code: function () {
              var prevPageIndex,
                  currentPageIndex,
                  rowIndex,
                  row,
                  offset,
                  state = this.state,
                  pages = state.pages;
  
              prevPageIndex = util.bisectRight(pages, state.scrollTop, 'y0') - 1;
              if (prevPageIndex < 0) {
                  return 1;
              }
              offset = state.scrollTop + pages[prevPageIndex].height / 2;
              currentPageIndex = util.bisectRight(pages, offset, 'y0') - 1;
              rowIndex = pages[currentPageIndex].rowIndex;
              row = state.rows[rowIndex];
              return 1 + row[0];
  
          }
docs: Calculate which page is currently the "focused" page.
  In vertical mode, this is the page at the top (and if multiple columns, the leftmost page),
  where at least half of the page is showing.
  @returns {int} The current page