fix(display): apply on-demand, brightness and schedule changes mid-screen (#618)

* fix(display): apply on-demand, brightness and schedule changes mid-screen

The main loop read the on-demand mailbox, the on/off schedule and the
brightness target once per pass -- once per screen. A dwell can be a minute
and a Vegas iteration runs for max_cycle_duration (240s), so on a Pi an
on-demand request posted at 10:54:27 was activated at 10:57:24, and two
brightness saves 12s apart inside one 30s screen never reached the panel.
During Vegas nothing read the mailbox at all: _check_vegas_interrupt only
checked on_demand_active, which only the main-loop read sets.

_service_pending_changes does the main loop's on-demand poll, expiry,
schedule and brightness steps, throttled to PENDING_CHANGES_INTERVAL (the
existing 0.25s mailbox floor), on the display thread. It runs from the Vegas
interrupt checker, the high-FPS and once-a-second render loops (replacing
their direct on-demand poll) and _sleep_with_plugin_updates; between passes
it costs one monotonic compare. A brightness change re-pushes the current
frame, since the panel only shows it from the next push.

Callers act on what it leaves behind: Vegas yields on an on-demand start or
the display being scheduled off (and the main loop then blanks instead of
rendering a screen), the render loops break on a schedule-off as they
already did on a mode change, and the dwell sleep returns early on an
on-demand start/stop or a schedule flip -- so the 60s scheduled-off sleep
now wakes for an on-demand request. The main loop no longer rotates after
a dwell that ended that way, which advanced a new on-demand session past
the mode that was asked for.

A brightness set_brightness() refuses is not retried until the target
changes, so the 4Hz pass doesn't log the same failure (fallback mode)
four times a second.

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

* test(display): a screen scheduled off midway stops rendering

Covers the schedule-off break added to the high-FPS and once-a-second
render loops: with the display scheduled off halfway through a 120s screen,
neither loop renders for more than one redraw plus one service interval
past the boundary.

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

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/display_controller.py

@@ -207,6 +207,12 @@ class DisplayController:
         # _poll_on_demand_requests. None means "never polled", so the first
         # call always goes through.
         self._last_on_demand_poll: Optional[float] = None
+        # Monotonic stamp of the last _service_pending_changes pass; same
+        # "None means never" convention as _last_on_demand_poll.
+        self._last_pending_service: Optional[float] = None
+        # A brightness set_brightness() refused, so the periodic service pass
+        # doesn't retry (and log) the same failure several times a second.
+        self._failed_brightness_target: Optional[int] = None
         self.on_demand_active = False
         self.on_demand_mode: Optional[str] = None
         self.on_demand_modes: List[str] = []  # All modes for the on-demand plugin
@@ -581,10 +587,22 @@ class DisplayController:
         Returns:
             True if Vegas should yield control, False to continue
         """
+        # A Vegas iteration runs for up to max_cycle_duration (240s by
+        # default) without returning to the main loop, and this is the only
+        # code of ours it calls while it does. Without servicing here, an
+        # on-demand request was never even read until the iteration ended --
+        # on_demand_active below is only set by that read -- and a saved
+        # brightness or a schedule boundary waited just as long.
+        self._service_pending_changes()
+
         # Check for pending on-demand request
         if self.on_demand_active:
             return True
 
+        # Scheduled off mid-iteration: hand back so the main loop blanks it.
+        if not self.is_display_active:
+            return True
+
         # Check for wifi status that needs display
         if self._check_wifi_status_message():
             return True
@@ -1004,12 +1022,22 @@ class DisplayController:
             self.sync_manager.send_frame(follower_frame)
 
     def _sleep_with_plugin_updates(self, duration: float, tick_interval: float = 1.0):
-        """Sleep while continuing to service plugin update schedules."""
+        """Sleep while continuing to service plugin update schedules.
+
+        Also services pending changes (see _service_pending_changes), and
+        returns early when one of them changes what the panel should show --
+        an on-demand start or stop, or the display schedule turning the panel
+        on or off -- so the caller can act on it instead of finishing a dwell
+        that could be a minute long (sixty seconds while scheduled off).
+        """
         if duration <= 0:
             return
 
         end_time = time.time() + duration
-        tick_interval = max(0.001, tick_interval)
+        tick_interval = max(0.001, min(tick_interval, self.PENDING_CHANGES_INTERVAL))
+        mode = self.current_display_mode
+        display_active = self.is_display_active
+        on_demand = self.on_demand_active
 
         while True:
             remaining = end_time - time.time()
@@ -1019,6 +1047,11 @@ class DisplayController:
             sleep_time = min(tick_interval, remaining)
             time.sleep(sleep_time)
             self._tick_plugin_updates()
+            self._service_pending_changes()
+            if (self.current_display_mode != mode
+                    or self.is_display_active != display_active
+                    or self.on_demand_active != on_demand):
+                break
 
     def _get_display_duration(self, mode_key):
         """Seconds to show a mode: the Rotation & Durations page's value for it
@@ -1215,6 +1248,83 @@ class DisplayController:
     #: perceptible, and it cuts the read rate by 30x.
     ON_DEMAND_POLL_INTERVAL = 0.25
 
