fix(startup): bound the initial plugin update so the panel lights sooner (#456)

* fix(startup): bound the initial plugin update so the panel lights sooner

DisplayController.__init__ calls _update_modules() once to populate
plugin data before the first frame. It walks every loaded plugin in
turn, and each update blocks the calling thread for up to the executor's
30s timeout, so the uncapped total is the sum of every slow plugin on
the system. The rig's own log:

    Initial plugin update completed in 82.255 seconds
    Initial plugin update completed in 55.123 seconds
    Initial plugin update completed in 25.975 seconds

The panel shows nothing for all of it.

Nothing is lost by stopping early. A plugin that has never updated is
immediately due, so run_scheduled_updates() collects it seconds later --
with the display already running rather than blank.

A deadline alone was not enough: it is checked before each plugin, so
the last one to start could still block for the full 30s, and a 20s
budget produced a 31.8s pass on the rig. The remaining budget is now
passed down as that update's timeout too, with a floor so a plugin
starting on the last sliver is not handed ~0s and recorded as having
timed out for a slot it never had. Measured after: 20.006s.

Found while profiling a scroll freeze with py-spy, which caught the main
thread 9.34s inside execute_with_timeout's join. Worth being clear that
this is startup latency, not the recurring stutter -- _update_modules
has exactly one caller and runtime updates already run off the display
thread.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

* feat(display): show the device address on the startup screen

That screen is what the panel holds for the whole initial plugin update,
and on a headless Pi it is the only place the address appears without
going looking for it -- so it now carries the address under
"Initializing".

The lookup connects a UDP socket, which sends no packets: it only asks
the kernel which source address it would route from. That costs 0.03ms
and works with the network down so long as a route exists. Deliberately
not `hostname -I` plus a systemctl probe for AP mode, which is how the
web launcher does it -- two subprocesses with multi-second timeouts, on
the startup path this branch exists to shorten.

Two things had to change for the address to be worth putting there.

The text is now sized to fit rather than fixed at 8px: "Initializing" is
96px in PressStart2P, drawn at x=10, so it already ran off the side of a
64px panel before an address was added. It falls back to 4x6 where that
does not fit, and both lines are centred.

And the test pattern is punched out from behind the block, with the text
drawn white rather than blue. The diagonal runs through the middle of
the panel, which is exactly where this sits, and blue on black reads
fine on a monitor but is marginal on a dim panel. An address that cannot
be read off the wall is not worth showing.

The rendering tests assert against pixels -- no green left behind the
text at any supported size, enough lit pixels to be visible -- rather
than against the geometry that produced them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

* fix(display): keep the startup text blue -- it is a channel reference

The test pattern lights one pure channel per element: red border, green
diagonal, blue text. That is how a glance at the panel tells you whether
led_rgb_sequence is right -- wire it BGR and the border comes up blue
and the text red. Drawing the text white, as the previous commit did for
contrast, lights all three channels and destroys the only blue reference
on the screen.

Reverted to blue, with the reason written down so it is not treated as a
style preference again, and with tests that pin it: the text must be
pure blue, nothing on the screen may be white, and all three primaries
must be present.

The punched-out backdrop stays. It only removes the diagonal from behind
the glyphs, which costs nothing diagnostically -- the diagonal is still
plainly visible across the rest of the panel -- and it is what makes the
address readable at all.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

* fix(startup): defer a plugin with too little budget, rather than clamp it

The per-plugin timeout was clamped up to a floor, so a plugin that began
with a sliver of budget left was granted the full floor and ran on past
the deadline: a 20s budget could take 22. The floor existed to stop a
plugin being handed a slot too short to use and then recorded as having
timed out, which is a real concern, but clamping solved it by breaking
the bound.

Deferring solves both. Below the floor the plugin is left to the update
tick, which was already the fate of everything after the deadline, so
nothing new is lost -- a plugin that has never updated is immediately
due. Above it, the timeout is the exact remainder, and the pass cannot
outlast its deadline.

Measured on the rig after the change: 20.002s, 5 plugins deferred.

