Pre-scan angle inspection: park the rig per angle, read the response on the scope

A 9-angle scan takes hours, and an angle that responds poorly still produces
rows that look structurally fine in the file — the SAW packet is just not
there. This lets the operator walk the angles first, parking the rig at a
random point in each, and judge the response before committing to the run.

Nothing reads the scope. The operator inspects the instrument directly, so
there is no transfer path, no plotting, and no waveform crossing the module
boundary — test_inspection_never_reads_a_waveform_back pins that, since it is
the kind of premise a later change erodes without noticing.

core/scope_inspect.py — the scope state worth looking at, which is not the
scan's state:
- plain rising-edge trigger on CH2 at 2.0 V, not the scan's logic AND of the
  laser pulse and the stage gate, so a stationary stage still triggers
- FastFrame off, SAMPLE (no averaging) — a weak or intermittent response is
  exactly what is being looked for, and averaging would hide it
- free-running (STOPAfter RUNSTop + STATE RUN) so the trace keeps updating
  while the operator looks at it
- CH1 keeps the acquisition front-end verbatim, so what is on screen is what
  a scan would record
- CH3/CH4 become bias monitors sharing one scale and position, since the
  comparison is by eye and only works if a division means the same on each.
  100 mV/div with ground 3.5 divisions below centre puts 0–700 mV on screen
  with headroom on an 8- or 10-division graticule (the signal never goes
  negative, hence moving the trace down).

core/angle_inspect.py — AngleInspector, headless and Qt-free like ScanEngine.
Points are drawn from the angle's own bounding box: Y from its actual row
positions and X uniformly across its data window, so the point is somewhere
the scan would really sample rather than merely inside the box. New Point
re-rolls without rotating, which is what separates a bad spot on the sample
from a bad angle. The stage gate is held off throughout, and the rotator goes
home on stop.

gui/inspect_bridge.py — QtAngleInspector on the existing QueueWorker base.
Inspection is click-driven rather than one long run, so the worker blocks on
its queue between commands and an open window costs nothing. BBD position
polling is suppressed while inspecting, for the same reason the scan does it:
the shared TX queue.

sc3_aui_app.py — AngleInspectWindow (angle list, prev/next, New Point) driven
off the plan currently entered in the scan panel, so it inspects exactly the
scan about to be run. Navigation locks while the stage moves. The list syncs
via itemClicked rather than currentRowChanged, so echoing the worker's
position back does not re-trigger the move it is reporting.

README picks up the new modules, and scope_burst.py which the previous merge
left out of the structure listing. 114 tests passing, ruff clean.

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

README.md

@@ -12,6 +12,8 @@ scanengine-3 is a unified platform for scanning acoustic microscopy and precisio
 - **Laser Systems**: Helios pulsed laser and Genesis CW laser control
 - **Data Acquisition**: Tektronix oscilloscope integration with fast-frame support
 - **Scan Planning**: Automated raster scan generation and execution
+- **Angle Inspection**: Park the rig at random points across a plan's angles
+  to check the SAW response on the scope before committing to a long scan
 - **Real-time Monitoring**: Live status updates and progress tracking
 
 ## Hardware Components
@@ -55,6 +57,9 @@ scanengine-3/
 │   ├── scan_geometry.py           # ScanPlan, rotated-bbox planning, limits
 │   ├── scan_resume.py             # Resume planning (frontier rule)
 │   ├── scope_sras.py              # Oscilloscope SCPI policy for SRAS
+│   ├── scope_burst.py             # Burst-mode FastFrame sizing + row splitting
+│   ├── scope_inspect.py           # Scope setup for pre-scan angle inspection
+│   ├── angle_inspect.py           # AngleInspector — park on a point per angle
 │   ├── rotation.py                # GR rotation axis settings + moves
 │   ├── sras_format.py             # v6 .sras writer/reader (memory-mapped)
 │   ├── sras_analysis.py           # Image reducers + SAW matched filter
@@ -72,6 +77,7 @@ scanengine-3/
 │
 ├── gui/                           # Shared PyQt6 layer
 │   ├── scan_bridge.py             # QtScanController over core.scan_engine
+│   ├── inspect_bridge.py          # QtAngleInspector over core.angle_inspect
 │   ├── qt_t3r.py                  # Qt adapter over the T3R driver
 │   ├── qt_workers.py              # QueueWorker / PollingQueueWorker bases
 │   └── widgets.py                 # ConnectionBar, LogConsole, PortSelector…

core/angle_inspect.py

@@ -0,0 +1,251 @@
+"""Pre-scan angle inspection: park the rig on a point and let the operator look.
+
+A multi-angle scan can take hours, and an angle that responds poorly produces
+rows that look fine in the file but carry no usable SAW packet.  This drives
+the rig through the same angles the scan will use, parking at a random point
+inside each angle's own bounding box so the response can be judged on the
+oscilloscope before committing to the run.
+
+Headless and Qt-free, like ScanEngine: gui/inspect_bridge.py wraps it.
+
+No waveform ever crosses this boundary.  The operator reads the scope screen
+directly; this module's job is only to put the hardware in the right place and
+the scope in a state worth looking at (see core.scope_inspect).
+"""
+from __future__ import annotations
+
+import logging
+import random
+from dataclasses import dataclass
+from typing import Callable
+
+from core import scope_inspect
+from core.rotation import RotationAxis
+from core.scan_engine import (
+    AXIS_X, AXIS_Y, SCAN_ACCEL_MM_S2, SCAN_VELOCITY_MM_S,
+)
+from core.scan_geometry import DEFAULT_STAGE_LIMITS, ScanPlan, StageLimits
+
+logger = logging.getLogger(__name__)
+
+# Positioning moves only — no data is taken while moving, so there is no
+# reason to cross the tray at full scan velocity.
+INSPECT_VELOCITY_MM_S = SCAN_VELOCITY_MM_S / 2.0
+
+
+@dataclass(frozen=True)
+class InspectionPoint:
+    """Where the rig is parked, and which angle it is parked for."""
+    angle_idx: int
+    angle_deg: float
+    x_mm: float
+    y_mm: float
+
+    def describe(self) -> str:
+        return (f"Angle {self.angle_idx + 1} ({self.angle_deg:.1f}°)  "
+                f"X={self.x_mm:.3f} mm  Y={self.y_mm:.3f} mm")
+
+
+@dataclass
+class InspectCallbacks:
+    """Progress reporting.  Defaults are no-ops so the core needs no front end."""
+    on_status: Callable[[str], None] = lambda msg: None
+    on_point: Callable[[InspectionPoint], None] = lambda pt: None
+    on_busy: Callable[[bool], None] = lambda busy: None
+
+
+@dataclass
+class _State:
+    angle_idx: int = 0
+    point: InspectionPoint | None = None
+    started: bool = False
+    rotator_ready: bool = False
+
+
+class AngleInspector:
+    """Drives stage + rotator to inspection points across a plan's angles."""
+
+    def __init__(self, stage, scope, rotator: RotationAxis | None,
+                 plan: ScanPlan,
+                 callbacks: InspectCallbacks | None = None,
+                 limits: StageLimits = DEFAULT_STAGE_LIMITS,
+                 rng: random.Random | None = None):
+        self._stage = stage
+        self._scope = scope
+        self._rotator = rotator
+        self._plan = plan
+        self._cb = callbacks if callbacks is not None else InspectCallbacks()
+        self._limits = limits
+        # Injectable so tests can pin the point selection.
+        self._rng = rng if rng is not None else random.Random()
+        self._st = _State()
+
+    # ── Introspection ─────────────────────────────────────────────────────────
+
+    @property
+    def n_angles(self) -> int:
+        return self._plan.n_angles
+
+    @property
+    def angle_idx(self) -> int:
+        return self._st.angle_idx
+
+    @property
+    def current_point(self) -> InspectionPoint | None:
+        return self._st.point
+
+    def angle_labels(self) -> list[str]:
+        return [f"Angle {i + 1}/{self.n_angles} — {pa.angle_deg:.2f}°"
+                for i, pa in enumerate(self._plan.per_angle)]
+
+    # ── Lifecycle ─────────────────────────────────────────────────────────────
+
+    def start(self) -> InspectionPoint:
+        """Configure the hardware and park on the first angle."""
+        if self._stage is None:
+            raise RuntimeError("BBD202 not connected")
+        if self._scope is None:
+            raise RuntimeError("Oscilloscope not connected")
+
+        self._st.rotator_ready = (self._rotator is not None
+                                  and self._rotator.is_available)
+        if self.n_angles > 1 and not self._st.rotator_ready:
+            raise RuntimeError(
+                f"Inspecting {self.n_angles} angles requires the T3R rotation "
+                "stage (GR-axis), but it is not connected. Connect T3R from "
+                "the T3R panel, or inspect a single-angle plan."
+            )
+
+        self._cb.on_busy(True)
+        try:
+            self._cb.on_status("Configuring stage for inspection …")
+            ctrl = self._stage
+            for axis in (AXIS_X, AXIS_Y):
+                ctrl.set_velocity_params(axis,
+                                         max_velocity=INSPECT_VELOCITY_MM_S,
+                                         acceleration=SCAN_ACCEL_MM_S2)
+            # Nothing here is gated, and an armed trigger output would keep
+            # driving the gate line on every positioning move.
+            ctrl.set_trigger_gate_off(AXIS_X)
+
+            if self._st.rotator_ready:
+                self._cb.on_status("Configuring GR axis …")
+                self._rotator.configure()
+
+            self._cb.on_status("Configuring oscilloscope for inspection …")
+            scope_inspect.configure_inspection(self._scope)
+
+            self._st.started = True
+            return self._goto(0, new_point=True)
+        finally:
+            self._cb.on_busy(False)
+
+    def stop(self) -> None:
+        """Stop the sweep and send the rotator home.  Safe to call twice."""
+        if not self._st.started:
+            return
+        self._st.started = False
+        self._cb.on_busy(True)
+        try:
+            try:
+                scope_inspect.stop_inspection(self._scope)
+            except Exception:
+                logger.exception("Could not stop the inspection acquisition")
+            if self._st.rotator_ready and abs(self._rotator.current_deg) > 0.001:
+                self._cb.on_status("Returning GR to home …")
+                try:
+                    self._rotator.return_to_zero()
+                except Exception:
+                    logger.exception("GR return-to-home failed")
+            self._cb.on_status("Inspection finished.")
+        finally:
+            self._cb.on_busy(False)
+
+    # ── Navigation ────────────────────────────────────────────────────────────
+
+    def goto_angle(self, angle_idx: int) -> InspectionPoint:
+        """Rotate to `angle_idx` and park on a fresh random point there."""
+        self._require_started()
+        self._cb.on_busy(True)
+        try:
+            return self._goto(angle_idx, new_point=True)
+        finally:
+            self._cb.on_busy(False)
+
+    def next_angle(self) -> InspectionPoint:
+        """Advance one angle, wrapping at the end."""
+        return self.goto_angle((self._st.angle_idx + 1) % self.n_angles)
+
+    def prev_angle(self) -> InspectionPoint:
+        return self.goto_angle((self._st.angle_idx - 1) % self.n_angles)
+
+    def new_point(self) -> InspectionPoint:
+        """Re-roll the point within the current angle, without rotating.
+
+        One point can be unrepresentative — a bad spot on the sample looks the
+        same as a bad angle.  Re-rolling a few times is how you tell them
+        apart, so this deliberately skips the rotation.
+        """
+        self._require_started()
+        self._cb.on_busy(True)
+        try:
+            return self._goto(self._st.angle_idx, new_point=True, rotate=False)
+        finally:
+            self._cb.on_busy(False)
+
+    # ── Internals ─────────────────────────────────────────────────────────────
+
+    def _require_started(self):
+        if not self._st.started:
+            raise RuntimeError("Inspection has not been started")
+
+    def _goto(self, angle_idx: int, new_point: bool,
+              rotate: bool = True) -> InspectionPoint:
+        if not 0 <= angle_idx < self.n_angles:
+            raise IndexError(
+                f"Angle {angle_idx} out of range (plan has {self.n_angles})")
+
+        pa = self._plan.per_angle[angle_idx]
+        self._st.angle_idx = angle_idx
+
+        if rotate and self._st.rotator_ready:
+            delta = pa.angle_deg - self._rotator.current_deg
+            if abs(delta) > 0.001:
+                self._cb.on_status(
+                    f"Rotating GR to {pa.angle_deg:.1f}° (Δ{delta:+.1f}°) …")
+                self._rotator.rotate_to(pa.angle_deg)
+
+        point = self._pick_point(angle_idx) if new_point else self._st.point
+
+        self._cb.on_status(f"Moving to {point.describe()} …")
+        # Y first, then X — the same order the scan uses to reach a row.
+        self._stage.move_axis_absolute(AXIS_Y, point.y_mm, timeout=60.0)
+        self._stage.move_axis_absolute(AXIS_X, point.x_mm, timeout=60.0)
+
+        self._st.point = point
+        self._cb.on_point(point)
+        self._cb.on_status(f"Parked at {point.describe()}")
+        return point
+
+    def _pick_point(self, angle_idx: int) -> InspectionPoint:
+        """A random point on this angle's scan grid.
+
+        Y is drawn from the angle's actual row positions and X uniformly from
+        its data window, so the point is somewhere the scan would really
+        sample — not merely inside the bounding box.
+        """
+        pa = self._plan.per_angle[angle_idx]
+        if not pa.y_positions:
+            raise ValueError(f"Angle {angle_idx + 1} has no rows to inspect")
+
+        y = self._rng.choice(pa.y_positions)
+        x = self._rng.uniform(pa.x_start, pa.x_start + pa.x_delta)
+
+        lim = self._limits
+        if not (lim.x_min <= x <= lim.x_max and lim.y_min <= y <= lim.y_max):
+            raise ValueError(
+                f"Inspection point X={x:.3f} Y={y:.3f} is outside the stage "
+                f"travel ({lim.x_min}–{lim.x_max} × {lim.y_min}–{lim.y_max} mm)"
+            )
+        return InspectionPoint(angle_idx=angle_idx, angle_deg=pa.angle_deg,
+                               x_mm=x, y_mm=y)

core/config.py

@@ -24,6 +24,7 @@ class ScanDefaults:
     save_dir: str = str(DEFAULTS_PATH.parent / "scans")
     helios_port: str = "/dev/ttyUSB2"
     burst_mode: bool = False
+    strict_rows: bool = False
 