+    #: Shortest gap between _service_pending_changes passes. The same floor as
+    #: the mailbox poll, since that read is the only real cost in the pass:
+    #: the schedule checks are gated to once per clock minute and the rest is
+    #: attribute compares. Callers run at frame rate, so between passes the
+    #: whole cost is one monotonic-clock compare.
+    PENDING_CHANGES_INTERVAL = ON_DEMAND_POLL_INTERVAL
+
+    def _service_pending_changes(self) -> None:
+        """Apply changes made elsewhere while the display thread is busy.
+
+        The main loop applies on-demand requests, the display on/off schedule
+        and brightness (a saved display.hardware.brightness, or a dim-schedule
+        transition) once per pass -- that is, once per screen. A screen can
+        dwell a minute and a Vegas iteration four, so those waited just as
+        long: an on-demand request sat unread for up to 240s, and a brightness
+        saved and reverted within one screen never reached the panel at all.
+
+        So the places the display thread spends long stretches -- the dwell
+        sleep, the per-screen render loops and Vegas's interrupt check -- call
+        this. It is throttled to PENDING_CHANGES_INTERVAL, runs on the display
+        thread (set_brightness must not be called from the config watcher),
+        and does what the main loop does, in the same order. Callers decide
+        what to do about the result from the state it leaves behind
+        (on_demand_active, current_display_mode, is_display_active).
+        """
+        now = time.monotonic()
+        last = self._last_pending_service
+        if last is not None and now - last < self.PENDING_CHANGES_INTERVAL:
+            return
+        self._last_pending_service = now
+
+        try:
+            self._poll_on_demand_requests()
+            self._check_on_demand_expiration()
+            self._evaluate_schedule()
+            self._apply_brightness_target(repaint=True)
+        except Exception:  # pylint: disable=broad-except
+            # Called from inside Vegas and the render loops; a failure here
+            # must not take the display loop down with it.
+            logger.exception("Error servicing pending display changes")
+
+    def _evaluate_schedule(self) -> None:
+        """Re-check the on/off schedule, letting an on-demand session override it."""
+        self._check_schedule()
+        if self.on_demand_active and not self.is_display_active:
+            if not self.on_demand_schedule_override:
+                logger.info("On-demand override keeping display active during scheduled downtime")
+            self.on_demand_schedule_override = True
+            self.is_display_active = True
+        elif not self.on_demand_active and self.on_demand_schedule_override:
+            self.on_demand_schedule_override = False
+
+    def _apply_brightness_target(self, repaint: bool = False) -> None:
+        """Push the brightness the config and dim schedule call for, if it changed.
+
+        Args:
+            repaint: Re-push the current frame afterwards. The panel only shows
+                a new brightness from the next frame pushed; the main loop is
+                about to render one, but a dwell or a static screen may not
+                push again for a minute.
+        """
+        if not self.is_display_active:
+            return
+        target = self._check_dim_schedule()
+        if target == self.current_brightness:
+            self._failed_brightness_target = None
+            return
+        if target == self._failed_brightness_target:
+            return
+        if self.display_manager.set_brightness(target):
+            self.current_brightness = target
+            self._failed_brightness_target = None
+            if repaint:
+                self.display_manager.update_display()
+        else:
+            self._failed_brightness_target = target
+
     def _select_startup_plugins(self, discovered_plugins: List[str],
                                 on_demand_config: Optional[Dict[str, Any]]) -> List[str]:
         """Which plugins to load at startup, restoring on-demand state if any.
@@ -1845,21 +1955,11 @@ class DisplayController:
                     self._log_memory_stats_if_due()
 
                 # Check the schedule
-                self._check_schedule()
+                self._evaluate_schedule()
-                if self.on_demand_active and not self.is_display_active:
-                    if not self.on_demand_schedule_override:
-                        logger.info("On-demand override keeping display active during scheduled downtime")
-                    self.on_demand_schedule_override = True
-                    self.is_display_active = True
-                elif not self.on_demand_active and self.on_demand_schedule_override:
-                    self.on_demand_schedule_override = False
 
