src/vs/workbench/browser/parts/editor/editorActions.ts/OpenPreviousEditorInGroup/navigate
class OpenPreviousEditorInGroup:	protected navigate(): IEditorIdentifier {
		const group = this.editorGroupService.activeGroup;
		const editors = group.getEditors(EditorsOrder.SEQUENTIAL);
		const index = group.activeEditor ? editors.indexOf(group.activeEditor) : -1;

		return { editor: index > 0 ? editors[index - 1] : editors[editors.length - 1], groupId: group.id };
	}