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

[anna-shakhova]

No description provided.

[Copilot]

Pull request overview

Fixes a DataGrid regression where grouped rows could disappear after collapsing/expanding groups while using virtual scrolling (local grouping with remote operations). The change refines how group offsets and virtual “loadIndex” are computed during group state transitions and adds coverage to prevent regressions.

Changes:

• Adjust virtual scrolling loadIndex progression for group rows to avoid incorrect index shifts.
• Rework expanded-group offset recalculation to account for “pending” expand/collapse state during loading.
• Add new typing helpers plus Jest + TestCafe regression tests for the disappearing-rows scenario.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/m_virtual_scrolling.ts	Tweaks group-row loadIndex increment conditions in the new virtual mode.
packages/devextreme/js/__internal/grids/data_grid/grouping/types.ts	Introduces TS types for grouping items and group info (used by expanded grouping helper).
packages/devextreme/js/__internal/grids/data_grid/grouping/m_grouping_expanded.ts	Updates group offset recalculation and loading-window logic using an isPending marker during expand/collapse.
packages/devextreme/js/__internal/grids/data_grid/grouping/m_grouping_expanded.test.ts	Adds Jest unit tests covering offset calculations and pending-state loading behavior.
packages/devextreme/js/__internal/grids/data_grid/grouping/m_grouping_core.ts	Adds a return type to findGroupInfo for the new grouping types.
e2e/testcafe-devextreme/tests/dataGrid/common/scrolling.ts	Adds an E2E regression test to ensure the last group doesn’t disappear after collapsing a subgroup in virtual mode.