     @classmethod
     def load(cls, path: Path = DEFAULTS_PATH) -> "ScanDefaults":

core/scan_engine.py

@@ -96,7 +96,7 @@ class ScanEngine:
                  plan: ScanPlan, out_path: Path,
                  resume: ResumeState | None = None,
                  callbacks: ScanCallbacks | None = None,
-                 burst_mode: bool = False):
+                 burst_mode: bool = False, strict_rows: bool = False):
         self._stage = stage
         self._scope = scope
         self._rotator = rotator
@@ -107,6 +107,9 @@ class ScanEngine:
         # Burst mode acquires as many whole rows per FastFrame acquisition as
         # the scope's frame memory holds, instead of one row per acquisition.
         self._burst_mode = burst_mode
+        # Strict row packing stops the scan on a frame-count mismatch
+        # instead of squaring the row up (see _check_frame_delta).
+        self._strict_rows = strict_rows
         self._max_frames = 0
         self._preflight_done = False
 
@@ -434,7 +437,7 @@ class ScanEngine:
             self._cb.on_status(f"Fetching CH{ch} data …")
             waveforms = scope_sras.transfer_channel(scope, ch)
             if ch == SCAN_CHANNELS[0]:
-                self._warn_frame_delta(row_idx, len(waveforms), n_frames)
+                self._check_frame_delta(row_idx, len(waveforms), n_frames)
             row = scope_burst.normalize_row(
                 b"".join(waveforms), 0, len(waveforms), n_frames, samples_per_frame)
             if ch == 4:
@@ -577,7 +580,7 @@ class ScanEngine:
         total_frames = sum(counts)
 
         for r, count in enumerate(counts):
-            self._warn_frame_delta(first_row + r, count, n_frames)
+            self._check_frame_delta(first_row + r, count, n_frames)
 
         for ch_idx, ch in enumerate(SCAN_CHANNELS):
             if ch == 3:
@@ -608,10 +611,26 @@ class ScanEngine:
 
         scan_file.seek(burst_start + len(counts) * row_bytes)
 
-    def _warn_frame_delta(self, row_idx: int, count: int, n_frames: int):
+    def _check_frame_delta(self, row_idx: int, count: int, n_frames: int):
+        """Decide what to do with a row that did not acquire n_frames frames.
+
+        v6 declares n_frames per row in the header and has no per-row length
+        field, so a mismatched row cannot just be written as-is — that would
+        shift every later row in the file.  The only two safe options are to
+        square it up or to stop, which is what strict_rows selects between.
+
+        Called before anything for the row is written (CH1 leads
+        SCAN_CHANNELS), so raising here leaves no partial row behind.
+        """
         if count == n_frames:
             return
         verb = "zero-padded" if count < n_frames else "truncated"
+        if self._strict_rows:
+            raise RuntimeError(
+                f"Row {row_idx + 1}: {count} frames acquired, {n_frames} "
+                f"expected. Strict row packing is on, so the scan stops here "
+                f"rather than writing a row that would be {verb}."
+            )
         msg = (f"Row {row_idx + 1}: {count} frames acquired, {n_frames} "
                f"expected — {verb} to keep the file layout intact.")
         logger.warning(msg)

core/scope_inspect.py

@@ -0,0 +1,104 @@
+"""Oscilloscope configuration for pre-scan angle inspection.
+
+Inspection is read-on-the-instrument: nothing in this module transfers or
+plots waveform data.  The app puts the scope into a free-running, edge-
+triggered state and drives the stage to the point being inspected; the
+operator judges the SAW response and the bias levels on the scope screen.
+
+That split is deliberate.  A scan's acquisition trigger is the logic AND of
+the laser pulse and the stage's max-velocity gate, and its transfers are
+FastFrame blocks — neither is useful for looking at one point by eye.  Here
+the trigger is a plain edge on the laser pulse, FastFrame is off, and the
+acquisition free-runs, so the display updates continuously while the stage
+sits still.
+
+CH1 keeps the acquisition front-end so what is on screen is what a scan would
+record.  CH3 and CH4 are rescaled as DC bias monitors (see BIAS_* below).
+"""
+from __future__ import annotations
+
+import logging
+from dataclasses import replace
+
+from core.scope_sras import SAMPLE_RATE_HZ, SRAS_CHANNELS, configure_channels
+
+logger = logging.getLogger(__name__)
+
+# CH2 carries the laser pulse.  The scan triggers it at 0.5 V as one term of a
+# logic AND; inspection triggers well above that so a slow edge or a noisy
+# baseline cannot free-run the display.
+INSPECT_TRIG_LEVEL_V = 2.0
+
+# CH3/CH4 are the DC bias monitors during inspection.  The signal never goes
+# negative and spans roughly 0–700 mV, so both channels get the *same* scale
+# and position — the point of inspecting them is comparing the two by eye, and
+# that only works if a division means the same thing on each.
+#
+# Ground sits BIAS_POSITION_DIV divisions below centre, which puts the whole
+# 0–700 mV range above the centre line with a little room underneath for
+# undershoot.  With 100 mV/div and ground 3.5 divisions low, the visible window
+# runs from about -50 mV to +750 mV on an 8-division display and wider on a
+# 10-division one, so 0–700 mV sits comfortably inside either.
+BIAS_CHANNELS = (3, 4)
+BIAS_WINDOW_V = 0.700
+BIAS_SCALE_V_DIV = 0.100
+BIAS_POSITION_DIV = -3.5
+
+BIAS_LABELS = {3: "Bias - A", 4: "Bias - B"}
+
+
+def inspect_channel_profiles() -> dict:
+    """Channel front-end config for inspection.
+
+    CH1 and CH2 are the acquisition profiles verbatim.  CH3 and CH4 differ
+    only in label, scale and position — termination, coupling and bandwidth
+    stay as the scan sets them, so the bias reading is the same measurement
+    the scan records, just displayed usefully.
+    """
+    profiles = dict(SRAS_CHANNELS)
+    for ch in BIAS_CHANNELS:
+        profiles[ch] = replace(
+            SRAS_CHANNELS[ch],
+            label=BIAS_LABELS[ch],
+            scale_v_div=BIAS_SCALE_V_DIV,
+            position_div=BIAS_POSITION_DIV,
+        )
+    return profiles
+
+
+def configure_inspection(scope) -> None:
+    """Put the scope into free-running inspection mode.
+
+    Leaves the acquisition running, so the display stays live while the
+    operator moves between angles and points.
+    """
+    configure_channels(scope, inspect_channel_profiles())
+
+    # Plain edge trigger on the laser pulse — no logic pattern, so the stage
+    # gate plays no part and a stationary stage still triggers.
+    scope.write("TRIGger:A:TYPe EDGE")
+    scope.set_trigger_source(2)
+    scope.set_trigger_slope("RISE")
+    scope.set_trigger_level(2, INSPECT_TRIG_LEVEL_V)
+    scope.set_trigger_mode("NORMAL")
+
+    # No averaging: a weak or intermittent SAW response is exactly what the
+    # operator is looking for, and averaging would hide it.
+    scope.set_acquire_mode("SAMPLE")
+    scope.set_fastframe_state(False)
+
+    scope.set_sample_rate(SAMPLE_RATE_HZ)
+    scope.write("HORizontal:POSition 30")
+
+    # Free-run rather than single-sequence, so the trace keeps updating.
+    scope.write("ACQuire:STOPAfter RUNSTop")
+    scope.write("ACQuire:STATE RUN")
+
+
+def stop_inspection(scope) -> None:
+    """Halt the free-running acquisition.
+
+    The next scan reconfigures the scope from scratch, so this only needs to
+    stop the sweep — it does not try to restore the acquisition profile.
+    """
+    scope.write("ACQuire:STATE STOP")

