib.adler32(dm.image.tobytes())
+        dm._last_snapshot_digest = digest      # queued earlier...
+        dm._saved_snapshot_digest = 12345      # ...but an older frame is on disk
+        # A touch is due; a write is not (no viewer: the idle interval).
+        stale = time.time() - snapshot_policy.TOUCH_INTERVAL - 1.0
+        dm._last_snapshot_ts = stale
+        dm._last_snapshot_touch_ts = stale
+        dm.update_display()
+        assert wrote.wait(5), "a frame not yet on disk must be written"
+        deadline = time.time() + 5
+        while dm._saved_snapshot_digest != digest and time.time() < deadline:
+            time.sleep(0.01)
+        assert touched == [] and len(saved) == 1
+
+        # Once it is on disk, the same frame is only touched.
+        dm._last_snapshot_ts = stale
+        dm._last_snapshot_touch_ts = stale
+        dm.set_scrolling_state(True)
+        dm.update_display()
+        assert len(touched) == 1 and len(saved) == 1
+
+    def test_the_first_static_frame_after_a_scroll_is_pushed_once(
+            self, dm, hashes):
+        # No digest is kept mid-scroll, so a static frame identical to the
+        # scroll's last one is pushed once (one swap), and then dirty
+        # tracking skips its repeats as before.
+        dm.set_scrolling_state(True)
+        dm.draw.rectangle([0, 0, 18, 18], fill=(90, 90, 0))
+        dm.update_display()                    # the scroll's last frame
+        dm.set_scrolling_state(False)
+        with _SwapSpy(dm.matrix) as spy:
+            dm.update_display()
+            dm.update_display()
+            dm.update_display()
+        assert spy.count == 1
+
+    def test_a_scrolled_push_forgets_the_last_static_frame(
+            self, dm, hashes):
+        # Scrolled frames carry no digest, so a scrolled push must clear the
+        # last static one. Were it kept, a static screen drawing what it
+        # showed before the scroll would be skipped as already on the panel,
+        # and the scroll's last frame would stay up until the content changed.
+        dm.set_scrolling_state(False)
+        dm.draw.rectangle([0, 0, 18, 18], fill=(10, 120, 30))
+        dm.update_display()                    # static frame A
+        dm.set_scrolling_state(True)
+        dm.draw.rectangle([0, 0, 18, 18], fill=(200, 0, 60))
+        dm.update_display()                    # a scrolled frame B
+        assert dm._last_pushed_digest is None
+        dm.set_scrolling_state(False)
+        dm.draw.rectangle([0, 0, 18, 18], fill=(10, 120, 30))
+        with _SwapSpy(dm.matrix) as spy:
+            dm.update_display()                # A again, over B
+        assert spy.count == 1, "A must replace the scroll's last frame"
+
+
+class TestOneScrollingAnswerPerFrame:
+    """update_display() asks is_currently_scrolling() once per frame and uses
+    that answer throughout. The call has side effects -- a scroll that has
+    gone quiet past its threshold is expired there and its frame hold
+    dropped -- so asking again partway through let one frame be presented at
+    the scroll's hold and recorded as static."""
+
+    def test_a_scroll_that_times_out_is_presented_without_its_hold(
+            self, dm, monkeypatch):
+        recorded = []
+        monkeypatch.setattr(
+            dm.frame_timing, "record",
+            lambda blit, wait, hold, scrolling, at: recorded.append(
+                (hold, scrolling)))
+        dm.set_scrolling_state(True, frame_hold=3)
+        # Quiet past the inactivity threshold, and nothing has asked since.
+        dm._scrolling_state['last_scroll_activity'] -= (
+            dm._scrolling_state['scroll_inactivity_threshold'] + 1.0)
+        dm.draw.rectangle([0, 0, 8, 8], fill=(31, 62, 93))
+        try:
+            with _SwapSpy(dm.matrix) as spy:
+                dm.update_display()