Also names an unused binding in the initializing-screen test.

Both reported by CodeRabbit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/display_controller.py

@@ -44,6 +44,20 @@ from src.common.sync_manager import DisplaySyncManager, SyncRole
 # Get logger with consistent configuration
 logger = get_logger(__name__)
 
+# How long startup will wait for plugins to fetch their first data before
+# showing anything. Each plugin's update blocks for up to the executor's 30s
+# timeout and they run one after another, so the uncapped total is the sum of
+# every slow plugin: 82 seconds on the worst boot measured, with a blank panel
+# throughout. Whatever does not finish in time is picked up by the scheduled
+# update tick moments later, with the display already running.
+_INITIAL_UPDATE_BUDGET_SECONDS = 20.0
+
+# The least budget worth starting a plugin with. Below this the plugin is
+# deferred instead: granting it a floor would let the pass run past its
+# deadline, and granting it the true remainder would record a timeout for a
+# slot it never had a chance to use.
+_MIN_INITIAL_UPDATE_TIMEOUT_SECONDS = 2.0
+
 # Vegas mode import (lazy loaded to avoid circular imports)
 _vegas_mode_imported = False
 VegasModeCoordinator = None
@@ -463,7 +477,7 @@ class DisplayController:
         # Initial data update for plugins (ensures data available on first display)
         logger.info("Performing initial plugin data update...")
         update_start = time.time()
-        self._update_modules()
+        self._update_modules(deadline=update_start + _INITIAL_UPDATE_BUDGET_SECONDS)
         logger.info("Initial plugin update completed in %.3f seconds", time.time() - update_start)
 
         # Initialize Vegas mode coordinator
@@ -819,14 +833,42 @@ class DisplayController:
             self._cached_target_brightness = normal_brightness  # persist for minute-gate
             return normal_brightness
 
-    def _update_modules(self):
-        """Update all plugin modules."""
+    def _update_modules(self, deadline: Optional[float] = None):
+        """Update all plugin modules.
+
+        Args:
+            deadline: Wall-clock time after which remaining plugins are left
+                for the scheduled update tick instead of being waited on. Each
+                update blocks this thread for up to the executor's timeout, and
+                they run one after another, so without a bound the total is the
+                sum of every slow plugin on the system. Measured at startup on
+                a live rig: 82 seconds, 55 and 26 on the two boots before -- all
+                of it with nothing on the panel.
+        """
         if not self.plugin_manager:
             return
 
         # Update all loaded plugins
         plugins_dict = getattr(self.plugin_manager, 'loaded_plugins', None) or getattr(self.plugin_manager, 'plugins', {})
+        deferred = []
         for plugin_id, plugin_instance in plugins_dict.items():
+            update_timeout = None
+            if deadline is not None:
+                update_timeout = deadline - time.time()
+                if update_timeout < _MIN_INITIAL_UPDATE_TIMEOUT_SECONDS:
+                    # Too little left to be worth starting. Deferring rather
+                    # than granting a floor keeps the budget a real ceiling --
+                    # clamping up to a minimum let a plugin that began with a
+                    # sliver left run on past the deadline -- and a plugin
+                    # handed a slot it cannot use would just be recorded as
+                    # having timed out.
+                    #
+                    # Nothing is lost either way: a plugin that has never
+                    # updated is immediately due, so run_scheduled_updates()
+                    # picks it up within seconds, with the display already
+                    # running.
+                    deferred.append(plugin_id)
+                    continue
             # Check circuit breaker before attempting update
             if hasattr(self.plugin_manager, 'health_tracker') and self.plugin_manager.health_tracker:
                 if self.plugin_manager.health_tracker.should_skip_plugin(plugin_id):
@@ -835,7 +877,13 @@ class DisplayController:
             
