Function scroll

static/bootstrap-select-1.13.18/js/bootstrap-select.js:1227–1411

(scrollTop, init)

      });

      function scroll (scrollTop, init) {
        var size = that.selectpicker.current.elements.length,
            chunks = [],
            chunkSize,
            chunkCount,
            firstChunk,
            lastChunk,
            currentChunk,
            prevPositions,
            positionIsDifferent,
            previousElements,
            menuIsDifferent = true,
            isVirtual = that.isVirtual();

        that.selectpicker.view.scrollTop = scrollTop;

        chunkSize = Math.ceil(that.sizeInfo.menuInnerHeight / that.sizeInfo.liHeight * 1.5); // number of options in a chunk
        chunkCount = Math.round(size / chunkSize) || 1; // number of chunks

        for (var i = 0; i < chunkCount; i++) {
          var endOfChunk = (i + 1) * chunkSize;

          if (i === chunkCount - 1) {
            endOfChunk = size;
          }

          chunks[i] = [
            (i) * chunkSize + (!i ? 0 : 1),
            endOfChunk
          ];

          if (!size) break;

          if (currentChunk === undefined && scrollTop - 1 <= that.selectpicker.current.data[endOfChunk - 1].position - that.sizeInfo.menuInnerHeight) {
            currentChunk = i;
          }
        }

        if (currentChunk === undefined) currentChunk = 0;

        prevPositions = [that.selectpicker.view.position0, that.selectpicker.view.position1];

        // always display previous, current, and next chunks
        firstChunk = Math.max(0, currentChunk - 1);
        lastChunk = Math.min(chunkCount - 1, currentChunk + 1);

        that.selectpicker.view.position0 = isVirtual === false ? 0 : (Math.max(0, chunks[firstChunk][0]) || 0);
        that.selectpicker.view.position1 = isVirtual === false ? size : (Math.min(size, chunks[lastChunk][1]) || 0);

        positionIsDifferent = prevPositions[0] !== that.selectpicker.view.position0 || prevPositions[1] !== that.selectpicker.view.position1;

        if (that.activeIndex !== undefined) {
          prevActive = that.selectpicker.main.elements[that.prevActiveIndex];
          active = that.selectpicker.main.elements[that.activeIndex];
          selected = that.selectpicker.main.elements[that.selectedIndex];

          if (init) {
            if (that.activeIndex !== that.selectedIndex) {