Problem

Three independent issues

1. notebookEditorWidget.ts:3061: this.viewModel?.viewCells.find(...) performs an O(n) linear scan whose result is never assigned, the cell was already resolved via the O(1) getCellByHandle() call on the previous line.

2. notebookEditorWidget.ts:3068+3074: cell.outputsViewModels.indexOf(key) is called twice for the same key: once for an existence check and again to retrieve the index.

3. notebookViewModelImpl.ts:163: selectionHandles getter creates a Set<number> for deduplication but never calls add(), so the Set stays empty and deduplication has no effect.

Fix

1. Remove the unused viewCells.find() call.
2. Cache the indexOf result and use it for both the existence check and the offset calculation. Return early when the output is gone.
3. Add the missing handlesSet.add(cell.handle) call.