+        finally:
+            dm.set_scrolling_state(False)
+        assert spy.count == 1
+        assert spy.last_frame_hold == 1
+        assert recorded == [(1, False)]
+
+    def test_scan_compensation_uses_the_frames_answer(
+            self, dm, tmp_path, monkeypatch):
+        from collections import deque
+        monkeypatch.setattr(dm, "_scan_lag_bands", [(16, 32, 1)])
+        monkeypatch.setattr(dm, "_scan_history", deque(maxlen=1))
+        # No preview and no snapshot due, so only the frame itself asks.
+        monkeypatch.setattr(dm, "_viewer_marker_path", str(tmp_path / "no-viewer"))
+        dm._viewer_check_ts = 0.0
+        dm._last_snapshot_ts = dm._last_snapshot_touch_ts = time.time()
+        asked = []
+        real = dm.is_currently_scrolling
+
+        def counting():
+            asked.append(1)
+            return real()
+
+        monkeypatch.setattr(dm, "is_currently_scrolling", counting)
+        dm.set_scrolling_state(True)
+        try:
+            dm.draw.rectangle([0, 0, 8, 8], fill=(17, 34, 51))
+            dm.update_display()
+        finally:
+            dm.set_scrolling_state(False)
+            dm._viewer_check_ts = 0.0
+        assert len(asked) == 1
+
+    @staticmethod
+    def _scan_banded(dm, tmp_path, monkeypatch):
+        """Scan-order compensation on, no preview, no snapshot due, and every
+        frame handed to the canvases kept (update_display alternates them)."""
+        from collections import deque
+        monkeypatch.setattr(dm, "_scan_lag_bands", [(16, 32, 1)])
+        monkeypatch.setattr(dm, "_scan_history", deque(maxlen=1))
+        monkeypatch.setattr(dm, "_viewer_marker_path", str(tmp_path / "no-viewer"))
+        dm._viewer_check_ts = 0.0
+        dm._last_snapshot_ts = dm._last_snapshot_touch_ts = time.time()
+        shown = []
+        for canvas in (dm.offscreen_canvas, dm.current_canvas):
+            def capture(image, *args, _real=canvas.SetImage, **kwargs):
+                shown.append(image.copy())
+                return _real(image, *args, **kwargs)
+            monkeypatch.setattr(canvas, "SetImage", capture)
+        return shown
+
+    def test_a_held_frame_is_split_on_the_frames_answer(
+            self, dm, tmp_path, monkeypatch):
+        # Scan-order compensation presents a held frame as two swaps, the
+        # lagging half stepping one refresh after the rest. The split is
+        # decided on the frame's one answer, exactly as when it asked itself.
+        shown = self._scan_banded(dm, tmp_path, monkeypatch)
+        asked, recorded = [], []
+        real = dm.is_currently_scrolling
+
+        def counting():
+            asked.append(1)
+            return real()
+
+        monkeypatch.setattr(dm, "is_currently_scrolling", counting)
+        monkeypatch.setattr(
+            dm.frame_timing, "record",
+            lambda blit, wait, hold, scrolling, at: recorded.append(
+                (hold, scrolling)))
+        dm.set_scrolling_state(True, frame_hold=2)
+        try:
+            # The fixture is shared: a slow blit left by an earlier test
+            # would send this frame out whole and drop the history.
+            dm._last_blit_seconds = 0.0
+            dm.draw.rectangle([0, 0, dm.width - 1, dm.height - 1],
+                              fill=(10, 0, 0))
+            dm.update_display()                # the previous frame
+            asked.clear()
+            recorded.clear()
+            before = len(shown)
+            dm._last_blit_seconds = 0.0        # a blit that fits a refresh
+            dm.set_scrolling_state(True, frame_hold=2)
+            dm.draw.rectangle([0, 0, dm.width - 1, dm.height - 1],
+                              fill=(20, 0, 0))
+            with _SwapSpy(dm.matrix) as spy:
+                dm.update_display()
+        finally:
