[bugfix] Preserve nested subgraph widget values during serialization (#5023)

When saving workflows with nested subgraphs, promoted widget values were not being synchronized back to the subgraph definitions before serialization. This caused widget values to revert to their original defaults when reloading the workflow.

The fix overrides the serialize() method in SubgraphNode to sync promoted widget values to their corresponding widgets in the subgraph definition before serialization occurs.

Fixes the issue where nested subgraph widget values would be lost after save/reload.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

src/lib/litegraph/src/subgraph/SubgraphNode.ts

@@ -16,7 +16,10 @@ import type {
   GraphOrSubgraph,
   Subgraph
 } from '@/lib/litegraph/src/subgraph/Subgraph'
-import type { ExportedSubgraphInstance } from '@/lib/litegraph/src/types/serialisation'
+import type {
+  ExportedSubgraphInstance,
+  ISerialisedNode
+} from '@/lib/litegraph/src/types/serialisation'
 import type { IBaseWidget } from '@/lib/litegraph/src/types/widgets'
 import type { UUID } from '@/lib/litegraph/src/utils/uuid'
 import { toConcreteWidget } from '@/lib/litegraph/src/widgets/widgetMap'
@@ -540,4 +543,36 @@ export class SubgraphNode extends LGraphNode implements BaseLGraph {
       }
     }
   }
+
+  /**
+   * Synchronizes widget values from this SubgraphNode instance to the
+   * corresponding widgets in the subgraph definition before serialization.
+   * This ensures nested subgraph widget values are preserved when saving.
+   */
+  override serialize(): ISerialisedNode {
+    // Sync widget values to subgraph definition before serialization
+    for (let i = 0; i < this.widgets.length; i++) {
+      const widget = this.widgets[i]
+      const input = this.inputs.find((inp) => inp.name === widget.name)
+
+      if (input) {
+        const subgraphInput = this.subgraph.inputNode.slots.find(
+          (slot) => slot.name === input.name
+        )
+
+        if (subgraphInput) {
+          // Find all widgets connected to this subgraph input
+          const connectedWidgets = subgraphInput.getConnectedWidgets()
+
+          // Update the value of all connected widgets
+          for (const connectedWidget of connectedWidgets) {
+            connectedWidget.value = widget.value
+          }
+        }
+      }
+    }
+
+    // Call parent serialize method
+    return super.serialize()
+  }
 }