ISTANBUL — During a tumultuous day in court in February, the Turkish businessman and philanthropist Osman Kavala was unexpectedly acquitted of trying to overthrow the government and then rearrested before he could walk free.

He described it as the best day of his life.

“We were acquitted,” he told his lawyers, referring to the eight others tried with him.

Never mind that the two years he had already spent in solitary confinement had been extended indefinitely — this time on specious charges of supporting a 2016 coup. He was happy that at least in one case, he and 15 others had been given the chance to show that the original charges against them were baseless.

“Nothing can affect that composure and attitude,” Murat Celikkan, a campaigning journalist and longtime friend and colleague, said of Mr. Kavala. “I would be furious, but in all the procedures he never raised his voice once.”