postprocessing: Use QgsLayerTreeRegistryBridge to add layers

When a profile tool is already opened, the output of a procssing is
not added to its layer tree. This is because it relies on a
`QgsLayerTreeRegistryBridge`. Indeed, `QgsLayerTreeRegistryBridge`
listens to the `QgsProject::legendLayersAdded()` signal in order to
update the elevation profile tree view. However this signal is not
triggered by the current logic in `Postprocessing.py`because
`QgsProject::addMaplayer` is called with `addToLegend` set to
False. Then, the layer is added to the tree by calling
`QgsLayerTreeGroup::insertChildNode`.

This issue is fixed by creating a
`QgsLayerTreeRegistryBridge::InsertionPoint` to set the insertion
point and then calling `QgsProject::addMaplayer` with `addToLegend`
set to True.

python/plugins/processing/gui/Postprocessing.py

@@ -35,6 +35,7 @@
     QgsLayerTreeLayer,
     QgsLayerTreeGroup,
     QgsLayerTreeNode,
+    QgsLayerTreeRegistryBridge,
 )
 from qgis.utils import iface
 
@@ -237,8 +238,6 @@ def handleAlgorithmResults(
                 # output layer already exists in the destination project
                 owned_map_layer = context.temporaryLayerStore().takeMapLayer(layer)
                 if owned_map_layer:
-                    details.project.addMapLayer(owned_map_layer, False)
-
                     # we don't add the layer to the tree yet -- that's done
                     # later, after we've sorted all added layers
                     layer_tree_layer = create_layer_tree_layer(owned_map_layer, details)
@@ -277,10 +276,15 @@ def handleAlgorithmResults(
         current_selected_node = iface.layerTreeView().currentNode()
         iface.layerTreeView().setUpdatesEnabled(False)
 
+    # store the current intersection point to restore it later
+    previous_insertion_point = (
+        details.project.layerTreeRegistryBridge().layerInsertionPoint()
+    )
     for group, layer_node in sorted_layer_tree_layers:
         layer_node.removeCustomProperty(SORT_ORDER_CUSTOM_PROPERTY)
+        insertion_point: Optional[QgsLayerTreeRegistryBridge.InsertionPoint] = None
         if group is not None:
-            group.insertChildNode(0, layer_node)
+            insertion_point = QgsLayerTreeRegistryBridge.InsertionPoint(group, 0)
         else:
             # no destination group for this layer, so should be placed
             # above the current layer
@@ -289,16 +293,32 @@ def handleAlgorithmResults(
                 current_node_index = current_node_group.children().index(
                     current_selected_node
                 )
-                current_node_group.insertChildNode(current_node_index, layer_node)
+                insertion_point = QgsLayerTreeRegistryBridge.InsertionPoint(
+                    current_node_group, current_node_index
+                )
             elif isinstance(current_selected_node, QgsLayerTreeGroup):
-                current_selected_node.insertChildNode(0, layer_node)
+                insertion_point = QgsLayerTreeRegistryBridge.InsertionPoint(
+                    current_selected_node, 0
+                )
             elif context.project():
-                context.project().layerTreeRoot().insertChildNode(0, layer_node)
+                insertion_point = QgsLayerTreeRegistryBridge.InsertionPoint()
+
+        if insertion_point:
+            details.project.layerTreeRegistryBridge().setLayerInsertionPoint(
+                insertion_point
+            )
+
+        details.project.addMapLayer(layer_node.layer())
 
         if not have_set_active_layer and iface is not None:
             iface.setActiveLayer(layer_node.layer())
             have_set_active_layer = True
 
+    # reset to the previous insertion point
+    details.project.layerTreeRegistryBridge().setLayerInsertionPoint(
+        previous_insertion_point
+    )
+
     # all layers have been added to the layer tree, so safe to call
     # postProcessors now
     for layer, details in layers_to_post_process: