LR's Target Adjustment Tool won't work if I drag it upwards

I have noticed the following problem  when using  Lightroom CC. When I drag down on the Targeted Adjustment Tool, it works perfectly. But when I drag up, nothing happens. I called Adobe and the tech support adviser was able to remotely drag the TAT upwards. He suggested I buy a new mouse. I did that, but it did not solve the problem. I am running the latest (as of July 26) versions of Mac OS and Lightroom CC. I have the same problem whether I use a wired mouse with a scroll wheel or a wireless Bluetooth mouse.

In all other respects, both mouses work fine. I can drag up and down on the Mac desktop without a problem.

I have reinstalled Mac OS Sierra and that did not help.

Any suggestions?