-                # Check dim schedule and apply brightness (only when display is active)
+                # Check dim schedule and apply brightness (only when display
-                if self.is_display_active:
+                # is active). No repaint: this screen's first frame pushes it.
-                    target_brightness = self._check_dim_schedule()
+                self._apply_brightness_target()
-                    if target_brightness != self.current_brightness:
-                        if self.display_manager.set_brightness(target_brightness):
-                            self.current_brightness = target_brightness
 
                 if not self.is_display_active:
                     # Clear display when schedule makes it inactive to ensure blank screen
@@ -2010,6 +2110,10 @@ class DisplayController:
                             else:
                                 # Vegas was interrupted (live priority), fall through to normal handling
                                 logger.debug("Vegas mode interrupted, falling back to normal rotation")
+                                if not self.is_display_active:
+                                    # Scheduled off mid-iteration: blank the
+                                    # panel now rather than render a screen.
+                                    continue
                         except Exception:
                             logger.exception("Vegas mode error")
                             # Fall through to normal rotation on error
@@ -2473,8 +2577,8 @@ class DisplayController:
                                 self._send_follower_frame(manager_to_display)
 
                                 self._tick_plugin_updates()
-                                self._poll_on_demand_requests()
+                                # Throttled: one clock compare between passes.
-                                self._check_on_demand_expiration()
+                                self._service_pending_changes()
 
                                 # Pace to the frame deadline rather than sleeping a flat
                                 # interval on top of the work. display() has already
@@ -2492,7 +2596,8 @@ class DisplayController:
                                 # 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:
+                                if (self.current_display_mode != active_mode
+                                        or not self.is_display_active):
                                     logger.debug("Mode changed during high-FPS loop, breaking early")
                                     break
 
@@ -2570,9 +2675,9 @@ class DisplayController:
                                 # Multi-display sync: send follower frame after each render
                                 self._send_follower_frame(manager_to_display)
 
-                                self._poll_on_demand_requests()
+                                self._service_pending_changes()
-                                self._check_on_demand_expiration()
+                                if (self.current_display_mode != active_mode
-                                if self.current_display_mode != active_mode:
+                                        or not self.is_display_active):
                                     logger.info("Mode changed during display loop from %s to %s, breaking early", active_mode, self.current_display_mode)
                                     break
 
@@ -2599,7 +2704,10 @@ class DisplayController:
                         # Removing it again will silently reintroduce both issues. _activate_on_demand
                         # already sets force_change=True and clears the display, so the next loop
                         # iteration renders the new mode immediately.
-                        if self.current_display_mode != active_mode:
+                        # Likewise if the schedule turned the display off
+                        # mid-screen: the next iteration blanks it.
+                        if (self.current_display_mode != active_mode
+                                or not self.is_display_active):
                             continue
 
                         # Ensure we honour minimum duration when not dynamic and loop ended early
@@ -2648,6 +2756,14 @@ class DisplayController:
                         # For non-plugin modes, use the original behavior
                         self._sleep_with_plugin_updates(max_duration)
 
+                # The dwell sleeps above return early when a pending change
+                # (on-demand started or stopped, display scheduled off) has
+                # already decided what comes next; rotating now would skip it
+                # -- an on-demand start would advance past the requested mode.
+                if (self.current_display_mode != active_mode
+                        or not self.is_display_active):
+                    continue
+
                 # Move to next mode
                 if self.on_demand_active:
                     # Guard against empty on_demand_modes to prevent ZeroDivisionError

