fix(gui): add "Show beam crosshair" toggle to the camera right-click menu

Right-clicking the sample camera now offers a checkable "Show beam
crosshair" entry. When on, two full-frame lines through the beam center
are drawn with the same pen as the beam box, so the crosshair follows
the beam state color. Off by default so the view stays as before.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/aare/gui/widgets/camera_image.py

@@ -139,6 +139,9 @@ class SampleCameraImageLabel(QGraphicsView):
         # toggle can still switch back to 1:1.
         self._autoscale = True
         self._show_coords = False
+        # Full-frame crosshair through the beam center; off by default so the
+        # default view stays as before, toggled from the right-click menu.
+        self._show_beam_crosshair = False
         self._helical_start = SmargonCoordinate()
         self._helical_end = SmargonCoordinate()
         self._raster_alpha = 127
@@ -682,6 +685,9 @@ class SampleCameraImageLabel(QGraphicsView):
         autofocus_action = menu.addAction("Auto-focus")
 
         beam_mark_action = menu.addAction("Mark beam center")
+        crosshair_action = menu.addAction("Show beam crosshair")
+        crosshair_action.setCheckable(True)
+        crosshair_action.setChecked(self._show_beam_crosshair)
 
         delete_action = None
         evaluate_action = None
@@ -730,6 +736,8 @@ class SampleCameraImageLabel(QGraphicsView):
         elif action == beam_mark_action:
             c = self.mapToScene(event.pos())
             self.update_beam_mark.emit(c.x(), c.y())
+        elif action == crosshair_action:
+            self._show_beam_crosshair = not self._show_beam_crosshair
         self.update()
 
     def _screenshot_with_dialog(self, overlay: bool):
@@ -1446,6 +1454,13 @@ class SampleCameraImageLabel(QGraphicsView):
                 int(beam_size_pxl.y),
             )
         )
+        if self._show_beam_crosshair:
+            # Same pen as the box so the crosshair follows the beam state color.
+            r = self.sceneRect()
+            cx = int(self._geom.beam_location_pxl.x)
+            cy = int(self._geom.beam_location_pxl.y)
+            painter.drawLine(int(r.left()), cy, int(r.right()), cy)
+            painter.drawLine(cx, int(r.top()), cx, int(r.bottom()))
 
     @staticmethod
     def _draw_circle(painter, coord: Coordinate, color: QColor, radius=10):

tests/unit/gui/test_camera_image.py

@@ -334,3 +334,27 @@ def test_mounted_hud_ink_follows_theme(camera):
     assert camera._mounted_hud_ink() == (SHADOW, WHITE)
     camera.set_theme(THEME_SUNSET)
     assert camera._mounted_hud_ink() == (WHITE, SHADOW)
+
+
+def test_beam_crosshair_toggle_draws(camera, monkeypatch):
+    from PySide6.QtWidgets import QMenu
+
+    assert camera._show_beam_crosshair is False
+    # Pick the crosshair action out of the real menu instead of exec'ing it.
+    monkeypatch.setattr(
+        QMenu,
+        "exec_",
+        lambda self, *_: next(a for a in self.actions() if a.text() == "Show beam crosshair"),
+    )
+    camera._camera_interaction_enabled = lambda: True
+    press = QMouseEvent(
+        QEvent.Type.MouseButtonPress,
+        QPointF(10, 10),
+        QPointF(camera.mapToGlobal(QPoint(10, 10))),
+        Qt.MouseButton.RightButton,
+        Qt.MouseButton.RightButton,
+        Qt.KeyboardModifier.NoModifier,
+    )
+    camera._right_click_menu(press)
+    assert camera._show_beam_crosshair is True
+    camera.grab()  # exercises the crosshair draw path