src/vs/base/browser/ui/grid/gridview.ts/BranchNode/_addChild
class BranchNode:	private _addChild(node: Node, index: number): void {
		const first = index === 0;
		const last = index === this.children.length;
		this.children.splice(index, 0, node);

		node.boundarySashes = {
			start: this.boundarySashes.orthogonalStart,
			end: this.boundarySashes.orthogonalEnd,
			orthogonalStart: first ? this.boundarySashes.start : this.splitview.sashes[index - 1],
			orthogonalEnd: last ? this.boundarySashes.end : this.splitview.sashes[index],
		};

		if (!first) {
			this.children[index - 1].boundarySashes = {
				...this.children[index - 1].boundarySashes,
				orthogonalEnd: this.splitview.sashes[index - 1]
			};
		}

		if (!last) {
			this.children[index + 1].boundarySashes = {
				...this.children[index + 1].boundarySashes,
				orthogonalStart: this.splitview.sashes[index]
			};
		}
	}