             # Use PluginExecutor if available for safe execution
             if hasattr(self.plugin_manager, 'plugin_executor'):
-                success = self.plugin_manager.plugin_executor.execute_update(plugin_instance, plugin_id)
+                # The remaining budget is the timeout, so the pass cannot
+                # run past its deadline. Bounding the loop alone did not do
+                # it: the last plugin to start could still block for the
+                # executor's full 30s, which turned a 20s budget into a 31.8s
+                # pass on the rig.
+                success = self.plugin_manager.plugin_executor.execute_update(
+                    plugin_instance, plugin_id, timeout=update_timeout)
                 if success and hasattr(self.plugin_manager, 'plugin_last_update'):
                     self.plugin_manager.plugin_last_update[plugin_id] = time.time()
             else:
@@ -854,6 +902,12 @@ class DisplayController:
                     if hasattr(self.plugin_manager, 'health_tracker') and self.plugin_manager.health_tracker:
                         self.plugin_manager.health_tracker.record_failure(plugin_id, exc)
 
+        if deferred:
+            logger.info(
+                "Initial update budget spent; %d plugin(s) left to the update "
+                "tick so the display can start: %s",
+                len(deferred), ", ".join(deferred))
+
     def _tick_plugin_updates_for_vegas(self) -> None:
         """Run scheduled plugin updates and tell Vegas mode which plugins
         actually got fresh data, so it can hot-swap them into the scroll

src/display_manager.py

@@ -25,6 +25,7 @@ the same object.
 
 import json
 import os
+import socket
 import tempfile
 if os.getenv("EMULATOR", "false") == "true":
     from RGBMatrixEmulator import RGBMatrix, RGBMatrixOptions
@@ -517,6 +518,91 @@ class DisplayManager:
             logger.warning(f"[BRIGHTNESS] Matrix does not support brightness property: {e}", exc_info=True)
             return -1
 
+    @staticmethod
+    def _local_ip() -> Optional[str]:
+        """This device's address on the network it routes through, or None.
+
+        Deliberately not `hostname -I` or a systemctl probe for AP mode, which
+        is how the web launcher does it: both spawn processes with multi-second
+        timeouts, and this runs on the startup path the rest of this change
+        exists to shorten. Connecting a UDP socket sends no packets -- it only
+        asks the kernel which source address it would use -- so it costs
+        microseconds and works with the network down, as long as a route
+        exists.
+        """
+        sock = None
+        try:
+            sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+            sock.settimeout(0.2)
+            sock.connect(("8.8.8.8", 80))  # nosec B104 - no traffic; selects a route
+            ip = sock.getsockname()[0]
+            return ip if ip and not ip.startswith("127.") else None
+        except OSError:
+            return None
+        finally:
+            if sock is not None:
+                try:
+                    sock.close()
+                except OSError:
+                    pass
+
+    def _fitting_font(self, lines, width):
+        """The largest font from the usual ladder that fits every line."""
+        candidates = [self.font,
+                      ("assets/fonts/4x6-font.ttf", 6)]
+        for candidate in candidates:
+            try:
+                font = candidate
+                if isinstance(candidate, tuple):
+                    font = ImageFont.truetype(candidate[0], candidate[1])
+                if all(self.draw.textlength(t, font=font) <= width for t in lines):
+                    return font
+            except (OSError, ValueError, AttributeError):
+                continue
+        return self.font
+
+    def _draw_startup_banner(self, lines, width: int, height: int) -> None:
+        """Centre `lines` over whatever the test pattern already drew.
+
+        This screen stays on the panel for the whole initial plugin update, and
+        on a headless Pi it is the only place the device's address appears
+        without going looking for it -- so it has to be readable off a wall,
+        not merely present.
+
+        The font is chosen to fit rather than fixed at 8px: "Initializing" is
+        96px in PressStart2P, which ran off the side of a 64px panel even
+        before an address was added. And the pattern is punched out behind the
+        text, because the diagonal runs through the middle of the panel, which
+        is exactly where this sits.
+
+        The text stays blue. It is not decoration: the pattern draws one pure
+        channel per element -- red border, green diagonal, blue text -- so that
+        a glance at the panel says whether led_rgb_sequence is right. Swap the
+        wiring to BGR and the border comes up blue and this text red. Drawing
+        it white would light all three channels and destroy the only blue
+        reference on the screen, which is why it is worth a comment rather
+        than a quiet preference.
+        """
+        if not lines:
+            return
+        font = self._fitting_font(lines, width - 2)
+        line_height = self.draw.textbbox((0, 0), "Ag", font=font)[3] + 1
+        block_height = line_height * len(lines)
+        block_top = max(1, (height - block_height) // 2)
+        block_width = max(self.draw.textlength(t, font=font) for t in lines)
+        block_left = max(0, (width - block_width) // 2)
+
+        self.draw.rectangle(
+            [block_left - 2, block_top - 1,
+             block_left + block_width + 1, block_top + block_height],
+            fill=(0, 0, 0))
+
+        for row, line in enumerate(lines):
+            line_width = self.draw.textlength(line, font=font)
+            self.draw.text(
+                (max(0, (width - line_width) // 2), block_top + row * line_height),
+                line, font=font, fill=(0, 0, 255))
+
     def _draw_test_pattern(self):
         """Draw a test pattern to verify the display is working."""
         try:
@@ -536,8 +622,11 @@ class DisplayManager:
             # Draw a diagonal line
             self.draw.line([0, 0, self.matrix.width-1, self.matrix.height-1], fill=(0, 255, 0))
             