+            dm.set_scrolling_state(False)
+            dm._viewer_check_ts = 0.0
+        assert len(asked) == 1
+        assert spy.holds == [1, 1]
+        first, second = shown[before:]
+        assert first.getpixel((0, 0)) == (20, 0, 0)
+        assert first.getpixel((0, 31)) == (10, 0, 0)    # lagging half: old
+        assert second.getpixel((0, 31)) == (20, 0, 0)   # a refresh later
+        assert recorded == [(2, True)]
+
+    def test_a_held_scroll_that_times_out_goes_out_whole_at_hold_1(
+            self, dm, tmp_path, monkeypatch):
+        # The expiry frame with scan-order compensation on: one swap of the
+        # frame as drawn, at hold 1, the history dropped -- and the pacing
+        # gate, the swap and frame timing all see that one hold. (Asking
+        # partway through instead had the swap use the dead scroll's hold
+        # while the gate and frame timing were told 1.)
+        shown = self._scan_banded(dm, tmp_path, monkeypatch)
+        gate_holds, recorded = [], []
+
+        class _Gate:
+            def before_swap(self, hold):
+                gate_holds.append(('before', hold))
+
+            def after_swap(self, hold):
+                gate_holds.append(('after', hold))
+
+        monkeypatch.setattr(
+            dm.frame_timing, "record",
+            lambda blit, wait, hold, scrolling, at: recorded.append(
+                (hold, scrolling)))
+        dm.set_scrolling_state(True, frame_hold=3)
+        try:
+            dm.draw.rectangle([0, 0, dm.width - 1, dm.height - 1],
+                              fill=(30, 0, 0))
+            dm.update_display()                # the scroll's last frame
+            recorded.clear()
+            before = len(shown)
+            # Quiet past the inactivity threshold, and nothing has asked since.
+            dm._scrolling_state['last_scroll_activity'] -= (
+                dm._scrolling_state['scroll_inactivity_threshold'] + 1.0)
+            dm._last_blit_seconds = 0.0        # a split would fit
+            monkeypatch.setattr(dm, "render_gate", _Gate())
+            dm.draw.rectangle([0, 0, dm.width - 1, dm.height - 1],
+                              fill=(40, 0, 0))
+            with _SwapSpy(dm.matrix) as spy:
+                dm.update_display()
+        finally:
+            dm.set_scrolling_state(False)
+            dm._viewer_check_ts = 0.0
+        assert spy.holds == [1]
+        assert gate_holds == [('before', 1), ('after', 1)]
+        assert recorded == [(1, False)]
+        (frame,) = shown[before:]
+        assert frame.getpixel((0, 31)) == (40, 0, 0)    # not composed
+        assert len(dm._scan_history) == 0
+
+
+class TestSnapshotEncoding:
+    def test_the_snapshot_is_written_at_the_fastest_level_and_lossless(
+            self, dm, tmp_path, monkeypatch):
+        # The encode is paid on the render thread for a static screen, and
+        # the file is read once; level 1 cuts the encode, and PNG is lossless
+        # at any level, so the preview shows exactly the frame.
+        from PIL import Image, ImageChops
+        levels = []
+        real_save = Image.Image.save
+
+        def recording(image, fp, format=None, **params):
+            levels.append(params.get("compress_level"))
+            return real_save(image, fp, format, **params)
+
+        monkeypatch.setattr(Image.Image, "save", recording)
+        monkeypatch.setattr(dm, "_snapshot_path", str(tmp_path / "snap.png"))
+        frame = Image.new("RGB", (dm.width, dm.height))
+        frame.paste((200, 10, 90), (3, 4, 40, 20))
+        frame.putpixel((0, 0), (1, 2, 3))
+        dm._save_snapshot(frame)
+        assert levels == [1]
+        with Image.open(dm._snapshot_path) as saved:
+            assert saved.format == "PNG"
+            assert ImageChops.difference(
+                saved.convert("RGB"), frame).getbbox() is None

