src/vs/base/browser/ui/tree/abstractTree.ts/AbstractTree/firstVisibleElement
class AbstractTree:	get firstVisibleElement(): T | undefined {
		const index = this.view.firstVisibleIndex;

		if (index < 0 || index >= this.view.length) {
			return undefined;
		}

		const node = this.view.element(index);
		return node.element;
	}