src/vs/workbench/services/editor/common/editorGroupsService.ts/preferredSideBySideGroupDirection
export function preferredSideBySideGroupDirection(configurationService: IConfigurationService): GroupDirection.DOWN | GroupDirection.RIGHT {
	const openSideBySideDirection = configurationService.getValue('workbench.editor.openSideBySideDirection');

	if (openSideBySideDirection === 'down') {
		return GroupDirection.DOWN;
	}

	return GroupDirection.RIGHT;
}