test/test_handover_scroll_state.py

@@ -151,12 +151,58 @@ class TestTheFirstStaticFrame:
         [(_blit, _wait, hold, scrolling, _at)] = records
         assert (hold, scrolling) == (3, True)
 
+    def test_it_is_decided_on_the_frames_one_answer_and_not_hashed(
+            self, dm, monkeypatch, tmp_path):
+        # The handover frame goes through update_display like any frame while
+        # the scroll state is set: it asks is_currently_scrolling() once, takes
+        # no digest, and that answer and its hold reach the swap and the
+        # record -- while the frame itself goes out as drawn.
+        import time
+        import zlib
+        import src.display_manager as display_manager_module
+        dm._scan_lag_bands = [(24, 48, 1)]
+        # No preview and no snapshot due: only the frame itself asks or hashes.
+        dm._viewer_marker_path = str(tmp_path / "no-viewer")
+        dm._viewer_check_ts = 0.0
+        dm.set_scrolling_state(True, 2)
+        _push(dm, (255, 0, 0))               # the ticker's last frame
+        dm._last_snapshot_ts = dm._last_snapshot_touch_ts = time.time()
+        asked, hashed, records = [], [], []
+        real_asked = dm.is_currently_scrolling
+
+        def asking():
+            asked.append(1)
+            return real_asked()
+
+        def hashing(data, *args):
+            hashed.append(len(data))
+            return zlib.adler32(data, *args)
+
+        real_record = dm.frame_timing.record
+
+        def record(*args, **kwargs):
+            records.append(args[2:4])
+            return real_record(*args, **kwargs)
+        monkeypatch.setattr(dm, "is_currently_scrolling", asking)
+        monkeypatch.setattr(display_manager_module, "zlib",
+                            types.SimpleNamespace(adler32=hashing))
+        monkeypatch.setattr(dm.frame_timing, "record", record)
+        holds = _watch_swaps(dm)
+        before = len(dm._presented)
+        dm._last_blit_seconds = 0.0          # fast enough to split
+        dm.end_scroll_for_static_screen()
+        _push(dm, (0, 0, 0))
+        assert len(asked) == 1
+        assert hashed == []
+        assert holds == [2]
+        assert records == [(2, True)]
+        assert len(dm._presented) - before == 1
+        assert dm._presented[-1].getpixel((10, 30)) == (0, 0, 0)
+
     def test_nor_its_own_first_frame_under_its_second(self, dm):
         # A first display() that pushes two frames: a clear, then the screen.
-        # Composed, the second would show the first's rows -- and once the
+        # Composed, the second would show the first's rows: a half-black frame
-        # scroll is over, dirty tracking (which compares frames as drawn, not
+        # up for the whole second until the 1 Hz loop's next redraw.
-        # as composed) skips every identical 1 Hz redraw, so that half-black
-        # frame would stay up for the whole turn.
         dm._scan_lag_bands = [(24, 48, 1)]
         dm.set_scrolling_state(True, 1)
         _push(dm, (255, 0, 0))               # the ticker's last frame
@@ -166,8 +212,13 @@ class TestTheFirstStaticFrame:
         assert shown.getpixel((10, 30)) == (0, 0, 255)
         dm.set_scrolling_state(False)        # the controller, after the dispatch
         pushed = len(dm._presented)
-        _push(dm, (0, 0, 255))               # the 1 Hz redraw: skipped
+        # The 1 Hz redraw is pushed once, as drawn: a frame pushed while the
-        assert len(dm._presented) == pushed
+        # scroll state was set leaves dirty tracking no digest to match. The
+        # next identical one is skipped.
+        assert _push(dm, (0, 0, 255)).getpixel((10, 30)) == (0, 0, 255)
+        assert len(dm._presented) == pushed + 1
+        _push(dm, (0, 0, 255))
+        assert len(dm._presented) == pushed + 1
 
