src/vs/base/browser/ui/grid/gridview.ts/BranchNode/boundarySashes
class BranchNode:	set boundarySashes(boundarySashes: IRelativeBoundarySashes) {
		this._boundarySashes = boundarySashes;

		this.splitview.orthogonalStartSash = boundarySashes.orthogonalStart;
		this.splitview.orthogonalEndSash = boundarySashes.orthogonalEnd;

		for (let index = 0; index < this.children.length; index++) {
			const child = this.children[index];
			const first = index === 0;
			const last = index === this.children.length - 1;

			child.boundarySashes = {
				start: boundarySashes.orthogonalStart,
				end: boundarySashes.orthogonalEnd,
				orthogonalStart: first ? boundarySashes.start : child.boundarySashes.orthogonalStart,
				orthogonalEnd: last ? boundarySashes.end : child.boundarySashes.orthogonalEnd,
			};
		}
	}