gui/inspect_bridge.py

@@ -0,0 +1,113 @@
+"""Qt bridge over the headless AngleInspector.
+
+Inspection is command-driven rather than one long run: the operator clicks an
+angle, waits for the stage to park, looks at the scope, clicks again.  That is
+exactly the shape QueueWorker exists for — it blocks on the queue between
+commands instead of polling, so an inspection window left open costs nothing.
+
+Every stage move and rotation blocks for seconds, so all of it runs on this
+worker's thread; the window only ever enqueues and reacts to signals.
+"""
+from __future__ import annotations
+
+import traceback
+
+from PyQt6.QtCore import pyqtSignal
+
+from core.angle_inspect import AngleInspector, InspectCallbacks
+from gui.qt_workers import QueueWorker
+
+
+class QtAngleInspector(QueueWorker):
+    """Runs an AngleInspector on its own QThread and republishes its events."""
+
+    ready = pyqtSignal(object)            # InspectionPoint — start() succeeded
+    start_failed = pyqtSignal(str)
+    point_changed = pyqtSignal(object)    # InspectionPoint
+    status_msg = pyqtSignal(str)
+    busy_changed = pyqtSignal(bool)       # True while a move is in flight
+    stopped = pyqtSignal()
+
+    def __init__(self, stage, scope, rotator, plan, on_inspect_active=None):
+        super().__init__()
+        self._on_inspect_active = on_inspect_active
+
+        callbacks = InspectCallbacks(
+            on_status=self.status_msg.emit,
+            on_point=self.point_changed.emit,
+            on_busy=self.busy_changed.emit,
+        )
+        self._inspector = AngleInspector(stage, scope, rotator, plan,
+                                         callbacks=callbacks)
+        self._handlers = {
+            "start": self._do_start,
+            "goto": self._do_goto,
+            "new_point": self._do_new_point,
+            "stop": self._do_stop,
+        }
+
+    # ── Introspection (safe from the GUI thread: reads the plan, not the rig) ──
+
+    def angle_labels(self) -> list[str]:
+        return self._inspector.angle_labels()
+
+    @property
+    def n_angles(self) -> int:
+        return self._inspector.n_angles
+
+    # ── Command submission (GUI thread) ───────────────────────────────────────
+
+    def request_start(self):
+        self._enqueue("start")
+
+    def request_goto(self, angle_idx: int):
+        self._enqueue("goto", angle_idx=angle_idx)
+
+    def request_new_point(self):
+        self._enqueue("new_point")
+
+    def request_stop(self):
+        self._enqueue("stop")
+
+    # ── Handlers (worker thread) ──────────────────────────────────────────────
+
+    def _do_start(self):
+        if self._on_inspect_active is not None:
+            self._on_inspect_active(True)
+        try:
+            point = self._inspector.start()
+        except Exception as exc:
+            traceback.print_exc()
+            if self._on_inspect_active is not None:
+                self._on_inspect_active(False)
+            self.start_failed.emit(str(exc))
+            return
+        self.ready.emit(point)
+
+    def _do_goto(self, angle_idx: int):
+        self._inspector.goto_angle(angle_idx)
+
+    def _do_new_point(self):
+        self._inspector.new_point()
+
+    def _do_stop(self):
+        try:
+            self._inspector.stop()
+        finally:
+            if self._on_inspect_active is not None:
+                self._on_inspect_active(False)
+            self.stopped.emit()
+
+    def _on_stop(self):
+        """Worker loop exiting — make sure the rig is left in a safe state.
+
+        Covers the case where the window is closed without a clean stop
+        command reaching the queue.
+        """
+        try:
+            self._inspector.stop()
+        except Exception:
+            traceback.print_exc()
+        finally:
+            if self._on_inspect_active is not None:
+                self._on_inspect_active(False)

gui/scan_bridge.py

@@ -32,7 +32,8 @@ class QtScanController(QObject):
     paused_changed = pyqtSignal(bool)                 # True while paused at a row boundary
 
     def __init__(self, stage, scope, rotator, plan, out_path,
-                 resume=None, on_scan_active=None, burst_mode=False):
+                 resume=None, on_scan_active=None, burst_mode=False,
+                 strict_rows=False):
         super().__init__()
         self._prompt_event = threading.Event()
         self._on_scan_active = on_scan_active
@@ -48,7 +49,8 @@ class QtScanController(QObject):
         )
         self._engine = ScanEngine(stage, scope, rotator, plan, out_path,
                                   resume=resume, callbacks=callbacks,
-                                  burst_mode=burst_mode)
+                                  burst_mode=burst_mode,
+                                  strict_rows=strict_rows)
 
     # ── Engine control (called from the GUI thread) ───────────────────────────
 

sc3-aui-main.ui

@@ -1007,6 +1007,26 @@
         </property>
        </widget>
       </item>
+      <item>
+       <widget class="QCheckBox" name="strict_rows_check">
+        <property name="toolTip">
+         <string>Stop the scan if a row does not acquire the expected number of frames, instead of zero-padding a short row or truncating a long one. Use for data runs where a silently squared-up row would be worse than a failed scan.</string>
+        </property>
+        <property name="text">
+         <string>Strict row packing (abort on frame-count mismatch)</string>
+        </property>
+       </widget>
+      </item>
+      <item>
+       <widget class="QPushButton" name="inspect_angles_btn">
+        <property name="toolTip">
+         <string>Rotate through the angles of the scan currently entered, parking at a random point in each so the SAW response can be checked on the oscilloscope before committing to the run.</string>
+        </property>
+        <property name="text">
+         <string>Inspect Angles…</string>
+        </property>
+       </widget>
+      </item>
       <item>
        <widget class="QPushButton" name="start_scan_btn">
         <property name="text">

sc3_aui_app.py

@@ -38,6 +38,7 @@ from core.scope_sras import SAMPLE_RATE_HZ, configure_channels
 from core.sras_format import SCAN_CHANNELS, SrasFile, plan_from_header
 from gui.qt_t3r import QtT3RAdapter
 from gui.qt_workers import PollingQueueWorker, QueueWorker
+from gui.inspect_bridge import QtAngleInspector
 from gui.scan_bridge import QtScanController
 from hardware.helios_laser import HeliosLaser
 from hardware.pybbd202 import AXIS_X, AXIS_Y, ThorlabsServoDriver
@@ -844,6 +845,104 @@ class ScanProgressWindow(QWidget):
 