     def test_the_next_scroll_is_compensated_again(self, dm):
         dm._scan_lag_bands = [(24, 48, 1)]

test/test_snapshot_policy.py

@@ -5,7 +5,9 @@ The invariants that matter:
   frames at 5 fps, 24/7)
 - the file mtime never goes stale enough to trip the health check's 60s
   degraded threshold (api_v3 get_hardware_status)
-- a viewer gets full cadence; no viewer drops to the idle keepalive
+- a viewer gets the viewer cadence; no viewer drops to the idle keepalive
+- decide() is monotone in frame_changed, which DisplayManager's lazy
+  frame hash relies on
 """
 
 import os
@@ -19,6 +21,8 @@ from src.common.snapshot_policy import (  # noqa: E402
     IDLE_INTERVAL,
     TOUCH_INTERVAL,
     VIEWER_INTERVAL,
+    VIEWER_MARKER_FRESH_SEC,
+    VIEWER_POLL_INTERVAL,
     SnapshotAction,
     decide,
 )
@@ -89,5 +93,45 @@ class TestHealthKeepalive:
                 assert action in (SnapshotAction.WRITE, SnapshotAction.TOUCH)
 
 
+class TestMonotoneInFrameChanged:
+    """DisplayManager asks decide() as if the frame had changed, and only
+    hashes the frame when that answer is not SKIP. That is exact only while
+    a SKIP for a changed frame is also a SKIP for an unchanged one."""
+
+    def test_skip_for_a_changed_frame_is_skip_for_an_unchanged_one(self):
+        # Ages that straddle every threshold decide() compares against, plus
+        # a timestamp in the future (a wall-clock step back) and "never".
+        eps = 1e-6
+        ages = sorted({0.0, -1.0, 1000.0, 100.0} | {
+            base + delta
+            for base in (VIEWER_INTERVAL, IDLE_INTERVAL, TOUCH_INTERVAL)
+            for delta in (-eps, 0.0, eps)
+        })
+        now = 1000.0
+        checked = 0
+        for write_age in ages:
+            for touch_age in ages:
+                for viewer in (True, False):
+                    args = (now, now - write_age, now - touch_age, viewer)
+                    if decide(*args, frame_changed=True) is SnapshotAction.SKIP:
+                        checked += 1
+                        assert decide(*args, frame_changed=False) is SnapshotAction.SKIP, args
+        assert checked, "the grid never reached a SKIP; it tests nothing"
+
+
+class TestPreviewPolling:
+    def test_the_reader_polls_faster_than_the_writer_writes(self):
+        """Equal periods alias: two unsynchronised clocks of the same period
+        leave the preview up to a period stale, with the odd double gap.
+        Polling at least twice per interval keeps each frame at most half an
+        interval late."""
+        assert VIEWER_POLL_INTERVAL <= VIEWER_INTERVAL / 2
+
+    def test_polling_keeps_the_viewer_marker_fresh(self):
+        # The SSE reader touches the marker about once a second however fast
+        # it polls; the display must still count that as a live viewer.
+        assert max(VIEWER_POLL_INTERVAL, 1.0) < VIEWER_MARKER_FRESH_SEC
+
+
 if __name__ == "__main__":
     sys.exit(pytest.main([__file__, "-v"]))

