Fix available height when adding modifier mesh in One at a Time printing

Cura lowers the available build volume height when more than 1 model is loaded. This count should include grouped nodes as one (since they will be slice as one and thus not affect the gantry), and should not include modifier meshes or support meshes (since support-meshes are always printed with the first model anyway).

Fixes Ultimaker#16566

cura/BuildVolume.py

@@ -120,6 +120,8 @@ def __init__(self, application: "CuraApplication", parent: Optional[SceneNode] =
 
         # Objects loaded at the moment. We are connected to the property changed events of these objects.
         self._scene_objects = set()  # type: Set[SceneNode]
+        # Number of toplevel printable meshes. If there is more than one, the build volume needs to take account of the gantry height in One at a Time printing.
+        self._root_printable_object_count = 0
 
         self._scene_change_timer = QTimer()
         self._scene_change_timer.setInterval(200)
@@ -151,6 +153,7 @@ def _onSceneChanged(self, source):
     def _onSceneChangeTimerFinished(self):
         root = self._application.getController().getScene().getRoot()
         new_scene_objects = set(node for node in BreadthFirstIterator(root) if node.callDecoration("isSliceable"))
+
         if new_scene_objects != self._scene_objects:
             for node in new_scene_objects - self._scene_objects: #Nodes that were added to the scene.
                 self._updateNodeListeners(node)
@@ -166,6 +169,26 @@ def _onSceneChangeTimerFinished(self):
             self.rebuild()
 
             self._scene_objects = new_scene_objects
+
+        # This also needs to be called when objects are grouped/ungrouped,
+        # which is not reflected in a change in self._scene_objects
+        self._updateRootPrintableObjectCount()
+
+    def _updateRootPrintableObjectCount(self):
+        # Get the number of models in the scene root, excluding modifier meshes and counting grouped models as 1
+        root = self._application.getController().getScene().getRoot()
+        scene_objects = set(node for node in BreadthFirstIterator(root) if node.callDecoration("isSliceable") or node.callDecoration("isGroup"))
+
+        new_root_printable_object_count = len(list(node for node in scene_objects if node.getParent() == root and not (
+            node_stack := node.callDecoration("getStack") and (
+                node.callDecoration("getStack").getProperty("anti_overhang_mesh", "value") or
+                node.callDecoration("getStack").getProperty("support_mesh", "value") or
+                node.callDecoration("getStack").getProperty("cutting_mesh", "value") or
+                node.callDecoration("getStack").getProperty("infill_mesh", "value")
+            ))
+        ))
+        if new_root_printable_object_count != self._root_printable_object_count:
+            self._root_printable_object_count = new_root_printable_object_count
             self._onSettingPropertyChanged("print_sequence", "value")  # Create fake event, so right settings are triggered.
 
     def _updateNodeListeners(self, node: SceneNode):
@@ -647,7 +670,7 @@ def _onStackChangeTimerFinished(self) -> None:
 
             self._width = self._global_container_stack.getProperty("machine_width", "value")
             machine_height = self._global_container_stack.getProperty("machine_height", "value")
-            if self._global_container_stack.getProperty("print_sequence", "value") == "one_at_a_time" and len(self._scene_objects) > 1:
+            if self._global_container_stack.getProperty("print_sequence", "value") == "one_at_a_time" and self._root_printable_object_count > 1:
                 new_height = min(self._global_container_stack.getProperty("gantry_height", "value") * self._scale_vector.z, machine_height)
 
                 if self._height > new_height:
@@ -689,9 +712,12 @@ def _onSettingChangeTimerFinished(self) -> None:
         update_extra_z_clearance = True
 
         for setting_key in self._changed_settings_since_last_rebuild:
+            if setting_key in ["print_sequence", "support_mesh", "infill_mesh", "cutting_mesh", "anti_overhang_mesh"]:
+                self._updateRootPrintableObjectCount()
+
             if setting_key == "print_sequence":
                 machine_height = self._global_container_stack.getProperty("machine_height", "value")
-                if self._application.getGlobalContainerStack().getProperty("print_sequence", "value") == "one_at_a_time" and len(self._scene_objects) > 1:
+                if self._application.getGlobalContainerStack().getProperty("print_sequence", "value") == "one_at_a_time" and self._root_printable_object_count > 1:
                     new_height = min(
                         self._global_container_stack.getProperty("gantry_height", "value") * self._scale_vector.z,
                         machine_height)