scrolling multi column jumps if ending item is n+1 % columnCount > 0

[DaveMBush]

At line 187 you check for maxStart in a way that assumes your items[] will always be evenly divisible by the number of items in each row. This causes the data to "jump" once you scroll to the bottom.

The fix for this is to adjust maxStart so that this doesn't happen.

        let maxStartEnd = end;
        let modEnd = end % d.itemsPerRow;
        if(modEnd) {
            maxStartEnd = end + d.itemsPerRow - modEnd;
        }
        var maxStart = Math.max(0, maxStartEnd - d.itemsPerCol * d.itemsPerRow - d.itemsPerRow);