world leaders were telling me in quiet asides that the meeting with Trump had gone brilliantly, that he had been very happy, that he had listened — actually listened — to every leader in the big closed-door confab and had left in a good mood.

Hang on, was this the same Donald Trump who had been berating partners only hours earlier?

Yes, apparently so.

I would say Trump was playing them, or playing the markets for his own financial ends. But he’s not that smart.

Such sudden and extreme changes in attitude and demeanor are a symptom of dementia. Or of bipolar or some other mental disorders.