-            # Draw some text - changed from "TEST" to "Initializing" with smaller font
-            self.draw.text((10, 10), "Initializing", font=self.font, fill=(0, 0, 255))
+            lines = ["Initializing"]
+            ip = self._local_ip()
+            if ip:
+                lines.append(ip)
+            self._draw_startup_banner(lines, self.matrix.width, self.matrix.height)
             
             # Update the display once after everything is drawn
             self.update_display()

test/test_initial_update_budget.py

@@ -0,0 +1,224 @@
+"""Tests that startup does not wait indefinitely for plugins to fetch data.
+
+DisplayController.__init__ calls _update_modules() once, to populate plugin
+data before the first frame. It walks every loaded plugin in turn, and each
+update blocks the calling thread for up to the executor's 30s timeout, so the
+uncapped total is the sum of every slow plugin on the system.
+
+Profiled on a live rig with py-spy, the main thread sat 9.34s in
+
+    display_controller._update_modules
+      -> plugin_executor.execute_update
+        -> execute_with_timeout -> threading.join
+
+and the controller's own log put the full pass at 82 seconds on the worst
+boot measured (55 and 26 on the two before). The panel shows nothing for all
+of it.
+
+Nothing is lost by stopping early: a plugin that has never updated is
+immediately due, so run_scheduled_updates() collects it seconds later with the
+display already running.
+"""
+
+import os
+import time
+from unittest.mock import Mock
+
+import pytest
+
+# display_controller imports display_manager, which binds the hardware
+# rgbmatrix module unless EMULATOR=true is set before import (same convention
+# as test_display_controller_vegas_tick.py).
+os.environ.setdefault("EMULATOR", "true")
+
+from src.display_controller import (  # noqa: E402
+    DisplayController, _INITIAL_UPDATE_BUDGET_SECONDS,
+    _MIN_INITIAL_UPDATE_TIMEOUT_SECONDS)
+
+
+class FakeExecutor:
+    """Records which plugins were updated, and can make some of them slow."""
+
+    def __init__(self, cost=0.0, slow=()):
+        self.updated = []
+        self.cost = cost
+        self.slow = set(slow)
+
+    def execute_update(self, plugin, plugin_id, timeout=None):
+        self.updated.append(plugin_id)
+        if plugin_id in self.slow:
+            time.sleep(self.cost)
+        return True
+
+
+@pytest.fixture
+def tiny_floor(monkeypatch):
+    """Shrink the "worth starting" floor so timing tests stay quick."""
+    import src.display_controller as mod
+    monkeypatch.setattr(mod, "_MIN_INITIAL_UPDATE_TIMEOUT_SECONDS", 0.01)
+
+
+def _controller(plugin_ids, executor):
+    c = DisplayController.__new__(DisplayController)
+    c.plugin_manager = Mock()
+    # Both attributes, because _update_modules reads
+    # `loaded_plugins or plugins` and an empty dict is falsy.
+    c.plugin_manager.loaded_plugins = {pid: Mock() for pid in plugin_ids}
+    c.plugin_manager.plugins = dict(c.plugin_manager.loaded_plugins)
+    c.plugin_manager.plugin_executor = executor
+    c.plugin_manager.plugin_last_update = {}
+    c.plugin_manager.health_tracker = None
+    return c
+
+
+class TestTheBudgetIsRespected:
+    def test_without_a_deadline_every_plugin_is_updated(self):