test/test_display_pending_changes.py

@@ -0,0 +1,592 @@
+"""Changes made from the web UI must reach the panel while a screen is showing.
+
+The main loop applies on-demand requests, the display schedule and brightness
+once per pass -- once per screen. Screens are long: a dwell can be a minute and
+a Vegas iteration runs for vegas_scroll.max_cycle_duration (240s here). On a
+real Pi on 2026-09-23:
+
+  * an on-demand request posted at 10:54:27 was activated at 10:57:24, when the
+    Vegas iteration it arrived during finally ended -- nothing read the mailbox
+    in between, because _check_vegas_interrupt only looked at a flag that the
+    main-loop read sets;
+  * two brightness saves 12s apart inside one 30s screen never showed at all.
+
+_service_pending_changes is the fix: a throttled pass the dwell sleep, the
+render loops and the Vegas interrupt check all call. These tests drive those
+long stretches on a fake clock and check a change lands within one throttle
+interval -- and that the throttle holds, since the callers run at frame rate.
+"""
+
+import threading
+import types
+from datetime import datetime, timedelta
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from src.vegas_mode.config import VegasModeConfig
+from src.vegas_mode.coordinator import VegasModeCoordinator
+
+VEGAS_ITERATION_SECONDS = 240
+REQUEST_KEY = 'display_on_demand_request'
+
+
+class FakeClock:
+    """A clock that only moves when the code under test sleeps.
+
+    Installed as the ``time`` module of display_controller and the Vegas
+    coordinator only -- patching the real module would also speed up every
+    background thread in the process.
+    """
+
+    def __init__(self, start=10_000.0):
+        self.t = start
+        self._events = []
+
+    def now(self):
+        return self.t
+
+    def sleep(self, seconds):
+        # Never zero, so a loop that sleeps "the rest of the frame" still
+        # advances when its fake frame took no time.
+        self.t += max(seconds, 0.0005)
+        while self._events and self._events[0][0] <= self.t:
+            _, fn = self._events.pop(0)
+            fn()
+
+    def after(self, seconds, fn):
+        """Run fn once the clock is `seconds` past now."""
+        self._events.append((self.t + seconds, fn))
+        self._events.sort(key=lambda e: e[0])
+
+    def module(self):
+        return types.SimpleNamespace(time=self.now, monotonic=self.now,
+                                     perf_counter=self.now, sleep=self.sleep)
+
+
+@pytest.fixture
+def clock(monkeypatch):
+    c = FakeClock()
+    fake_time = c.module()
+    monkeypatch.setattr('src.display_controller.time', fake_time)
+    monkeypatch.setattr('src.vegas_mode.coordinator.time', fake_time)
+    return c
+
+
+@pytest.fixture
+def controller(test_display_controller, clock):
+    """A controller at rest: no schedule, full brightness, empty mailbox."""
+    c = test_display_controller
+    c._refresh_config_cache({'display': {'hardware': {'brightness': 90}}})
+    c.current_brightness = 90
+    c.is_display_active = True
+    c._check_wifi_status_message = MagicMock(return_value=None)
+
+    c.mailbox = {}  # what the web process has written
+
+    def cache_get(key, *args, **kwargs):
+        if key == REQUEST_KEY:
+            return c.mailbox.get('request')
+        return None
+
+    def cache_delete(key):
+        if key == REQUEST_KEY:
+            c.mailbox.pop('request', None)
+
+    c.cache_manager.get = MagicMock(side_effect=cache_get)
+    c.cache_manager.delete = MagicMock(side_effect=cache_delete)
+    c.cache_manager.set = MagicMock()
+    c.display_manager.set_brightness = MagicMock(return_value=True)
+    c.display_manager.update_display = MagicMock()
+
+    # Two loaded plugins: the rotation is on one, on-demand can ask for either.
+    for mode in ('clock', 'weather'):
+        c.plugin_display_modes[mode] = [mode]
+        c.plugin_modes[mode] = MagicMock(spec=[])
+        c.mode_to_plugin_id[mode] = mode
+    c.available_modes = ['clock', 'weather']
+    c.current_mode_index = 0
+    c.current_display_mode = 'clock'
+    return c
+
+
+def post_request(controller, request_id='r1', mode='clock'):
+    controller.mailbox['request'] = {
+        'request_id': request_id, 'action': 'start',
+        'plugin_id': mode, 'mode': mode,
+    }
+
+
+def save_brightness(controller, brightness):
+    """What the config watcher thread does when the web UI saves."""
+    controller._controller_config_change(
+        controller.config,
+        {'display': {'hardware': {'brightness': brightness}}})
+
+
+def mailbox_reads(controller):
+    return sum(1 for call in controller.cache_manager.get.call_args_list
+               if call.args and call.args[0] == REQUEST_KEY)
+
+
+def vegas_coordinator(controller):
+    """A coordinator that runs real run_iteration() over a stub renderer.
+
+    Wired to the controller's interrupt checker the way _initialize_vegas_mode
+    wires it (every 10 frames).
+    """
+    coord = VegasModeCoordinator.__new__(VegasModeCoordinator)
+    coord.vegas_config = VegasModeConfig.from_config({'display': {'vegas_scroll': {
