protected long getHeaderId(int position) {
        return translatePosition(position).mHeader;
    }
Counts the number of items that would be need to fill out the last row in
the group of items with the given header.

@param header Header set of items are grouped by.
@return The count of unfilled spaces in the last row.
Library/src/com/tonicartos/widget/stickygridheaders/StickyGridHeadersBaseAdapterWrapper.java