+        ex = FakeExecutor()
+        _controller(['a', 'b', 'c'], ex)._update_modules()
+        assert ex.updated == ['a', 'b', 'c']
+
+    def test_a_passed_deadline_stops_the_pass(self):
+        ex = FakeExecutor()
+        _controller(['a', 'b', 'c'], ex)._update_modules(deadline=time.time() - 1)
+        assert ex.updated == [], "updated %r after the deadline" % ex.updated
+
+    def test_slow_plugins_do_not_drag_in_the_rest(self, tiny_floor):
+        # One plugin burns the whole budget; the remainder must be left alone
+        # rather than each adding its own wait.
+        ex = FakeExecutor(cost=0.3, slow={'slow'})
+        c = _controller(['slow'] + ['p%d' % i for i in range(20)], ex)
+        started = time.time()
+        c._update_modules(deadline=started + 0.2)
+        elapsed = time.time() - started
+
+        assert ex.updated == ['slow'], "updated %r" % ex.updated
+        # Bounded by the one in-flight update, not by twenty more.
+        assert elapsed < 1.0, "%.2fs" % elapsed
+
+    def test_a_generous_deadline_still_gets_everything(self):
+        ex = FakeExecutor()
+        c = _controller(['a', 'b', 'c'], ex)
+        c._update_modules(deadline=time.time() + 30)
+        assert ex.updated == ['a', 'b', 'c']
+
+    def test_the_deadline_is_checked_before_each_plugin(self, tiny_floor):
+        # Not just once up front: the budget can be spent partway through.
+        ex = FakeExecutor(cost=0.15, slow={'a', 'b', 'c', 'd'})
+        c = _controller(['a', 'b', 'c', 'd'], ex)
+        c._update_modules(deadline=time.time() + 0.2)
+        assert 0 < len(ex.updated) < 4, "updated %r" % ex.updated
+
+
+class TestThePassIsBoundedInPractice:
+    def test_the_last_plugin_cannot_overrun_the_budget(self):
+        # Checking the deadline before each plugin is not enough on its own:
+        # one that starts with a moment left could still block for the
+        # executor's full timeout. On the rig that turned a 20s budget into a
+        # 31.8s pass, so the remaining budget is passed down as the timeout.
+        seen = []
+
+        class Executor:
+            def execute_update(self, plugin, plugin_id, timeout=None):
+                seen.append(timeout)
+                return True
+
+        c = _controller(['a', 'b', 'c'], Executor())
+        deadline = time.time() + 5
+        c._update_modules(deadline=deadline)
+
+        assert seen and all(t is not None for t in seen), seen
+        assert all(t <= 5.01 for t in seen), seen
+        # The exact remainder, never clamped up: clamping would let the pass
+        # run past its deadline. Anything below the floor is deferred instead,
+        # so what does start always has a usable slot.
+        assert all(t >= _MIN_INITIAL_UPDATE_TIMEOUT_SECONDS for t in seen), seen
+
+    def test_without_a_deadline_the_executor_default_is_left_alone(self):
+        seen = []
+
+        class Executor:
+            def execute_update(self, plugin, plugin_id, timeout=None):
+                seen.append(timeout)
+                return True
+
+        _controller(['a'], Executor())._update_modules()
+        assert seen == [None], seen
+
+
+class TestTheBudgetItself:
