DataGrid: fix rows on collapse group and virtual scroll (T1325181)

e2e/testcafe-devextreme/tests/dataGrid/common/scrolling.ts

@@ -1495,6 +1495,54 @@ test.meta({ runInTheme: Themes.genericLight })('New virtual mode. Navigation to
   });
 });
 
+// T1284002
+test('Last group should not disappear after collapsing another subgroup with virtual scrolling, local grouping and remote operations (T1284002)', async (t) => {
+  const dataGrid = new DataGrid('#container');
+  await t.expect(dataGrid.isReady()).ok();
+
+  await dataGrid.apiCollapseRow(['Cell phones', 'Touch Screen Phones']);
+
+  const visibleRows = await dataGrid.apiGetVisibleRows();
+  const dataRows = visibleRows.filter((r) => r.rowType === 'data');
+
+  await t.expect(dataRows.length).eql(2, 'Computers category should still show 2 data rows');
+}).before(async () => createWidget('dxDataGrid', {
+  height: 500,
+  dataSource: [
+    {
+      Id: 1, Category: 'Cell phones', Subcategory: 'Touch Screen Phones', Store: 'Europe Online Store', Date: '2024-01-10',
+    },
+    {
+      Id: 2, Category: 'Cell phones', Subcategory: 'Touch Screen Phones', Store: 'Europe Online Store', Date: '2024-02-15',
+    },
+    {
+      Id: 3, Category: 'Computers', Subcategory: 'Computers Accessories', Store: 'North America Reseller', Date: '2024-03-20',
+    },
+    {
+      Id: 4, Category: 'Computers', Subcategory: 'Computers Accessories', Store: 'North America Online Store', Date: '2024-04-25',
+    },
+  ],
+  keyExpr: 'Id',
+  remoteOperations: {
+    filtering: true,
+    sorting: true,
+    paging: true,
+  },
+  scrolling: {
+    mode: 'virtual',
+  },
+  grouping: {
+    autoExpandAll: true,
+  },
+  columns: [
+    { dataField: 'Id', dataType: 'number' },
+    { dataField: 'Category', dataType: 'string', groupIndex: 0 },
+    { dataField: 'Subcategory', dataType: 'string', groupIndex: 1 },
+    { dataField: 'Store', dataType: 'string', groupIndex: 2 },
+    { dataField: 'Date', dataType: 'date', format: 'yyyy-MM-dd' },
+  ],
+}));
+
 // T1152498
 // TODO: fix unstable tests
 // ['infinite', 'virtual'].forEach((scrollingMode) => {

packages/devextreme/js/__internal/grids/data_grid/grouping/__tests__/m_grouping_expanded.helpers.ts

@@ -0,0 +1,101 @@
+import { GroupingHelper, updateGroupOffsets } from '../m_grouping_expanded';
+import type { DataItem, GroupInfoData, GroupItemData } from '../types';
+
+export interface GroupConfig {
+  key: string;
+  count: number;
+  serverCount?: number;
+}
+
+/**
+ * Test helper that simulates the grid's expand/collapse flow for grouped data.
+ *
+ * Constructed from a configuration array — each entry generates a group
+ * with `count` leaf items (ids like `"A0"`, `"A1"`, …).
+ * When `serverCount` is provided it is used as the group's total instead of
+ * the visible leaf count, mirroring the real server-side paging scenario.
+ *
+ * - `collapse(path)` — nullifies `group.items` and registers the group as collapsed.
+ * - `expand(path)` — restores the saved children and marks the group as expanded.
+ *
+ * After each operation all group offsets are recalculated via `updateGroupOffsets`,
+ * exactly matching the `changeRowExpand` flow in the production code.
+ */
+
+export class GroupingTestHelper {
+  public readonly grouping: GroupingHelper;
+
+  public readonly items: DataItem[];
+
+  private readonly groupsByKey: Map<string, GroupItemData>;
+
+  private readonly leafCounts: Map<string, number>;
+
+  /** Saved children snapshots so expand can restore them. */
+  private readonly savedChildren = new Map<string, GroupItemData[] | null>();
+
+  constructor(groups: GroupConfig[]) {
+    this.grouping = new GroupingHelper({ option: (): undefined => undefined });
+    this.groupsByKey = new Map();
+    this.leafCounts = new Map();
+
+    // Initialize group items
+    this.items = groups.map(({ key, count, serverCount }) => {
+      const items = Array.from({ length: count }, (_, i) => ({ id: `${key}${i}` }));
+      const group = { key, items } as unknown as GroupItemData;
+      this.groupsByKey.set(key, group);
+      this.leafCounts.set(key, serverCount ?? count);
+
+      return group;
+    });
+  }
+
+  public collapse(path: string[]): void {
+    const key = path[0];
+    const group = this.groupsByKey.get(key);
+    const count = this.leafCounts.get(key) ?? 0;
+
+    this.simulateChangeRowExpand(path, count);
+
+    if (group) {
+      this.savedChildren.set(key, group.items);
+      group.items = null;
+    }
+  }
+
+  public expand(path: string[]): void {
+    const groupInfo = this.grouping.findGroupInfo(path);
+    const count = groupInfo ? groupInfo.count : 0;
+
+    this.simulateChangeRowExpand(path, count);
+
+    const key = path[0];
+    const group = this.groupsByKey.get(key);
+    if (group) {
+      group.items = this.savedChildren.get(key) ?? [];
+    }
+  }
+
+  private simulateChangeRowExpand(
+    path: unknown[],
+    count: number,
+  ): void {
+    const groupInfo = this.grouping.findGroupInfo(path);
+
+    const pendingGroupInfo: GroupInfoData = {
+      offset: groupInfo ? groupInfo.offset : -1,
+      path: groupInfo ? groupInfo.path : path,
+      isExpanded: groupInfo ? !groupInfo.isExpanded : false,
+      count,
+    };
+
+    updateGroupOffsets(this.grouping, this.items, [], 0, pendingGroupInfo);
+
+    if (groupInfo) {
+      groupInfo.isExpanded = !groupInfo.isExpanded;
+      groupInfo.count = count;
+    } else if (pendingGroupInfo.offset >= 0) {
+      this.grouping.addGroupInfo(pendingGroupInfo);
+    }
+  }
+}

packages/devextreme/js/__internal/grids/data_grid/grouping/__tests__/m_grouping_expanded.mock.ts

@@ -0,0 +1,8 @@
+import { GroupingHelper } from '../m_grouping_expanded';
+
+/** Subclass that exposes the protected handleDataLoading method for testing. */
+export class GroupingHelperMock extends GroupingHelper {
+  public testHandleDataLoading(options: unknown): void {
+    this.handleDataLoading(options);
+  }
+}