src/vs/workbench/contrib/notebook/browser/notebookEditorServiceImpl.ts/NotebookEditorWidgetService/_freeWidget
class NotebookEditorWidgetService:	private _freeWidget(input: NotebookEditorInput, sourceID: GroupIdentifier, targetID: GroupIdentifier): void {
		const targetWidget = this._borrowableEditors.get(targetID)?.get(input.resource);
		if (targetWidget) {
			// not needed
			return;
		}

		const widget = this._borrowableEditors.get(sourceID)?.get(input.resource);
		if (!widget) {
			throw new Error('no widget at source group');
		}
		this._borrowableEditors.get(sourceID)?.delete(input.resource);
		widget.token = undefined;

		let targetMap = this._borrowableEditors.get(targetID);
		if (!targetMap) {
			targetMap = new ResourceMap();
			this._borrowableEditors.set(targetID, targetMap);
		}
		targetMap.set(input.resource, widget);
	}