[ui] Additive mode for select following Nodes

Implement additive selection behavior when selecting downstream
nodes from a node, using Alt+Shift+Click.

Author: yann-lty

meshroom/ui/graph.py

@@ -952,9 +952,12 @@ def selectNodes(self, nodes, command=QItemSelectionModel.SelectionFlag.ClearAndS
         self.selectNodesByIndices(indices, command)
 
     @Slot(Node)
-    def selectFollowing(self, node: Node):
+    @Slot(Node, int)
+    def selectFollowing(self, node: Node, command=QItemSelectionModel.SelectionFlag.ClearAndSelect):
         """Select all the nodes that depend on `node`."""
-        self.selectNodes(self._graph.dfsOnDiscover(startNodes=[node], reverse=True, dependenciesOnly=True)[0])
+        self.selectNodes(
+            self._graph.dfsOnDiscover(startNodes=[node], reverse=True, dependenciesOnly=True)[0], command
+        )
         self.selectedNode = node
 
     @Slot(int)

meshroom/ui/qml/GraphEditor/GraphEditor.qml

@@ -864,7 +864,12 @@ Item {
                                 selectionMode = ItemSelectionModel.Toggle;
                             }
                             if(mouse.modifiers & Qt.AltModifier) {
-                                uigraph.selectFollowing(node);
+                                let selectFollowingMode = ItemSelectionModel.ClearAndSelect;
+                                if(mouse.modifiers & Qt.ShiftModifier) {
+                                    selectFollowingMode = ItemSelectionModel.Select;
+                                }
+                                uigraph.selectFollowing(node, selectFollowingMode);
+                                // Indicate selection has been dealt with by setting conservative Select mode.
                                 selectionMode = ItemSelectionModel.Select;
                             }
                         }