+        'enabled': True, 'max_cycle_duration': VEGAS_ITERATION_SECONDS}}})
+    assert coord.vegas_config.continuous_scroll
+    coord.render_pipeline = MagicMock()
+    coord.stream_manager = MagicMock()
+    coord.display_manager = controller.display_manager
+    coord.stats = {'cycles_completed': 0, 'interruptions': 0}
+    coord._state_lock = threading.Lock()
+    coord._is_active = True
+    coord._is_paused = False
+    coord._should_stop = False
+    coord._fps_last_health_log = 0.0
+    coord._fps_was_degraded = False
+    coord._live_priority_check = None
+    coord._live_priority_active = False
+    coord._update_callback = None
+    coord._update_tick_running = False
+    coord.sync_manager = None
+    coord._update_static_mode_plugins = lambda: None
+    coord._check_static_plugin_trigger = lambda: None
+    coord.frames = 0
+
+    def run_frame():
+        coord.frames += 1
+        return True
+
+    coord.run_frame = run_frame
+    coord.set_interrupt_checker(controller._check_vegas_interrupt, check_interval=10)
+    return coord
+
+
+def frame_budget(coord):
+    """Longest a change can wait for the next interrupt check."""
+    return coord._interrupt_check_interval * coord.vegas_config.get_frame_interval()
+
+
+class TestOnDemandDuringVegas:
+    def test_a_request_mid_iteration_ends_the_iteration_promptly(self, controller, clock):
+        coord = vegas_coordinator(controller)
+        start = clock.t
+        posted = {}
+
+        def post():
+            posted['at'] = clock.t
+            post_request(controller, mode='weather')
+
+        clock.after(30, post)
+        completed = coord.run_iteration()
+        latency = clock.t - posted['at']
+
+        assert completed is False, "Vegas ran the whole iteration past the request"
+        assert controller.on_demand_active
+        assert controller.current_display_mode == 'weather'
+        assert latency <= controller.PENDING_CHANGES_INTERVAL + frame_budget(coord) + 0.01, (
+            f"request took {latency:.2f}s to interrupt Vegas")
+        assert clock.t - start < VEGAS_ITERATION_SECONDS
+
+    def test_the_interrupt_checker_answers_within_one_interval(self, controller, clock):
+        controller._service_pending_changes()  # the main loop's last pass
+        post_request(controller)
+        waited = 0.0
+        step = 0.01
+        while not controller._check_vegas_interrupt():
+            clock.t += step
+            waited += step
+            assert waited <= controller.PENDING_CHANGES_INTERVAL + step, (
+                "interrupt checker never saw the request")
+        assert controller.on_demand_active
+
+    def test_an_empty_mailbox_lets_the_iteration_run_out(self, controller, clock):
+        coord = vegas_coordinator(controller)
+        assert coord.run_iteration() is True
+        assert not controller.on_demand_active
+        # And the read is throttled: at most one per interval, not per check.
+        max_reads = VEGAS_ITERATION_SECONDS / controller.PENDING_CHANGES_INTERVAL + 1
+        assert 0 < mailbox_reads(controller) <= max_reads
+        checks = coord.frames // coord._interrupt_check_interval
+        assert mailbox_reads(controller) < checks / 2
+
+
+class TestBrightnessIsAppliedMidScreen:
+    def _applied_at(self, controller, clock, brightness):
+        """Clock time set_brightness(brightness) was first called, or None."""
+        return controller.applied.get(brightness)
+
+    def _record_brightness(self, controller, clock):
+        controller.applied = {}
+
+        def set_brightness(value):
+            controller.applied.setdefault(value, clock.t)
+            return True
+
+        controller.display_manager.set_brightness = MagicMock(side_effect=set_brightness)
+
+    def test_during_a_long_dwell(self, controller, clock):
+        """The observed failure: two saves 12s apart inside one screen."""
+        self._record_brightness(controller, clock)
+        saves = {}
+
+        def save(value):
+            saves[value] = clock.t
+            save_brightness(controller, value)
+
+        clock.after(5, lambda: save(40))
+        clock.after(17, lambda: save(90))
+        controller._sleep_with_plugin_updates(30)
+
+        interval = controller.PENDING_CHANGES_INTERVAL
+        assert 40 in controller.applied, "brightness 40 never reached the panel"
+        assert controller.applied[40] - saves[40] <= interval + 0.01
+        assert controller.applied[90] - saves[90] <= interval + 0.01
+        assert controller.current_brightness == 90
+
+    def test_the_frame_is_repushed_so_the_change_shows(self, controller, clock):
+        clock.after(1, lambda: save_brightness(controller, 40))
+        controller._sleep_with_plugin_updates(5)
+        calls = controller.display_manager.method_calls
+        names = [c[0] for c in calls]
+        assert 'set_brightness' in names
+        assert 'update_display' in names[names.index('set_brightness'):], (
+            "nothing pushed a frame after the brightness change")
+
+    def test_during_a_vegas_iteration(self, controller, clock):
+        self._record_brightness(controller, clock)
+        coord = vegas_coordinator(controller)
+        saved = {}
+
+        def save():
+            saved['at'] = clock.t
+            save_brightness(controller, 25)
+
+        clock.after(60, save)
+        assert coord.run_iteration() is True  # brightness does not end the scroll
+        assert 25 in controller.applied, "brightness waited for the iteration to end"
+        assert controller.applied[25] - saved['at'] <= (
+            controller.PENDING_CHANGES_INTERVAL + frame_budget(coord) + 0.01)
+
+    def test_a_dim_schedule_transition_mid_dwell(self, controller, clock):
+        self._record_brightness(controller, clock)
+        controller._refresh_config_cache({
+            'display': {'hardware': {'brightness': 90}},
+            'dim_schedule': {'enabled': True, 'start_time': '20:00',
+                             'end_time': '07:00', 'dim_brightness': 20},
+        })
+        wall_start = datetime(2026, 9, 21, 19, 59, 30)
+        t0 = clock.t
+        with patch('src.display_controller.datetime') as mock_dt:
+            mock_dt.strptime = datetime.strptime
+            mock_dt.now.side_effect = lambda tz=None: wall_start + timedelta(seconds=clock.t - t0)
+            controller._sleep_with_plugin_updates(60)
+
+        assert 20 in controller.applied, "the dim period started mid-screen and was missed"
+        dimmed_at = t0 + 30  # 20:00:00
+        assert controller.applied[20] - dimmed_at <= controller.PENDING_CHANGES_INTERVAL + 0.01
+
+
+class TestThrottle:
+    def test_no_reads_or_brightness_calls_between_passes(self, controller, clock):
+        controller._service_pending_changes()
+        reads = mailbox_reads(controller)
+        save_brightness(controller, 40)
+        post_request(controller)
+        for _ in range(500):  # a few seconds of frames, all inside one interval
+            controller._service_pending_changes()
+            clock.t += controller.PENDING_CHANGES_INTERVAL / 1000
+        assert mailbox_reads(controller) == reads
+        controller.display_manager.set_brightness.assert_not_called()
+        assert not controller.on_demand_active
+
+        clock.t += controller.PENDING_CHANGES_INTERVAL
+        controller._service_pending_changes()
+        # The poll, plus the consume step's re-read of the request it acted on.
+        assert mailbox_reads(controller) > reads
+        controller.display_manager.set_brightness.assert_called_once_with(40)
+        assert controller.on_demand_active
+
+    def test_between_passes_it_does_no_work_at_all(self, controller, clock):
+        """The high-FPS and Vegas loops call this every frame."""
+        controller._service_pending_changes()
+        controller._poll_on_demand_requests = MagicMock()
+        controller._check_dim_schedule = MagicMock(return_value=90)
+        controller._check_schedule = MagicMock()
+        for _ in range(100):
+            controller._service_pending_changes()
+            controller._check_vegas_interrupt()
+        controller._poll_on_demand_requests.assert_not_called()
+        controller._check_dim_schedule.assert_not_called()
+        controller._check_schedule.assert_not_called()
+
+    def test_an_unchanged_brightness_is_not_re_sent(self, controller, clock):
+        for _ in range(20):
+            controller._service_pending_changes()
+            clock.t += controller.PENDING_CHANGES_INTERVAL
+        controller.display_manager.set_brightness.assert_not_called()
+
+    def test_a_refused_brightness_is_not_retried_every_pass(self, controller, clock):
+        controller.display_manager.set_brightness = MagicMock(return_value=False)
+        save_brightness(controller, 40)
+        for _ in range(20):
+            controller._service_pending_changes()
+            clock.t += controller.PENDING_CHANGES_INTERVAL
+        controller.display_manager.set_brightness.assert_called_once_with(40)
+        assert controller.current_brightness == 90
+        # A different target is still tried.
+        save_brightness(controller, 50)
+        controller._service_pending_changes()
+        controller.display_manager.set_brightness.assert_called_with(50)
+
+    def test_a_refused_brightness_is_tried_again_after_going_back(self, controller, clock):
+        controller.display_manager.set_brightness = MagicMock(return_value=False)
+        for value in (40, 90, 40):
+            save_brightness(controller, value)
+            controller._service_pending_changes()
+            clock.t += controller.PENDING_CHANGES_INTERVAL
+        assert controller.display_manager.set_brightness.call_count == 2
+
+
+class TestScheduleAndDwells:
+    def _schedule_until(self, controller, end_time):
+        controller._refresh_config_cache({
+            'display': {'hardware': {'brightness': 90}},
+            'schedule': {'enabled': True, 'start_time': '07:00', 'end_time': end_time},
+        })
+
+    def test_vegas_hands_back_when_the_display_is_scheduled_off(self, controller, clock):
+        self._schedule_until(controller, '22:59')
+        coord = vegas_coordinator(controller)
