src/vs/workbench/browser/parts/editor/editorActions.ts/OpenFirstEditorInGroup/navigate
class OpenFirstEditorInGroup:	protected navigate(): IEditorIdentifier {
		const group = this.editorGroupService.activeGroup;
		const editors = group.getEditors(EditorsOrder.SEQUENTIAL);

		return { editor: editors[0], groupId: group.id };
	}