Pass whole numbers to PrefixSumComputer for nb (#226568)

Pass whole numbers to PrefixSumComputer for mbÏ

src/vs/workbench/contrib/notebook/browser/view/notebookCellListView.ts

@@ -210,7 +210,7 @@ export class NotebookCellsLayout implements IRangeMap {
 			return this.count;
 		}
 
-		return this._prefixSumComputer.getIndexOf(offset).index;
+		return this._prefixSumComputer.getIndexOf(Math.trunc(offset)).index;
 	}
 
 	indexAfter(position: number): number {