src/vs/base/browser/ui/splitview/splitview.ts/ViewItem/constructor
class ViewItem:	constructor(
		protected container: HTMLElement,
		private view: IView<TLayoutContext>,
		size: ViewItemSize,
		private disposable: IDisposable
	) {
		if (typeof size === 'number') {
			this._size = size;
			this._cachedVisibleSize = undefined;
			container.classList.add('visible');
		} else {
			this._size = 0;
			this._cachedVisibleSize = size.cachedVisibleSize;
		}
	}