 # ── Main window ───────────────────────────────────────────────────────────────
 
+class AngleInspectWindow(QWidget):
+    """Click through a plan's angles, parking the rig at a point in each.
+
+    Deliberately shows no waveform.  The operator reads the SAW response and
+    the bias levels off the oscilloscope itself — this window only says where
+    the rig is and lets them move it somewhere else.
+    """
+
+    goto_requested = pyqtSignal(int)
+    new_point_requested = pyqtSignal()
+    stop_requested = pyqtSignal()
+
+    def __init__(self, angle_labels: list[str], parent: QWidget | None = None):
+        super().__init__(parent, Qt.WindowType.Window)
+        self.setWindowTitle("Inspect Angles")
+        self.resize(420, 460)
+
+        layout = QVBoxLayout(self)
+        layout.addWidget(QLabel(
+            "Select an angle to rotate to it and park at a random point in "
+            "its scan area.\nRead the SAW response on the oscilloscope."
+        ))
+
+        self.angle_list = QListWidget(self)
+        for i, label in enumerate(angle_labels):
+            item = QListWidgetItem(label)
+            item.setData(Qt.ItemDataRole.UserRole, i)
+            self.angle_list.addItem(item)
+        self.angle_list.setCurrentRow(0)
+        # currentRowChanged would also fire when the code syncs the highlight
+        # back after a move, re-triggering the move it is reporting.
+        self.angle_list.itemClicked.connect(self._on_item_clicked)
+        layout.addWidget(self.angle_list)
+
+        nav_row = QHBoxLayout()
+        self.prev_btn = QPushButton("◀ Previous")
+        self.next_btn = QPushButton("Next ▶")
+        self.new_point_btn = QPushButton("New Point")
+        self.new_point_btn.setToolTip(
+            "Pick another random point at this angle without rotating — a bad "
+            "spot on the sample looks the same as a bad angle until you move."
+        )
+        self.prev_btn.clicked.connect(self._on_prev)
+        self.next_btn.clicked.connect(self._on_next)
+        self.new_point_btn.clicked.connect(self.new_point_requested.emit)
+        nav_row.addWidget(self.prev_btn)
+        nav_row.addWidget(self.next_btn)
+        nav_row.addWidget(self.new_point_btn)
+        layout.addLayout(nav_row)
+
+        self.point_label = QLabel("—")
+        self.point_label.setStyleSheet("font-weight: bold;")
+        layout.addWidget(self.point_label)
+
+        self.status_label = QLabel("Starting …")
+        self.status_label.setWordWrap(True)
+        layout.addWidget(self.status_label)
+
+        self.close_btn = QPushButton("Close")
+        self.close_btn.clicked.connect(self.close)
+        layout.addWidget(self.close_btn)
+
+        self._n_angles = len(angle_labels)
+        self._angle_idx = 0
+        self.set_busy(True)
+
+    # ── Worker → window ───────────────────────────────────────────────────────
+
+    def set_busy(self, busy: bool):
+        """Lock navigation while the stage is moving; the rig is not re-entrant."""
+        for w in (self.angle_list, self.prev_btn, self.next_btn,
+                  self.new_point_btn):
+            w.setEnabled(not busy)
+
+    def on_status(self, msg: str):
+        self.status_label.setText(msg)
+
+    def on_point(self, point):
+        self._angle_idx = point.angle_idx
+        self.angle_list.setCurrentRow(point.angle_idx)
+        self.point_label.setText(point.describe())
+
+    # ── Window → worker ───────────────────────────────────────────────────────
+
+    def _on_item_clicked(self, item):
+        self.goto_requested.emit(item.data(Qt.ItemDataRole.UserRole))
+
+    def _on_prev(self):
+        self.goto_requested.emit((self._angle_idx - 1) % self._n_angles)
+
+    def _on_next(self):
+        self.goto_requested.emit((self._angle_idx + 1) % self._n_angles)
+
+    def closeEvent(self, event):
+        self.stop_requested.emit()
+        super().closeEvent(event)
+
+
 class MainWindow(QMainWindow):
     def __init__(self):
         super().__init__()
@@ -878,6 +977,9 @@ class MainWindow(QMainWindow):
         self._scan_progress = ScanProgressWindow()
 
         self._scan_worker: QtScanController | None = None
+        self._inspect_thread: QThread | None = None
+        self._inspect_worker: QtAngleInspector | None = None
+        self._inspect_window: AngleInspectWindow | None = None
         self._scan_thread:  QThread | None = None
 
         self._bbd_active_jog: tuple[str, int] | None = None   # (axis, direction)
@@ -918,6 +1020,7 @@ class MainWindow(QMainWindow):
         self.scan_prefix_edit.setText("scan")
         self.scan_save_dir_edit.setText(DEFAULTS.save_dir)
         self.burst_mode_check.setChecked(DEFAULTS.burst_mode)
+        self.strict_rows_check.setChecked(DEFAULTS.strict_rows)
 
         # Hide the old T3R manual controls; the connect toggle becomes the panel button.
         for w in (
@@ -979,6 +1082,7 @@ class MainWindow(QMainWindow):
         self.bbd202_comport_edit.editingFinished.connect(self._persist_defaults)
         self.oscope_ip_edit.editingFinished.connect(self._persist_defaults)
         self.burst_mode_check.toggled.connect(self._persist_defaults)
+        self.strict_rows_check.toggled.connect(self._persist_defaults)
 
         # Camera
         self.show_camera_toggle.toggled.connect(self._on_camera_toggle)
@@ -997,6 +1101,7 @@ class MainWindow(QMainWindow):
 
         # Scan
         self.start_scan_btn.clicked.connect(self._on_start_scan)
+        self.inspect_angles_btn.clicked.connect(self._on_inspect_angles)
         self.save_dir_browse_btn.clicked.connect(self._on_browse_save_dir)
         self._scan_progress.abort_requested.connect(self._on_abort_scan)
         self._scan_progress.pause_toggled.connect(self._on_pause_scan)
@@ -1137,6 +1242,7 @@ class MainWindow(QMainWindow):
         DEFAULTS.oscope_ip = self.oscope_ip_edit.text().strip()
         DEFAULTS.save_dir = self.scan_save_dir_edit.text().strip()
         DEFAULTS.burst_mode = self.burst_mode_check.isChecked()
+        DEFAULTS.strict_rows = self.strict_rows_check.isChecked()
         DEFAULTS.save()
 
     # ── Camera toggle ─────────────────────────────────────────────────────────
@@ -1169,6 +1275,79 @@ class MainWindow(QMainWindow):
             self.scan_save_dir_edit.setText(d)
             self._persist_defaults()
 
