fix(gui): wait for hw lock before auto-centering after mount

The server publishes the new sample while the mount still holds the hw
lock, and /alc/center_loop takes that lock non-blocking, so the click
fired on the sample-id change was rejected as busy and never retried.
Arm on mount, fire on the first non-busy status tick instead.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/aare/gui/panels/data_collection_settings.py

@@ -195,6 +195,8 @@ class DataCollectionSettings(QFrame):
         # startup must not trigger an auto-centering (hardware would move
         # uninvited on every restart).
         self._status_seen = False
+        # Set on mount, fired once the hw lock frees (see _track_sample).
+        self._center_after_mount_pending = False
 
         self._tab_bar.currentChanged.connect(self._stack.setCurrentIndex)
         self._tab_bar.currentChanged.connect(self._on_tab_changed)
@@ -242,23 +244,31 @@ class DataCollectionSettings(QFrame):
         self.raster.update_daq_status(s)
         self.screening.update_daq_status(s)
         self.simple.update_daq_status(s)
-        self._track_sample(None if s.sample is None else s.sample.db_id)
+        self._track_sample(None if s.sample is None else s.sample.db_id, s.busy)
 
-    def _track_sample(self, db_id: int | None):
+    def _track_sample(self, db_id: int | None, busy: bool = False):
         # Separate from update_daq_status so tests can drive mount detection
         # without building a full DAQStatusModel.
         if db_id is None:
             # Forget on unmount so remounting the same sample counts as a
             # fresh mount below.
             self._sample_id = None
+            self._center_after_mount_pending = False
         elif db_id != self._sample_id:
             self._sample_id = db_id
             self._tab_bar.setCurrentIndex(0)
-            if self._status_seen and self.auto_center_after_mount.isChecked():
-                # Reuse the button wiring (clicked -> daq.center_loop in
-                # main_window) instead of a second signal path.
-                self.find_tip.click()
+            self._center_after_mount_pending = (
+                self._status_seen and self.auto_center_after_mount.isChecked()
+            )
         self._status_seen = True
+        # The server publishes the new sample while the mount still holds the
+        # hw lock, and /alc/center_loop refuses (non-blocking lock) while busy,
+        # so firing on the id change alone was silently rejected. Wait for idle.
+        if self._center_after_mount_pending and not busy:
+            self._center_after_mount_pending = False
+            # Reuse the button wiring (clicked -> daq.center_loop in
+            # main_window) instead of a second signal path.
+            self.find_tip.click()
 
     @Slot(int)
     def _on_tab_changed(self, idx: int):

tests/unit/gui/test_data_collection_settings.py

@@ -353,6 +353,29 @@ def test_auto_center_fires_on_mount_only_when_armed(settings_panel):
     assert len(clicks) == 3
 
 
+def test_auto_center_waits_for_mount_to_release_hw_lock(settings_panel):
+    # Server shows the new sample while the mount still holds the hw lock;
+    # center_loop would be refused then, so the click must wait for idle.
+    clicks = []
+    settings_panel.find_tip.clicked.connect(lambda: clicks.append(1))
+    settings_panel.auto_center_after_mount.setChecked(True)
+    settings_panel._track_sample(1)
+    settings_panel._track_sample(None)
+
+    settings_panel._track_sample(2, busy=True)
+    settings_panel._track_sample(2, busy=True)
+    assert not clicks
+    settings_panel._track_sample(2, busy=False)
+    assert len(clicks) == 1
+    settings_panel._track_sample(2, busy=False)
+    assert len(clicks) == 1
+
+    # Unmount before the lock frees drops the pending centering.
+    settings_panel._track_sample(3, busy=True)
+    settings_panel._track_sample(None, busy=False)
+    assert len(clicks) == 1
+
+
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