test/web_interface/test_display_preview_stream.py

@@ -0,0 +1,135 @@
+"""The /stream/display generator's pace (web_interface/app.py).
+
+display_preview_generator() used to sleep a flat second between passes, while
+the display wrote a snapshot up to five times a second for it: four encodes
+in five were overwritten unread. The display now writes about once a second
+(snapshot_policy.VIEWER_INTERVAL), so the generator checks the snapshot's
+mtime every snapshot_policy.VIEWER_POLL_INTERVAL instead -- a stat -- and
+sends each frame soon after it lands. Polling equal to the write period would
+alias the two clocks into stale frames and the odd two-second gap.
+
+What must not change with it: the viewer marker is touched about once a
+second (more would only be extra writes to /tmp), and a pass with no snapshot
+or an error still sends its message once a second, not four times.
+"""
+
+import os
+import sys
+import types
+from contextlib import nullcontext
+from pathlib import Path
+from unittest.mock import MagicMock
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[2]))
+
+from src.common import snapshot_policy  # noqa: E402
+from web_interface import display_preview  # noqa: E402
+
+MARKER = "/tmp/led_matrix_preview_viewer"  # nosec B108 - the path the generator touches
+
+
+class _Stop(Exception):
+    """Raised from the fake sleep to end the generator's endless loop."""
+
+
+class _Stream:
+    """Drives display_preview_generator() on a fake clock.
+
+    The marker is a fixed /tmp path shared with a display service that may be
+    running on this machine, so touches are recorded rather than made.
+    """
+
+    def __init__(self, monkeypatch, snapshot_path, os_path=None):
+        import web_interface.app as web_app
+        self.now = 1000.0
+        self.sleeps = []
+        self.touches = []
+        self.at_sleep = {}
+        self.stop_after = 0
+        monkeypatch.setattr(web_app, "time", types.SimpleNamespace(
+            sleep=self._sleep, monotonic=lambda: self.now, time=lambda: self.now))
+        monkeypatch.setattr(web_app, "open", self._open, raising=False)
+        monkeypatch.setattr(web_app, "os", types.SimpleNamespace(
+            path=os_path or os.path, utime=lambda *a, **k: None))
+        monkeypatch.setattr(web_app, "config_manager",
+                            MagicMock(**{"load_config.return_value": {}}))
+        monkeypatch.setattr(display_preview, "SNAPSHOT_PATH", str(snapshot_path))
+        self._generator = web_app.display_preview_generator
+
+    def _open(self, path, mode="r", *args, **kwargs):
+        assert path == MARKER and mode == "a"
+        self.touches.append(self.now)
+        return nullcontext()
+
+    def _sleep(self, seconds):
+        self.sleeps.append(seconds)
+        self.now += seconds
+        if len(self.sleeps) >= self.stop_after:
+            raise _Stop
+        action = self.at_sleep.get(len(self.sleeps))
+        if action:
+            action()
+
+    def run(self, passes):
+        """The messages sent over ``passes`` passes, each with the pass number."""
+        self.stop_after = passes
+        sent = []
+        try:
+            for message in self._generator():
+                sent.append((len(self.sleeps), message))
+        except _Stop:
+            pass
+        return sent
+
+
+def _write(path, mtime):
+    path.write_bytes(b"\x89PNG not really")
+    os.utime(path, (mtime, mtime))
+
+
+def test_frames_go_out_on_the_next_poll_and_the_marker_is_touched_once_a_second(
+        monkeypatch, tmp_path):
+    snapshot = tmp_path / "led_matrix_preview.png"
+    _write(snapshot, 500.0)
+    stream = _Stream(monkeypatch, snapshot)
+    # The display writes a new frame once a second: every fourth poll.
+    for n, sleep_no in enumerate((4, 8, 12)):
+        stream.at_sleep[sleep_no] = lambda m=501.0 + n: _write(snapshot, m)
+
+    sent = stream.run(16)
+
+    assert stream.sleeps == [snapshot_policy.VIEWER_POLL_INTERVAL] * 16
+    # The first pass sends what is there; each write goes out on the poll
+    # straight after it, not up to a second later.
+    assert [pass_no for pass_no, _ in sent] == [0, 4, 8, 12]
+    assert all(message["image"] for _, message in sent)
+    # Sixteen quarter-second polls are four seconds: four touches, a second
+    # apart, well inside VIEWER_MARKER_FRESH_SEC.
+    assert stream.touches == [1000.0, 1001.0, 1002.0, 1003.0]
+
+
+def test_without_a_snapshot_the_placeholder_still_goes_out_once_a_second(
+        monkeypatch, tmp_path):
+    stream = _Stream(monkeypatch, tmp_path / "missing.png")
+
+    sent = stream.run(5)
+
+    assert stream.sleeps == [1.0] * 5
+    assert [pass_no for pass_no, _ in sent] == [0, 1, 2, 3, 4]
+    assert all(message["image"] is None for _, message in sent)
+    assert stream.touches == [1000.0, 1001.0, 1002.0, 1003.0, 1004.0]
+
+
+def test_an_error_keeps_the_one_second_pace(monkeypatch, tmp_path):
+    def broken(_path):
+        raise RuntimeError("stat failed")
+
+    stream = _Stream(monkeypatch, tmp_path / "snap.png",
+                     os_path=types.SimpleNamespace(exists=lambda _p: True,
+                                                   getmtime=broken))
+
+    sent = stream.run(3)
+
+    assert stream.sleeps == [1.0] * 3
+    assert [message for _, message in sent] == [
+        {"error": "An error occurred; see server logs"}] * 3

