src/vs/base/browser/ui/grid/gridview.ts/BranchNode/addChild
class BranchNode:	addChild(node: Node, size: number | Sizing, index: number, skipLayout?: boolean): void {
		if (index < 0 || index > this.children.length) {
			throw new Error('Invalid index');
		}

		this.splitview.addView(node, size, index, skipLayout);
		this._addChild(node, index);
		this.onDidChildrenChange();
	}