onScroll(evt) {
var scrollTop = 0;
if (evt != null) {
var scrollDiv = evt.target;
if (this.paddingDiv == null) {
this.paddingDiv = scrollDiv.querySelector('#padding');
}
if (this.innerDiv == null) {
this.innerDiv = scrollDiv.querySelector('#inner');
this.innerDiv.style.setProperty('width', `${ROW_WIDTH}px`);
}
scrollTop = scrollDiv.scrollTop;
}
var iStart = Math.floor(scrollTop / ITEM_HEIGHT);
var iEnd = Math.min(iStart + VISIBLE_ITEMS + 1, this._fullList.length);
var padding = iStart * ITEM_HEIGHT;
if (this.innerDiv != null) {
this.innerDiv.style.setProperty('height', `${HEIGHT - padding}px`);
}
if (this.paddingDiv != null) {
this.paddingDiv.style.setProperty('height', `${padding}px`);
}
this.visibleItems = ListWrapper.slice(this._fullList, iStart, iEnd);
}