fix: improve row selection handling and clean up code, fixes: #1504

[kevenleone]

Fixes: #1504

Screen.Recording.2026-05-02.at.23.35.36.mov

[sorenbs]

Hi @kevenleone — thanks a lot for tackling this, and for the clear demo video! Restoring the checkboxes as presentation-only elements (pointer-events-none) while keeping all pointer handling in DataGrid is exactly the right architecture, and the toggle-on-click behavior also addresses the "can't unselect" complaint from #1488. A few changes needed before we can merge:

1. Drag-select still overwrites multi-selection. selectRowsInRange replaces the entire selection, so after checking rows 1 and 3, click-dragging rows 6–8 silently drops 1 and 3 — the same "overwritten" symptom from #1504, just via the drag path. Could you make range-drag merge into the existing selection (or clear the drag anchor when a click toggles a row off)? Related: clicking a selected row deselects it but still arms the drag ref, so a tiny mouse movement re-selects it.
2. Reuse the existing components. ui/studio/cell/SelectHeaderCell.tsx and SelectRowCell.tsx already exist (currently unused) and are almost exactly the JSX you added inline in three places — please adapt/reuse them (or delete them) rather than duplicating.
3. Indeterminate header state. With a partial selection the header checkbox looks unchecked — table.getIsSomeRowsSelected() ? "indeterminate" : false would be clearer.
4. Lint. The two re-formatting hunks in DataGrid.tsx fail the repo's pinned prettier (looks like an editor prettier-version mismatch) — please drop them. There's also a new exhaustive-deps warning: selectSingleRowMode is still used in the right-click branch but was removed from the deps array. And with plain click now toggling, the Shift+click branch is identical code — worth unifying.
5. Regression test + changeset. Per repo convention, bugfixes need a test that reproduces the bug (checkboxes render / additive selection would be great additions to DataGrid.interactions.test.tsx) and a .changeset/*.md entry so the fix ships in a release.

Happy to re-review once these are in — the core approach is solid and this will be a nice usability win. Thanks again!

[kevenleone]

Thanks @sorenbs for your review, working on the changes