+    def _on_inspect_angles(self):
+        """Open the pre-scan angle inspector for the plan currently entered."""
+        if self._scan_thread is not None and self._scan_thread.isRunning():
+            QMessageBox.warning(
+                self, "Scan In Progress",
+                "A scan is running — abort it before inspecting angles."
+            )
+            return
+        if self._inspect_thread is not None and self._inspect_thread.isRunning():
+            self._inspect_window.raise_()
+            self._inspect_window.activateWindow()
+            return
+
+        try:
+            plan, _, _ = self._build_scan_plan()
+        except ValueError as e:
+            QMessageBox.warning(self, "Invalid Scan Parameters", str(e))
+            return
+
+        rotator = RotationAxis(self._t3r_driver.driver, DEFAULT_ROTATION)
+        self._inspect_thread = QThread(self)
+        self._inspect_worker = QtAngleInspector(
+            stage=self._bbd_worker.controller,
+            scope=self._oscope_worker.scope,
+            rotator=rotator,
+            plan=plan,
+            # Same reason the scan does it: the inspector drives the stage from
+            # its own thread, and the position poll shares the BBD TX queue.
+            on_inspect_active=lambda active: setattr(
+                self._bbd_worker, "scanning_active", active),
+        )
+        self._inspect_worker.moveToThread(self._inspect_thread)
+
+        window = AngleInspectWindow(self._inspect_worker.angle_labels(), self)
+        self._inspect_window = window
+
+        window.goto_requested.connect(self._inspect_worker.request_goto)
+        window.new_point_requested.connect(self._inspect_worker.request_new_point)
+        window.stop_requested.connect(self._on_inspect_window_closed)
+
+        self._inspect_worker.status_msg.connect(window.on_status)
+        self._inspect_worker.point_changed.connect(window.on_point)
+        self._inspect_worker.busy_changed.connect(window.set_busy)
+        self._inspect_worker.start_failed.connect(self._on_inspect_failed)
+        self._inspect_worker.error_occurred.connect(
+            lambda m: QMessageBox.warning(self, "Inspection Error", m))
+
+        self._inspect_thread.started.connect(self._inspect_worker.run)
+        self.start_scan_btn.setEnabled(False)
+        self.inspect_angles_btn.setEnabled(False)
+        window.show()
+        self._inspect_thread.start()
+        self._inspect_worker.request_start()
+
+    def _on_inspect_failed(self, message: str):
+        QMessageBox.warning(self, "Cannot Inspect Angles", message)
+        if self._inspect_window is not None:
+            self._inspect_window.close()
+
+    def _on_inspect_window_closed(self):
+        """Tear the worker down and hand the hardware back to the scan panel."""
+        if self._inspect_worker is not None:
+            self._inspect_worker.request_stop()
+            self._inspect_worker.stop_worker()
+        if self._inspect_thread is not None:
+            self._inspect_thread.quit()
+            self._inspect_thread.wait(10000)
+        self._inspect_thread = None
+        self._inspect_worker = None
+        self._inspect_window = None
+        self.start_scan_btn.setEnabled(True)
+        self.inspect_angles_btn.setEnabled(True)
+
     def _on_start_scan(self):
         try:
             plan, prefix, save_dir = self._build_scan_plan()
@@ -1261,6 +1440,7 @@ class MainWindow(QMainWindow):
             on_scan_active=lambda active: setattr(
                 self._bbd_worker, "scanning_active", active),
             burst_mode=self.burst_mode_check.isChecked(),
+            strict_rows=self.strict_rows_check.isChecked(),
         )
         self._scan_worker.moveToThread(self._scan_thread)
         self._scan_thread.started.connect(self._scan_worker.run)

scan_format.md

@@ -222,9 +222,23 @@ re-armed for each acquiring pass.  Row boundaries inside the burst come from
 `ACQuire:NUMFRAMESACQuired?` sampled after each pass — the burst itself carries
 no row markers.  See `core/scope_burst.py`.
 
-Either path squares each row up to the declared `n_frames` (zero-padding a
-short row, dropping the tail of a long one), because the format has no per-row
-length field and a mismatch would shift every later row.
+### Row packing
+
+The format has no per-row length field, so a row that over- or under-triggers
+cannot be written as it arrived — that would shift every later row.  Two
+policies are selectable (`ScanEngine(strict_rows=…)`, a checkbox in the app),
+and the choice is not recorded in the file:
+
+| | Pad (default) | Strict |
+|---|---|---|
+| Short row | zero-padded to `n_frames`, warned | scan stops |
+| Long row | trailing frames dropped, warned | scan stops |
+
+Pad keeps a scan running through an occasional mis-trigger, at the cost that
+the affected row is indistinguishable from a good one afterwards — nothing in
+the file records that it was padded.  Strict is for data runs where that
+ambiguity is worse than a failed scan: it aborts before writing the row, so
+the file always ends on a whole-row boundary.
 
