src/vs/base/browser/ui/splitview/splitview.ts/SplitView/isViewVisible
class SplitView:	isViewVisible(index: number): boolean {
		if (index < 0 || index >= this.viewItems.length) {
			throw new Error('Index out of bounds');
		}

		const viewItem = this.viewItems[index];
		return viewItem.visible;
	}