fix(sync): handle missing position events without breaking sync stream

frontend/src/app/workspace/service/workflow-graph/model/joint-graph-wrapper.spec.ts

@@ -818,4 +818,40 @@ describe("JointGraphWrapperService", () => {
       expect(emitted).toEqual([false, true, false]);
     });
   });
+
+  it(
+    "should ignore position change events when old position is missing and continue emitting future events",
+    marbles(m => {
+      const operator = jointUIService.getJointOperatorElement(mockScanPredicate, mockPoint);
+      jointGraph.addCell(operator);
+
+      const events: any[] = [];
+      const subscription = jointGraphWrapper.getElementPositionChangeEvent().subscribe(event => events.push(event));
+
+      // remove operator position from the internal map to force bug condition
+      (jointGraphWrapper as any).elementPositions.delete(mockScanPredicate.operatorID);
+
+      // trigger position change with missing oldPosition
+      operator.position(150, 150);
+
+      // restore position tracking
+      (jointGraphWrapper as any).elementPositions.set(mockScanPredicate.operatorID, {
+        currPos: { x: 100, y: 100 },
+        lastPos: undefined,
+      });
+
+      // trigger another valid position change
+      operator.position(200, 200);
+
+      // the first event should be skipped,but the second event should still emit
+      expect(events.length).toBe(1);
+      expect(events[0].elementID).toBe(mockScanPredicate.operatorID);
+      expect(events[0].newPosition).toEqual({
+        x: 200,
+        y: 200,
+      });
+
+      subscription.unsubscribe();
+    })
+  );
 });

frontend/src/app/workspace/service/workflow-graph/model/joint-graph-wrapper.ts

@@ -307,7 +307,7 @@ export class JointGraphWrapper {
         const oldPosition = this.elementPositions.get(elementID);
         const newPosition = { x: e[1].x, y: e[1].y };
         if (!oldPosition) {
-          throw new Error(`internal error: cannot find element position for ${elementID}`);
+          return undefined;
         }
         if (
           !oldPosition.lastPos ||
@@ -325,7 +325,8 @@ export class JointGraphWrapper {
           oldPosition: oldPosition.lastPos,
           newPosition: newPosition,
         };
-      })
+      }),
+      filter((event): event is { elementID: string; oldPosition: Point; newPosition: Point } => event !== undefined)
     );
   }