I’ve not had this issue as I don’t use it that way, but I can confirm that the behaviour has changed between 2017 and 2018. In 2017 you could drag the mouse over the text and change it. I also don’t drag with the tape tool (or any other), I prefer to click once to start then a 2nd click to finish. You can still just type the dimension as before, its the editing of dimensions that has changed. At least with mm anyway, it is much quicker to just type the correct value than to select part of the value with the mouse then correcting it.
He’s not clicking in the measurements window, but dragging the mouse over some of it to select the wrong part then typing the correct value.