+        wall_start = datetime(2026, 9, 21, 22, 59, 0)
+        t0 = clock.t
+        with patch('src.display_controller.datetime') as mock_dt:
+            mock_dt.strptime = datetime.strptime
+            mock_dt.now.side_effect = lambda tz=None: wall_start + timedelta(seconds=clock.t - t0)
+            completed = coord.run_iteration()
+        assert completed is False
+        assert not controller.is_display_active
+        off_at = t0 + 60  # 23:00:00
+        assert clock.t - off_at <= controller.PENDING_CHANGES_INTERVAL + frame_budget(coord) + 0.01
+
+    def test_on_demand_wakes_a_scheduled_off_display_within_an_interval(self, controller, clock):
+        """The display-off branch sleeps 60s at a time between checks."""
+        controller.is_display_active = False
+        controller._evaluate_schedule = MagicMock()  # hold the schedule "off"
+        posted = {}
+
+        def post():
+            posted['at'] = clock.t
+            post_request(controller)
+
+        clock.after(3, post)
+        start = clock.t
+        controller._sleep_with_plugin_updates(60)
+        assert controller.on_demand_active
+        assert clock.t - start < 60, "the off-schedule sleep ran its full minute"
+        assert clock.t - posted['at'] <= controller.PENDING_CHANGES_INTERVAL + 0.01
+
+    def test_the_dwell_returns_early_on_an_on_demand_stop(self, controller, clock):
+        post_request(controller)
+        controller._service_pending_changes()
+        assert controller.on_demand_active
+        clock.t += controller.PENDING_CHANGES_INTERVAL
+        controller.mailbox['request'] = {'request_id': 'r2', 'action': 'stop'}
+        start = clock.t
+        controller._sleep_with_plugin_updates(30)
+        assert not controller.on_demand_active
+        assert clock.t - start <= controller.PENDING_CHANGES_INTERVAL + 0.01
+
+    def test_a_quiet_dwell_runs_its_full_length(self, controller, clock):
+        start = clock.t
+        controller._sleep_with_plugin_updates(30)
+        assert clock.t - start >= 30
+
+
+class _Plugin:
+    """A static (non-scrolling) plugin mode that records what was shown."""
+
+    needs_high_fps = False
+
+    def __init__(self, mode, plugin_id, shown, results=()):
+        self.mode = mode
+        self.plugin_id = plugin_id
+        self._shown = shown
+        self._results = iter(results)
+
+    def display(self, force_clear=False):
+        self._shown.append(self.mode)
+        if len(self._shown) > 6:
+            raise KeyboardInterrupt  # ends run(); it catches this and cleans up
+        return next(self._results, True)
+
+
+class TestRunLoopDoesNotSkipTheRequestedMode:
+    def test_on_demand_during_the_minimum_duration_sleep(self, controller, clock):
+        """A dwell sleep that ends early on an on-demand start must not rotate.
+
+        The first screen shows once, then reports no content, so run() sleeps
+        out the rest of its minimum duration. The request lands during that
+        sleep. Falling through to "move to next mode" advanced the new
+        on-demand rotation past the mode that was asked for.
+        """
+        c = controller
+        shown = []
+        c.plugin_modes.clear()
+        c.mode_to_plugin_id.clear()
+        c.plugin_display_modes.clear()
+        modes = {'clock': ('clock', [True, False]), 'news': ('news', []),
+                 'weather': ('weather', []), 'weather_hourly': ('weather', [])}
+        for mode, (pid, results) in modes.items():
+            c.plugin_modes[mode] = _Plugin(mode, pid, shown, results)
+            c.mode_to_plugin_id[mode] = pid
+            c.plugin_display_modes.setdefault(pid, []).append(mode)
+        c.available_modes = list(modes)
+        c.current_mode_index = 0
+        c._cleanup_expired_wifi_status = MagicMock()
+        c.config.setdefault('display', {})['display_durations'] = {'clock': 30}
+        c.plugin_manager.plugin_executor.execute_display.side_effect = (
+            lambda target, plugin_id, force_clear=False, display_mode=None, **kw:
+            target.display(force_clear=force_clear))
+        clock.after(10, lambda: post_request(c, mode='weather'))
+
+        c.run()
+
+        assert c.on_demand_modes[:2] == ['weather', 'weather_hourly']
+        assert shown[:2] == ['clock', 'clock']
+        assert shown[2] == 'weather', f"on-demand opened on {shown[2]}, not the requested mode"
+
+
+class _Stop(KeyboardInterrupt):
+    """Ends run(): it catches KeyboardInterrupt and cleans up."""
+
+
+class TestRunLoopBlanksWhenVegasHandsBack:
+    def test_scheduled_off_mid_iteration_blanks_instead_of_rendering(self, controller, clock):
+        c = controller
+        shown = []
+        c.plugin_modes['clock'] = _Plugin('clock', 'clock', shown)
+        c.plugin_modes['weather'] = _Plugin('weather', 'weather', shown)
+        c._cleanup_expired_wifi_status = MagicMock()
+        c._refresh_config_cache({
+            'display': {'hardware': {'brightness': 90}},
+            'schedule': {'enabled': True, 'start_time': '07:00', 'end_time': '22:59'},
+        })
+        c.vegas_coordinator = vegas_coordinator(c)