web_interface/app.py

@@ -33,6 +33,7 @@ from src.web_interface.error_handler import describe_exception
 from src.common.path_safety import (
     resolve_under, safe_path_component, safe_relative_parts,
 )
+from src.common import snapshot_policy
 from werkzeug.exceptions import HTTPException
 from src.exceptions import ConfigError
 from src.plugin_system.plugin_catalog import PluginCatalog
@@ -797,12 +798,19 @@ def display_preview_generator():
     """Generate display preview updates from snapshot file"""
     snapshot_path = display_preview.SNAPSHOT_PATH
     # Viewer marker: this generator only runs while the broadcaster has
-    # subscribers (it exits with no clients), so touching the marker each
+    # subscribers (it exits with no clients), so touching the marker tells
-    # loop tells the DISPLAY service a browser is actually watching — it
+    # the DISPLAY service a browser is actually watching — it only pays for
-    # only pays for full-rate PNG snapshot encodes while this stays fresh
+    # viewer-rate PNG snapshot encodes while this stays fresh
     # (see src/common/snapshot_policy.py).
     viewer_marker_path = "/tmp/led_matrix_preview_viewer"  # nosec B108 - fixed path matches display_manager
     last_modified = None
+    # While there is a snapshot, its mtime is checked every
+    # VIEWER_POLL_INTERVAL, so a new frame goes out soon after it lands
+    # rather than up to a second later. The rest keeps its old once-a-second
+    # pace: the marker touch (the display counts it fresh for
+    # VIEWER_MARKER_FRESH_SEC), and passes with no snapshot or an error,
+    # each of which sends a message.
+    last_marker_touch = None
 
     def _touch_viewer_marker():
         try:
@@ -821,10 +829,15 @@ def display_preview_generator():
         width, height = logical_size({})
     
     while True:
+        delay = 1.0
         try:
-            _touch_viewer_marker()
+            now = time.monotonic()
+            if last_marker_touch is None or now - last_marker_touch >= 1.0:
+                _touch_viewer_marker()
+                last_marker_touch = now
             # Check if snapshot file exists and has been modified
             if os.path.exists(snapshot_path):
+                delay = snapshot_policy.VIEWER_POLL_INTERVAL
                 current_modified = os.path.getmtime(snapshot_path)
                 
                 # Only read if file is new or has been updated
@@ -842,10 +855,11 @@ def display_preview_generator():
                 yield display_preview.preview_payload(width, height, None)
                 
         except Exception:
+            delay = 1.0
             app.logger.error("SSE generator error", exc_info=True)
             yield {'error': 'An error occurred; see server logs'}
         
-        time.sleep(1.0)  # the snapshot is re-read only when its mtime changes
+        time.sleep(delay)  # the snapshot is re-read only when its mtime changes
 
 # Logs generator for SSE
 def logs_generator():