fixes an old issue opened in SlickGrid (6pac) project: 6pac/SlickGrid#345

vibe coded a fix with Copilot

Summary

This change fixes checkbox row selection behavior so it correctly follows the grid multiSelect option.

• When multiSelect is true:
  • Checkbox clicks keep existing toggle behavior (add/remove row from selection).
• When multiSelect is false:
  • Checkbox clicks enforce single selection (only one row selected at a time).
  • Clicking an already selected checkbox clears selection.
• Active cell is still updated after checkbox toggle to preserve keyboard/editor flow.
• Row-mode Shift+Arrow selection in the hybrid selection model now clamps to a single row when multiSelect is false.

Root cause

Checkbox toggling previously always used additive selection logic, even when multiSelect was disabled.
Also, row keyboard range selection had logic gaps for single-select mode in the merged hybrid selection model.

Repro steps (before fix)

1. Open a grid with checkbox selector enabled and set multiSelect to false.
2. Click checkbox on row A.
3. Click checkbox on row B.
4. Observe both rows can remain selected (incorrect for single-select mode).

Optional keyboard repro:

1. Keep multiSelect false.
2. Enter row selection mode.
3. Use Shift+Arrow Up/Down.
4. Observe range-style behavior instead of single-row behavior.

Expected behavior (after fix)

1. With multiSelect false, selecting row B after row A keeps only row B selected.
2. Clicking an already selected checkbox clears selection.
3. Shift+Arrow in row mode keeps selection to one row.
4. Active cell remains synchronized on checkbox toggle.

What was changed

• Updated checkbox toggle logic to branch on multiSelect :
  • true: preserve existing toggle semantics.
  • false: enforce [row] or [] selection only.
• Kept active-cell update after toggle.
• Updated hybrid row keyboard handling so Shift+Arrow still works, but clamps to a single row when multiSelect is false.
• Added/updated unit tests for:
  • checkbox single-select behavior
  • keyboard single-select clamp behavior

Validation

• Unit tests pass.
• Coverage checks pass.
• Cypress run in progress by maintainer.

I'll need to add a note warning on the next release:

⚠️ Breaking Change - multiSelect Option Now Properly Enforced

The multiSelect option is now properly respected. If you previously set multiSelect: false expecting it to work differently, this will now correctly enforce single-row-only selection. To allow multiple row selection while preventing the currently-active row from being automatically selected during navigation, use selectionOptions.selectActiveRow: false instead (keeping multiSelect at its default true).

This makes it clearer that selectActiveRow: false is specifically about navigation behavior (not auto-selecting as you arrow through rows), whereas multiSelect is about how many rows you can select in total.