Drawing on a federal report about policing in Chicago as well as a Massachusetts court’s caution against the probative value of people fleeing police, a state panel has ruled a man’s arrest and conviction for a gun offense violated the Fourth Amendment.The 1st District Appellate Court deemed Chicago police illegally arrested Markell Horton after spotting him with what they thought was a gun and pursuing him to the second floor of a townhouse.An officer’s glimpse of chrome at Horton’s waistband did …