+        c.vegas_coordinator._pending_config_update = False
+
+        def stop():
+            raise _Stop()
+
+        wall_start = datetime(2026, 9, 21, 22, 59, 0)
+        t0 = clock.t
+        clock.after(90, stop)
+        with patch('src.display_controller.datetime') as mock_dt:
+            mock_dt.strptime = datetime.strptime
+            mock_dt.now.side_effect = lambda tz=None: wall_start + timedelta(seconds=clock.t - t0)
+            c.run()
+
+        assert c.vegas_coordinator.frames > 0  # Vegas was running first
+        assert not c.is_display_active
+        assert shown == [], f"rendered {shown} after the display was scheduled off"
+        c.display_manager.clear.assert_called()
+
+    @pytest.mark.parametrize('needs_high_fps', [True, False])
+    def test_a_screen_scheduled_off_midway_stops_rendering(self, controller, clock,
+                                                           needs_high_fps):
+        c = controller
+        shown_at = []
+        plugin = _Plugin('ticker', 'ticker', [])
+        plugin.needs_high_fps = needs_high_fps
+        plugin.display = lambda force_clear=False: shown_at.append(clock.t) or True
+        c.plugin_modes.clear()
+        c.mode_to_plugin_id.clear()
+        c.plugin_display_modes.clear()
+        c.plugin_modes['ticker'] = plugin
+        c.mode_to_plugin_id['ticker'] = 'ticker'
+        c.plugin_display_modes['ticker'] = ['ticker']
+        c.available_modes = ['ticker']
+        c.current_mode_index = 0
+        c._cleanup_expired_wifi_status = MagicMock()
+        c._refresh_config_cache({
+            'display': {'hardware': {'brightness': 90},
+                        'display_durations': {'ticker': 120}},
+            'schedule': {'enabled': True, 'start_time': '07:00', 'end_time': '22:59'},
+        })
+        c.plugin_manager.plugin_executor.execute_display.side_effect = (
+            lambda target, plugin_id, force_clear=False, display_mode=None, **kw:
+            target.display(force_clear=force_clear))
+
+        def stop():
+            raise _Stop()
+
+        wall_start = datetime(2026, 9, 21, 22, 59, 0)
+        t0 = clock.t
+        clock.after(150, stop)
+        with patch('src.display_controller.datetime') as mock_dt:
+            mock_dt.strptime = datetime.strptime
+            mock_dt.now.side_effect = lambda tz=None: wall_start + timedelta(seconds=clock.t - t0)
+            c.run()
+
+        off_at = t0 + 60  # 23:00:00, halfway through a 120s screen
+        assert shown_at and shown_at[0] < off_at
+        assert shown_at[-1] - off_at <= 1.0 + c.PENDING_CHANGES_INTERVAL, (
+            f"kept rendering {shown_at[-1] - off_at:.1f}s after the display was scheduled off")
+        assert not c.is_display_active
+
+
+class TestRunLoopAppliesBrightnessMidScreen:
+    """The render loops of run() itself, not just the helpers they call."""
+
+    def _run_screen(self, c, clock, needs_high_fps):
+        shown = []
+        plugin = _Plugin('ticker', 'ticker', shown)
+        plugin.needs_high_fps = needs_high_fps
+        plugin.display = lambda force_clear=False: shown.append('ticker') or True
+        c.plugin_modes.clear()
+        c.mode_to_plugin_id.clear()
+        c.plugin_display_modes.clear()
+        c.plugin_modes['ticker'] = plugin
+        c.mode_to_plugin_id['ticker'] = 'ticker'
+        c.plugin_display_modes['ticker'] = ['ticker']
+        c.available_modes = ['ticker']
+        c.current_mode_index = 0
+        c._cleanup_expired_wifi_status = MagicMock()
+        c.config.setdefault('display', {})['display_durations'] = {'ticker': 60}
+        c.plugin_manager.plugin_executor.execute_display.side_effect = (
+            lambda target, plugin_id, force_clear=False, display_mode=None, **kw:
+            target.display(force_clear=force_clear))
+        applied = {}
+
+        def set_brightness(value):
+            applied.setdefault(value, clock.t)
+            return True
+
+        c.display_manager.set_brightness = MagicMock(side_effect=set_brightness)
+        start = clock.t
+        clock.after(5, lambda: save_brightness(c, 30))
+
+        def stop():
+            raise _Stop()
+
+        clock.after(20, stop)
+        c.run()
+        return start, applied, shown
+
+    def test_a_scrolling_screen(self, controller, clock):
+        start, applied, shown = self._run_screen(controller, clock, needs_high_fps=True)
+        assert len(shown) > 100  # really was the high-FPS loop
+        assert 30 in applied, "brightness waited for the scrolling screen to end"
+        assert applied[30] - (start + 5) <= controller.PENDING_CHANGES_INTERVAL + 0.02
+
+    def test_a_static_screen(self, controller, clock):
+        start, applied, shown = self._run_screen(controller, clock, needs_high_fps=False)
+        assert len(shown) < 30  # really was the once-a-second loop
+        assert 30 in applied, "brightness waited for the static screen to end"
+        # This loop redraws once a second, and services changes after each redraw.
+        assert applied[30] - (start + 5) <= 1.0 + controller.PENDING_CHANGES_INTERVAL