Fix incorrect data node rendering for modular pipelines (#1439)

The viz flowchart is actually a representation of modular pipelines. The root modular pipeline is the first thing we see in the collapsed flowchart. It must include:

All top-level modular pipelines (e.g., level1.node, not level2.level1.node).
Any tasks, datasets, or parameters not part of a modular pipeline.
Inputs and outputs to the top-level modular pipeline (also called external_inputs and external_outputs in the code).
Lastly, all parameters, even if they belong to nested modular pipelines. The flowchart design ensures these parameters remain visible from the root modular pipeline, even when we expand the view.
In the code, there was a variable named root_children_ids, which was causing issues as reported by users. It's a bit confusing what it does but removing it resulted in the parameters not being shown at the root level anymore.

To address this, the variable root_children_ids has been renamed to root_parameters, and it has been modified to include all parameters that are part of a modular pipeline.

After making this change, the code correctly satisfies the above four conditions, ensuring all relevant components are present in the root modular pipeline.

RELEASE.md

@@ -6,6 +6,11 @@ Please follow the established format:
 - Include the ID number for the related PR (or PRs) in parentheses
 -->
 
+
+## Bug fixes and other changes
+
+- Fix incorrect rendering of datasets in modular pipelines. (#1439) 
+
 # Release 6.3.4
 
 ## Bug fixes and other changes

package/kedro_viz/data_access/managers.py

@@ -371,7 +371,7 @@ def create_modular_pipelines_tree_for_registered_pipeline(
         modular_pipelines_tree = modular_pipelines_services.expand_tree(
             modular_pipelines.as_dict()
         )
-        root_children_ids = set()
+        root_parameters = set()
 
         # turn all modular pipelines in the tree into a graph node for visualisation,
         # except for the artificial root node
@@ -397,32 +397,24 @@ def create_modular_pipelines_tree_for_registered_pipeline(
             )
 
             # only keep the modular pipeline's inputs belonging to the current registered pipeline
-            inputs_in_registered_pipeline = set()
             for input_id in modular_pipeline_node.inputs:
                 input_node = self.nodes.get_node_by_id(input_id)
                 if input_node.belongs_to_pipeline(registered_pipeline_id):
                     edges.add_edge(
                         GraphEdge(source=input_id, target=modular_pipeline_id)
                     )
                     node_dependencies[input_id].add(modular_pipeline_id)
-                    inputs_in_registered_pipeline.add(input_id)
-            root_children_ids.update(
-                modular_pipeline_node.external_inputs & inputs_in_registered_pipeline
-            )
+                if isinstance(input_node, ParametersNode):
+                    root_parameters.add(input_id)
 
             # only keep the modular pipeline's outputs belonging to the current registered pipeline
-            outputs_in_registered_pipeline = set()
             for output_id in modular_pipeline_node.outputs:
                 output_node = self.nodes.get_node_by_id(output_id)
                 if output_node.belongs_to_pipeline(registered_pipeline_id):
                     edges.add_edge(
                         GraphEdge(source=modular_pipeline_id, target=output_id)
                     )
                     node_dependencies[modular_pipeline_id].add(output_id)
-                    outputs_in_registered_pipeline.add(output_id)
-            root_children_ids.update(
-                modular_pipeline_node.external_outputs & outputs_in_registered_pipeline
-            )
 
         # After adding modular pipeline nodes into the graph,
         # There is a chance that the graph with these nodes contains cycles if
@@ -456,7 +448,7 @@ def create_modular_pipelines_tree_for_registered_pipeline(
                 or not node.belongs_to_pipeline(registered_pipeline_id)
             ):
                 continue
-            if not node.modular_pipelines or node_id in root_children_ids:
+            if not node.modular_pipelines or node_id in root_parameters:
                 modular_pipelines_tree[ROOT_MODULAR_PIPELINE_ID].children.add(
                     ModularPipelineChild(
                         node_id, self.nodes.get_node_by_id(node_id).type

package/tests/test_api/test_rest/test_responses.py

@@ -200,6 +200,7 @@ def assert_example_data(response_data):
             "children": [
                 {"id": "0ecea0de", "type": "data"},
                 {"id": "f1f1425b", "type": "parameters"},
+                {"id": "f0ebef01", "type": "parameters"},
                 {"id": "uk", "type": "modularPipeline"},
             ],
             "id": "__root__",