+    def test_it_is_short_enough_to_be_worth_having(self):
+        # The measured uncapped worst case was 82s; a budget near that would
+        # not bound anything.
+        assert _INITIAL_UPDATE_BUDGET_SECONDS <= 30
+
+    def test_it_is_long_enough_for_a_quick_plugin_or_two(self):
+        assert _INITIAL_UPDATE_BUDGET_SECONDS >= 5
+
+
+class TestNothingIsSilentlyDropped:
+    def test_deferred_plugins_are_named_in_the_log(self, caplog):
+        ex = FakeExecutor()
+        c = _controller(['a', 'b'], ex)
+        with caplog.at_level('INFO'):
+            c._update_modules(deadline=time.time() - 1)
+        text = "\n".join(r.getMessage() for r in caplog.records)
+        assert 'a' in text and 'b' in text, text
+        assert 'budget' in text.lower(), text
+
+    def test_nothing_is_logged_when_all_of_them_ran(self, caplog):
+        ex = FakeExecutor()
+        c = _controller(['a'], ex)
+        with caplog.at_level('INFO'):
+            c._update_modules(deadline=time.time() + 30)
+        assert not any('budget' in r.getMessage().lower() for r in caplog.records)
+
+
+class TestItDoesNotBreakTheOrdinaryPaths:
+    def test_no_plugin_manager_is_harmless(self):
+        c = DisplayController.__new__(DisplayController)
+        c.plugin_manager = None
+        c._update_modules(deadline=time.time() - 1)      # must not raise
+
+    def test_an_empty_plugin_set_is_harmless(self):
+        ex = FakeExecutor()
+        _controller([], ex)._update_modules(deadline=time.time() + 5)
+        assert ex.updated == []
+
+
+class TestTooLittleBudgetDefersRatherThanClamps:
+    def test_a_plugin_starting_below_the_floor_is_deferred(self):
+        ex = FakeExecutor()
+        c = _controller(['a'], ex)
+        # Just under the floor: previously this was clamped up to the floor and
+        # run anyway, which pushed the pass past its deadline.
+        c._update_modules(
+            deadline=time.time() + _MIN_INITIAL_UPDATE_TIMEOUT_SECONDS - 0.05)
+        assert ex.updated == [], "started a plugin it could not give a slot to"
+
+    def test_a_plugin_starting_above_the_floor_still_runs(self):
+        ex = FakeExecutor()
+        c = _controller(['a'], ex)
+        c._update_modules(
+            deadline=time.time() + _MIN_INITIAL_UPDATE_TIMEOUT_SECONDS + 1)
+        assert ex.updated == ['a']
+
+    def test_the_timeout_is_the_remainder_not_the_floor(self):
+        seen = []
+
+        class Executor:
+            def execute_update(self, plugin, plugin_id, timeout=None):
+                seen.append(timeout)
+                return True
+
+        c = _controller(['a'], Executor())
+        c._update_modules(deadline=time.time() + 9)
+        assert seen and 8.5 <= seen[0] <= 9.01, seen
+
+    def test_the_pass_cannot_outlast_its_deadline(self, tiny_floor):
+        # Every plugin sleeps well past the budget; the deferral keeps the
+        # whole pass inside it rather than overrunning by a floor's worth.
+        ex = FakeExecutor(cost=0.4, slow={'a', 'b', 'c', 'd', 'e'})
+        c = _controller(['a', 'b', 'c', 'd', 'e'], ex)
+        started = time.time()
+        c._update_modules(deadline=started + 0.5)
+        assert time.time() - started < 1.2, "%.2fs" % (time.time() - started)

