set orthogonalStartSash(sash) {
        if (this._orthogonalStartSash === sash) {
            return;
        }
        this.orthogonalStartDragHandleDisposables.clear();
        this.orthogonalStartSashDisposables.clear();
        if (sash) {
            const onChange = (state) => {
                this.orthogonalStartDragHandleDisposables.clear();
                if (state !== 0 /* SashState.Disabled */) {
                    this._orthogonalStartDragHandle = append(this.el, $('.orthogonal-drag-handle.start'));
                    this.orthogonalStartDragHandleDisposables.add(toDisposable(() => this._orthogonalStartDragHandle.remove()));
                    this.orthogonalStartDragHandleDisposables.add(new DomEmitter(this._orthogonalStartDragHandle, 'mouseenter')).event(() => Sash.onMouseEnter(sash), undefined, this.orthogonalStartDragHandleDisposables);
                    this.orthogonalStartDragHandleDisposables.add(new DomEmitter(this._orthogonalStartDragHandle, 'mouseleave')).event(() => Sash.onMouseLeave(sash), undefined, this.orthogonalStartDragHandleDisposables);
                }
            };
            this.orthogonalStartSashDisposables.add(sash.onDidEnablementChange.event(onChange, this));
            onChange(sash.state);
        }
        this._orthogonalStartSash = sash;
    }
A reference to another sash, perpendicular to this one, which
aligns at the start of this one. A corner sash will be created
automatically at that location.

The start of a horizontal sash is its left-most position.
The start of a vertical sash is its top-most position.