fix: コードレビューのフィードバックに対応し、start_time更新の検出を改善

Co-authored-by: nannany <14943122+nannany@users.noreply.github.com>

Author: Copilot

ui/src/reducers/taskReducer.test.ts

@@ -67,6 +67,28 @@ describe("taskReducer", () => {
       expect(newState[2].start_time).toBe("2024-01-01T10:00:00Z");
     });
 
+    it("should re-sort tasks when start_time is set to null", () => {
+      const task1WithStart = { ...mockTask1, start_time: "2024-01-01T10:00:00Z" };
+      const initialState = [mockTask2, mockTask3, task1WithStart];
+
+      // Task1のstart_timeをnullに設定
+      const action = {
+        type: "UPDATE_TASK" as const,
+        payload: {
+          id: "1",
+          start_time: null,
+        },
+      };
+
+      const newState = taskReducer(initialState, action);
+
+      // Task1がstart_time: nullになるため、task_orderに基づいて先頭に移動
+      expect(newState[0].id).toBe("1");
+      expect(newState[1].id).toBe("2");
+      expect(newState[2].id).toBe("3");
+      expect(newState[0].start_time).toBeNull();
+    });
+
     it("should sort tasks by start_time when multiple tasks have start_time", () => {
       const task1WithStart = { ...mockTask1, start_time: "2024-01-01T12:00:00Z" };
       const task2WithStart = { ...mockTask2, start_time: "2024-01-01T10:00:00Z" };

ui/src/reducers/taskReducer.ts

@@ -38,7 +38,7 @@ export const taskReducer = (state: Task[], action: TaskAction): Task[] => {
         task.id === action.payload.id ? { ...task, ...action.payload } : task,
       );
       // start_timeが更新された場合、タスクを再ソート
-      if (action.payload.start_time !== undefined) {
+      if ("start_time" in action.payload) {
         return sortTasks(updatedTasks);
       }
       return updatedTasks;