test/test_initializing_screen.py

@@ -0,0 +1,190 @@
+"""Tests the startup screen that shows while plugins fetch their first data.
+
+That screen is on the panel for the whole initial-update window, and on a
+headless Pi it is the only place the device's address appears without going
+looking for it -- so it now carries the address as well as "Initializing".
+
+Two things have to hold. It must fit every supported panel: the old fixed
+8px PressStart2P drew "Initializing" 96px wide at x=10, which ran off the
+side of a 64px panel before an address was ever added. And the lookup must be
+cheap, because this runs on the startup path that the rest of this change
+exists to shorten.
+"""
+
+import os
+import time
+
+from PIL import Image, ImageDraw, ImageFont
+import pytest
+
+os.environ.setdefault("EMULATOR", "true")
+
+from src.display_manager import DisplayManager  # noqa: E402
+
+SIZES = [(64, 32), (128, 32), (128, 64), (256, 32), (512, 64)]
+
+
+class FakeMatrix:
+    def __init__(self, width, height):
+        self.width, self.height = width, height
+
+
+def _manager(width, height):
+    dm = DisplayManager.__new__(DisplayManager)
+    dm.image = Image.new('RGB', (width, height))
+    dm.draw = ImageDraw.Draw(dm.image)
+    dm.matrix = FakeMatrix(width, height)
+    dm.font = ImageFont.truetype('assets/fonts/PressStart2P-Regular.ttf', 8)
+    return dm
+
+
+def _layout(dm, lines):
+    """The geometry _draw_startup_banner uses."""
+    font = dm._fitting_font(lines, dm.matrix.width - 2)
+    line_height = dm.draw.textbbox((0, 0), "Ag", font=font)[3] + 1
+    top = max(1, (dm.matrix.height - line_height * len(lines)) // 2)
+    widths = [dm.draw.textlength(t, font=font) for t in lines]
+    return font, widths, top, top + line_height * len(lines)
+
+
+def _render_over_pattern(width, height, lines):
+    """Draw the test pattern, then the banner over it, as startup does."""
+    dm = _manager(width, height)
+    dm.draw.rectangle([0, 0, width - 1, height - 1], outline=(255, 0, 0))
+    dm.draw.line([0, 0, width - 1, height - 1], fill=(0, 255, 0))
+    dm._draw_startup_banner(lines, width, height)
+    return dm
+
+
+class TestTheAddressLookup:
+    def test_it_never_reports_loopback(self):
+        # A loopback address on the panel would be actively misleading -- it is
+        # not something anyone can browse to.
+        ip = DisplayManager._local_ip()
+        assert ip is None or not ip.startswith("127."), ip
+
+    def test_it_looks_like_an_address_when_there_is_one(self):
+        ip = DisplayManager._local_ip()
+        if ip is None:
+            pytest.skip("host has no routable address")
+        parts = ip.split(".")
+        assert len(parts) == 4 and all(p.isdigit() for p in parts), ip
+
+    def test_it_is_cheap_enough_for_the_startup_path(self):
+        DisplayManager._local_ip()          # warm anything cacheable
+        started = time.perf_counter()
+        for _ in range(20):
+            DisplayManager._local_ip()
+        per_call = (time.perf_counter() - started) / 20
+        # `hostname -I` with its 2s timeout, which the web launcher uses, would
+        # be thousands of times this.
+        assert per_call < 0.05, "%.1f ms per call" % (per_call * 1000)
+
+    def test_it_returns_none_rather_than_raising(self, monkeypatch):
+        import src.display_manager as mod
+
+        def no_network(*a, **k):
+            raise OSError("network is unreachable")
+
+        monkeypatch.setattr(mod.socket, "socket", no_network)
+        assert DisplayManager._local_ip() is None
+
+
+class TestItFitsEveryPanel:
+    @pytest.mark.parametrize("width,height", SIZES)
+    def test_both_lines_fit_with_an_address(self, width, height):
+        dm = _manager(width, height)
+        _font, widths, top, bottom = _layout(dm, ["Initializing", "255.255.255.255"])
+        assert all(w <= width - 2 for w in widths), (width, widths)
+        assert bottom <= height and top >= 0, (top, bottom, height)
+
+    @pytest.mark.parametrize("width,height", SIZES)
+    def test_it_still_fits_with_no_address(self, width, height):
+        dm = _manager(width, height)
+        _font, widths, _top, bottom = _layout(dm, ["Initializing"])
+        assert all(w <= width - 2 for w in widths), (width, widths)
+        assert bottom <= height, (bottom, height)
+
+    def test_the_smallest_panel_drops_to_a_narrower_font(self):
+        # The regression this guards: PressStart2P at 8px is 96px wide for
+        # "Initializing", which does not fit 64px however it is positioned.
+        dm = _manager(64, 32)
+        font, widths, _t, _b = _layout(dm, ["Initializing", "10.0.20.104"])
+        assert font is not dm.font, "kept a font that cannot fit"
+        assert max(widths) <= 62, widths
+
+    def test_a_roomy_panel_keeps_the_larger_font(self):
+        dm = _manager(256, 32)
+        font, _w, _t, _b = _layout(dm, ["Initializing", "10.0.20.104"])
+        assert font is dm.font, "needlessly shrank on a panel with room"
+
+
+class TestPlacement:
+    @pytest.mark.parametrize("width,height", SIZES)
+    def test_the_lines_are_centred(self, width, height):
+        dm = _manager(width, height)
+        lines = ["Initializing", "10.0.20.104"]
+        _font, widths, _t, _b = _layout(dm, lines)
+        for w in widths:
+            left = max(0, (width - w) // 2)
+            assert abs((left + (left + w)) - width) <= 2, (left, w, width)
+
+    def test_the_address_sits_under_the_word(self):
+        dm = _manager(128, 64)
+        font, _w, top, bottom = _layout(dm, ["Initializing", "10.0.20.104"])
+        line_height = dm.draw.textbbox((0, 0), "Ag", font=font)[3] + 1
+        assert bottom - top == line_height * 2
+
+
+class TestItIsActuallyReadable:
+    """The point of the address is that someone can read it off the wall."""
+
+    @pytest.mark.parametrize("width,height", SIZES)
+    def test_the_diagonal_does_not_cross_the_text(self, width, height):
+        lines = ["Initializing", "10.0.20.104"]
+        dm = _render_over_pattern(width, height, lines)
+        _font, widths, top, bottom = _layout(dm, lines)
+        # textlength returns a float, so these must be floored before they
+        # can index pixels.
+        block_width = int(max(widths))
+        left = int(max(0, (width - block_width) // 2))
+
+        px = dm.image.load()
+        green = 0
+        for y in range(int(top), min(int(bottom), height)):
+            for x in range(left, min(left + block_width, width)):
+                r, g, b = px[x, y]
+                if g > 128 and r < 128 and b < 128:
+                    green += 1
+        assert green == 0, "%d green pixels behind the text at %dx%d" % (
+            green, width, height)
+
+    @pytest.mark.parametrize("width,height", SIZES)
+    def test_the_text_stays_pure_blue(self, width, height):
+        # Not a style choice. The pattern lights one channel per element --
+        # red border, green diagonal, blue text -- so a glance says whether
+        # led_rgb_sequence is right: wire it BGR and the border comes up blue
+        # and this text red. White text would light all three and destroy the
+        # only blue reference on the screen.
+        dm = _render_over_pattern(width, height, ["Initializing", "10.0.20.104"])
+        px = dm.image.load()
+        blue = sum(1 for y in range(height) for x in range(width)
+                   if px[x, y] == (0, 0, 255))
+        assert blue > 20, "only %d blue pixels at %dx%d" % (blue, width, height)
+        white = sum(1 for y in range(height) for x in range(width)
+                    if px[x, y] == (255, 255, 255))
+        assert white == 0, "%d white pixels would muddy the channel check" % white
+
+    def test_each_element_lights_one_channel(self):
+        # The whole point of the pattern: three pure primaries on screen.
+        dm = _render_over_pattern(128, 64, ["Initializing", "10.0.20.104"])
+        seen = set(dm.image.getdata())
+        assert (255, 0, 0) in seen, "no pure red border"
+        assert (0, 255, 0) in seen, "no pure green diagonal"
+        assert (0, 0, 255) in seen, "no pure blue text"
+
+    def test_nothing_is_drawn_for_no_lines(self):
+        dm = _manager(128, 64)
+        before = dm.image.tobytes()
+        dm._draw_startup_banner([], 128, 64)
+        assert dm.image.tobytes() == before