 ---
 

tests/test_angle_inspect.py

@@ -0,0 +1,292 @@
+"""Pre-scan angle inspection, driven entirely by fake hardware.
+
+The feature's defining constraint is that it reads nothing back from the
+scope — the operator looks at the instrument. These tests pin that, the scope
+state the app is responsible for putting the instrument into, and the motion
+sequence across angles.
+"""
+import random
+
+import pytest
+
+from core.angle_inspect import AngleInspector, InspectCallbacks
+from core.rotation import RotationAxis, RotationSettings
+from core.scan_engine import AXIS_X, AXIS_Y
+from core.scan_geometry import build_plan
+from core.scope_inspect import (
+    BIAS_CHANNELS, BIAS_POSITION_DIV, BIAS_SCALE_V_DIV, BIAS_WINDOW_V,
+    INSPECT_TRIG_LEVEL_V, inspect_channel_profiles,
+)
+from core.scope_sras import SRAS_CHANNELS
+from fakes import FakeScope, FakeStage, FakeT3R, Trace
+
+SPF = 8
+
+
+def make_plan(num_angles=3):
+    return build_plan(40.0, 30.0, 2.0, 1.0, num_angles, 0.25,
+                      laser_freq_hz=20000.0, velocity_mm_s=100.0)
+
+
+def build(num_angles=3, seed=1234, callbacks=None, rotator_open=True):
+    trace = Trace()
+    scope = FakeScope(trace, samples_per_frame=SPF)
+    stage = FakeStage(trace, scope=scope)
+    t3r = FakeT3R(trace, is_open=rotator_open)
+    rotator = RotationAxis(t3r, RotationSettings())
+    plan = make_plan(num_angles)
+    insp = AngleInspector(stage, scope, rotator, plan,
+                          callbacks=callbacks or InspectCallbacks(),
+                          rng=random.Random(seed))
+    return insp, trace, plan
+
+
+def writes(trace):
+    return [c[1] for c in trace.of("write")]
+
+
+# ── The defining constraint ──────────────────────────────────────────────────
+
+def test_inspection_never_reads_a_waveform_back():
+    """The operator reads the scope; the app must not pull data off it.
+
+    If this fails, someone has added a transfer path to a feature whose whole
+    premise is that there isn't one.
+    """
+    insp, trace, plan = build()
+    insp.start()
+    for i in range(plan.n_angles):
+        insp.goto_angle(i)
+        insp.new_point()
+    insp.stop()
+
+    forbidden = {"transfer_fastframe", "transfer_fastframe_bulk",
+                 "transfer_curve", "set_data_source", "query_wfmoutpre"}
+    assert forbidden.isdisjoint(set(trace.names()))
+    assert "CURVe?" not in writes(trace)
+
+
+# ── Scope configuration ──────────────────────────────────────────────────────
+
+def test_start_sets_an_edge_trigger_on_ch2_above_the_scan_level():
+    insp, trace, _ = build()
+    insp.start()
+
+    assert "TRIGger:A:TYPe EDGE" in writes(trace)
+    assert trace.of("set_trigger_source")[-1][1] == 2
+    assert trace.of("set_trigger_slope")[-1][1] == "RISE"
+    ch, level = trace.of("set_trigger_level")[-1][1:3]
+    assert (ch, level) == (2, INSPECT_TRIG_LEVEL_V)
+    assert INSPECT_TRIG_LEVEL_V >= 2.0
+
+
+def test_start_disables_fastframe_averaging_and_the_logic_trigger():
+    """Everything the scan needs and inspection must not inherit."""
+    insp, trace, _ = build()
+    insp.start()
+
+    assert trace.of("set_fastframe_state")[-1][1] is False
+    assert trace.of("set_acquire_mode")[-1][1] == "SAMPLE"
+    w = writes(trace)
+    assert not any("LOGIc" in cmd or "LOGICPattern" in cmd for cmd in w)
+
+
+def test_start_leaves_the_acquisition_free_running():
+    """The display has to keep updating while the operator looks at it."""
+    insp, trace, _ = build()
+    insp.start()
+
+    w = writes(trace)
+    assert "ACQuire:STOPAfter RUNSTop" in w
+    assert w.index("ACQuire:STOPAfter RUNSTop") < w.index("ACQuire:STATE RUN")
+    assert "ACQuire:STATE STOP" not in w
+
+
+def test_bias_channels_are_directly_comparable():
+    """CH3/CH4 must share scale and position or the eye comparison is a lie."""
+    profiles = inspect_channel_profiles()
+    a, b = (profiles[ch] for ch in BIAS_CHANNELS)
+    assert a.scale_v_div == b.scale_v_div
+    assert a.position_div == b.position_div
+    # Same front end as the scan records — only the display changes.
+    for ch in BIAS_CHANNELS:
+        assert profiles[ch].termination_ohm == SRAS_CHANNELS[ch].termination_ohm
+        assert profiles[ch].coupling == SRAS_CHANNELS[ch].coupling
+        assert profiles[ch].bandwidth_hz == SRAS_CHANNELS[ch].bandwidth_hz
+
+
+@pytest.mark.parametrize("n_divisions", [8, 10])
+def test_bias_window_shows_zero_to_700mv_with_headroom(n_divisions):
+    """0–700 mV must fit on screen, above ground, on either graticule size.
+
+    Ground sits BIAS_POSITION_DIV divisions below centre, so the visible
+    window runs from (-N/2 - pos)*scale to (+N/2 - pos)*scale.
+    """
+    half = n_divisions / 2
+    bottom = (-half - BIAS_POSITION_DIV) * BIAS_SCALE_V_DIV
+    top = (half - BIAS_POSITION_DIV) * BIAS_SCALE_V_DIV
+
+    assert bottom < 0.0, "no room below ground for undershoot"
+    assert top > BIAS_WINDOW_V, "700 mV is clipped or sitting on the top edge"
+    # The point of moving the trace down: most of the screen is above ground.
+    assert abs(bottom) < top
+
+
+def test_ch1_keeps_the_acquisition_front_end():
+    """What you see at a point is what a scan would record there."""
+    assert inspect_channel_profiles()[1] == SRAS_CHANNELS[1]
+
+
+# ── Stage and rotation ───────────────────────────────────────────────────────
+
+def test_start_parks_on_the_first_angle():
+    insp, _, plan = build()
+    point = insp.start()
+
+    assert point.angle_idx == 0
+    assert point.angle_deg == plan.per_angle[0].angle_deg
+    assert insp.current_point == point
+
+
+def test_the_gate_is_off_for_the_whole_inspection():
+    """Nothing here is gated, and an armed output keeps driving the line."""
+    insp, trace, _ = build()
+    insp.start()
+    insp.goto_angle(2)
+    insp.new_point()
+
+    assert trace.count("set_trigger_gate_off") >= 1
+    assert trace.count("set_trigger_trigout_maxv") == 0
+    assert [c[2] for c in trace.of("arm_scan_gate") if c[2]] == []
+
+
+def test_points_land_on_the_scan_grid():
+    """A point the scan would never sample tells you nothing about the scan."""
+    insp, _, plan = build()
+    insp.start()
+
+    for i in range(plan.n_angles):
+        pa = plan.per_angle[i]
+        for _ in range(5):
+            pt = insp.new_point() if insp.angle_idx == i else insp.goto_angle(i)
+            assert pt.angle_idx == i
+            assert pt.y_mm in pa.y_positions
+            assert pa.x_start <= pt.x_mm <= pa.x_start + pa.x_delta
+
+
+def test_goto_angle_rotates_then_moves():
+    insp, trace, plan = build()
+    insp.start()
+    trace.calls.clear()
+
+    insp.goto_angle(2)
+
+    # t3r_rotate carries the delta, so assert the resulting absolute angle.
+    assert trace.count("t3r_rotate") == 1, "expected exactly one rotation"
+    assert insp._rotator.current_deg == pytest.approx(plan.per_angle[2].angle_deg)
+    moves = trace.of("move_axis_absolute")
+    assert [m[1] for m in moves] == [AXIS_Y, AXIS_X], "Y then X, as the scan does"
+
+
+def test_new_point_re_rolls_without_rotating():
+    """Distinguishing a bad spot from a bad angle depends on not rotating."""
+    insp, trace, _ = build()
+    insp.start()
+    insp.goto_angle(1)
+    trace.calls.clear()
+
+    first = insp.current_point
+    second = insp.new_point()
+
+    assert second.angle_idx == first.angle_idx == 1
+    assert (second.x_mm, second.y_mm) != (first.x_mm, first.y_mm)
+    assert trace.count("t3r_rotate") == 0, "new_point must not rotate"
+    assert [m[1] for m in trace.of("move_axis_absolute")] == [AXIS_Y, AXIS_X]
+
+
+def test_next_and_prev_wrap_around():
+    insp, _, plan = build(num_angles=3)
+    insp.start()
+
+    assert insp.next_angle().angle_idx == 1
+    assert insp.next_angle().angle_idx == 2
+    assert insp.next_angle().angle_idx == 0, "should wrap forward"
+    assert insp.prev_angle().angle_idx == plan.n_angles - 1, "should wrap back"
+
+
+def test_angle_labels_cover_every_angle():
+    insp, _, plan = build(num_angles=9)
+    labels = insp.angle_labels()
+    assert len(labels) == 9
+    assert labels[0].startswith("Angle 1/9")
+
+
+# ── Guards ───────────────────────────────────────────────────────────────────
+
+def test_multi_angle_inspection_requires_the_rotator():
+    insp, _, _ = build(num_angles=3, rotator_open=False)
+    with pytest.raises(RuntimeError, match="T3R rotation stage"):
+        insp.start()
+
+
+def test_single_angle_inspection_works_without_the_rotator():
+    insp, _, _ = build(num_angles=1, rotator_open=False)
+    point = insp.start()
+    assert point.angle_idx == 0
+
+
+def test_navigation_before_start_is_rejected():
+    insp, _, _ = build()
+    with pytest.raises(RuntimeError, match="not been started"):
+        insp.goto_angle(1)
+    with pytest.raises(RuntimeError, match="not been started"):
+        insp.new_point()
+
+
+def test_out_of_range_angle_is_rejected():
+    insp, _, _ = build(num_angles=3)
+    insp.start()
+    with pytest.raises(IndexError):
+        insp.goto_angle(3)
+
+
+def test_stop_halts_the_sweep_and_sends_the_rotator_home():
+    insp, trace, _ = build()
+    insp.start()
+    insp.goto_angle(2)
+    trace.calls.clear()
+
+    insp.stop()
+
+    assert "ACQuire:STATE STOP" in writes(trace)
+    assert trace.count("t3r_rotate") == 1, "GR not sent home"
+    assert insp._rotator.current_deg == pytest.approx(0.0)
+
+
+def test_stop_is_idempotent():
+    insp, trace, _ = build()
+    insp.start()
+    insp.stop()
+    trace.calls.clear()
+
+    insp.stop()      # must not re-issue anything or raise
+
+    assert trace.calls == []
+
+
+def test_busy_callback_brackets_every_move():
+    """The window disables its controls on this, so it has to pair up."""
+    events = []
+    insp, _, _ = build(callbacks=InspectCallbacks(on_busy=events.append))
+    insp.start()
+    insp.goto_angle(1)
+    insp.new_point()
+    insp.stop()
+
+    assert events, "no busy events emitted"
+    assert events[0] is True and events[-1] is False
+    depth = 0
+    for e in events:
+        depth += 1 if e else -1
+        assert depth in (0, 1), f"unbalanced busy events: {events}"
+    assert depth == 0

