Fix: Shift+Click deselection range not working properly in Datagrid

[Jszigeti]

Problem

While investigating #11010, I discovered that the Shift+Click deselection behavior actually exists in the Datagrid, but was only partially working in some cases.

• Basic range selection worked fine
• But in specific scenarios (e.g. after “Select All”, or when alternating select/deselect actions), Shift+Click didn’t correctly update the selection state.

This led to UX inconsistencies compared to common list-selection patterns in tools like Gmail, Google Sheets, or file explorers.

---

Solution

• Fixed the range toggle logic in DataTableBase.tsx:
  • Detects whether the clicked item is already selected (instead of relying on event.target.checked)
  • Always updates lastSelected.current, even when deselecting
  • Ensures consistent Shift+Select and Shift+Deselect behavior

Added unit tests in DataTable.spec.tsx
Added E2E tests in list.cy.js

Both confirm correct behavior for:

• Alternating Shift+Select and Shift+Deselect
• Shift+Deselect after “Select All”

---

How To Test

1. Go to any list view with bulk selection enabled.
2. Select a first item, then Shift+Click another to select a range.
3. Shift+Click one of the selected items again → all items in that range should now be deselected.
4. Run tests:
   • make test
   • make e2e
     All DataTable and E2E tests should pass.

---

Additional Checks

• The PR targets master for a bugfix
• The PR includes unit tests (DataTable.spec.tsx)
• The PR includes E2E tests (list.cy.js)
• No documentation change required (behavior aligns with existing feature)
• No story update required (visual behavior unchanged)

---

Notes

• Fixes partial inconsistencies mentioned in #11010.
• No breaking changes.
• Improves UX consistency with standard multi-selection behavior.
• A couple of unrelated tests occasionally fail on master (confirmed unrelated to this change).

I'm happy to adjust anything if needed 👍

[djhi]

Sweet! Thanks!