src/vs/base/browser/ui/splitview/paneview.ts/Pane/orientation
class Pane:	set orientation(orientation: Orientation) {
		if (this._orientation === orientation) {
			return;
		}

		this._orientation = orientation;

		if (this.element) {
			this.element.classList.toggle('horizontal', this.orientation === Orientation.HORIZONTAL);
			this.element.classList.toggle('vertical', this.orientation === Orientation.VERTICAL);
		}

		if (this.header) {
			this.updateHeader();
		}
	}