tests/test_scan_engine.py

@@ -27,7 +27,8 @@ def make_plan(num_angles=1, y_delta=0.005):
 
 
 def build(tmp_path, num_angles=1, callbacks=None, resume=None, plan=None,
-          burst_mode=False, max_frames=4096, out_name="out.sras", **kw):
+          burst_mode=False, max_frames=4096, out_name="out.sras",
+          strict_rows=False, **kw):
     trace = Trace()
     scope = FakeScope(trace, samples_per_frame=SPF, max_frames=max_frames)
     stage = FakeStage(trace, scope=scope)
@@ -37,7 +38,7 @@ def build(tmp_path, num_angles=1, callbacks=None, resume=None, plan=None,
     engine = ScanEngine(stage, scope, rotator, plan, tmp_path / out_name,
                         resume=resume,
                         callbacks=callbacks or ScanCallbacks(),
-                        burst_mode=burst_mode)
+                        burst_mode=burst_mode, strict_rows=strict_rows)
     return engine, trace, plan
 
 
@@ -356,6 +357,26 @@ def test_burst_preflight_rejects_a_dark_laser(tmp_path):
         engine.run()
 
 
+def _clip_one_row(engine, which_pass=2, lost=1):
+    """Make one acquiring pass come up `lost` frames short.
+
+    `which_pass` counts acquiring passes from 1, so the default clips the
+    second data row (row 2) — far enough in that a mishandled short row shows
+    up as a shift in the rows after it.
+    """
+    scope = engine._scope
+    real_acquire = scope.acquire_frames
+    passes = {"n": 0}
+
+    def clipped(n):
+        if scope._running:
+            passes["n"] += 1
+            if passes["n"] == which_pass:
+                n -= lost
+        real_acquire(n)
+    scope.acquire_frames = clipped
+
+
 @pytest.mark.parametrize("burst_mode", [False, True])
 def test_short_row_is_padded_to_declared_frame_count(tmp_path, burst_mode):
     """A clipped row must not shift every later row in the file.
@@ -374,17 +395,7 @@ def test_short_row_is_padded_to_declared_frame_count(tmp_path, burst_mode):
     # acquiring-pass count below identical in both modes.
     engine._preflight_done = True
 
-    scope = engine._scope
-    real_acquire = scope.acquire_frames
-    passes = {"n": 0}
-
-    def clipped(n):
-        if scope._running:
-            passes["n"] += 1
-            if passes["n"] == 2:        # second data row loses one frame
-                n -= 1
-        real_acquire(n)
-    scope.acquire_frames = clipped
+    _clip_one_row(engine)
 
     result = engine.run()
 
@@ -398,6 +409,62 @@ def test_short_row_is_padded_to_declared_frame_count(tmp_path, burst_mode):
     assert bytes(sras.load_row(0, 2, 0)[0]) != bytes(SPF)
 
 
+# ── Strict row packing ───────────────────────────────────────────────────────
+
+@pytest.mark.parametrize("burst_mode", [False, True])
+def test_strict_row_packing_aborts_on_a_short_row(tmp_path, burst_mode):
+    """Strict mode fails the scan instead of silently squaring a row up.
+
+    The default padding keeps the file readable but makes a mis-triggered row
+    indistinguishable from a good one after the fact, since v6 records no
+    per-row frame count. Strict mode trades the salvaged rows for knowing.
+    """
+    plan = make_plan(**BURST_PLAN)
+    engine, _, _ = build(tmp_path, plan=plan, burst_mode=burst_mode,
+                         max_frames=BURST_MAX_FRAMES, strict_rows=True)
+    engine._preflight_done = True
+    _clip_one_row(engine)
+
+    with pytest.raises(RuntimeError, match="Row 2: 3 frames acquired, 4 expected"):
+        engine.run()
+
+
+@pytest.mark.parametrize("burst_mode", [False, True])
+def test_strict_row_packing_does_not_disturb_a_clean_scan(tmp_path, burst_mode):
+    """Strict mode is inert when every row acquires what it declared."""
+    plan = make_plan(**BURST_PLAN)
+    engine, _, _ = build(tmp_path, plan=plan, burst_mode=burst_mode,
+                         max_frames=BURST_MAX_FRAMES, strict_rows=True)
+    engine._preflight_done = True
+
+    result = engine.run()
+
+    assert result.rows_written == plan.total_rows
+    sras = SrasFile(tmp_path / "out.sras")
+    assert [s.status for s in sras.angle_status()] == ["OK"]
+
+
+def test_strict_row_packing_writes_nothing_for_the_failed_row(tmp_path):
+    """The abort must not leave a half-written row behind.
+
+    CH1 leads SCAN_CHANNELS, so the frame count is known before any of the
+    row's channels are written — the file should end on a whole-row boundary.
+    """
+    plan = make_plan(**BURST_PLAN)
+    engine, _, _ = build(tmp_path, plan=plan, strict_rows=True)
+    engine._preflight_done = True
+    _clip_one_row(engine)
+
+    with pytest.raises(RuntimeError, match="Strict row packing"):
+        engine.run()
+
+    # Row 1 was written in full; row 2 aborted before writing anything, so
+    # the file ends exactly on a row boundary.
+    sras = SrasFile(tmp_path / "out.sras")
+    written = (tmp_path / "out.sras").stat().st_size - sras.data_start_offset
+    assert written == sras.row_bytes(0)
+
+
 def test_engine_imports_without_qt():
     """The engine must be usable from a non-Qt front end."""
     import subprocess