As Mayor Lori Lightfoot prepares to present her critical first city budget, a key indicator of the health of the downtown office market has begun to flash red—a sign of the difficulties the mayor may face as she struggles to raise new revenue without stalling growth.  The indicator is not the number of jobs or the vacancy rate, both of which continue to be solid by most counts. Rather, the weakness is in capital flows, the amount of money spent to buy or sometimes build new structures. Consider it a sort of leading indicator as to whether investors consider a given area worth the risk of plunking down their money in exchange for returns that can be many years away.  Click here to read more at Crain’s Chicago Business…