I did the reinstall as instructed - reinstall was successful but the problem with TaskView reappeared as soon as I activated the 'darken background windows' feature just as before. If/when I disable the feature TaskView immediate works as designed.
I reduced to a single monitor - that doesn't fix it.
I disabled/quit other extensions - that didn't fix it.
Since you can't repeat it perhaps it's my Windows build, I'm on 1803 OS Build 17133.73. I'm also running on Nvidia GPU and drivers.
I have a number of computers here, so perhaps tomorrow I'll try it on another system.
On a single monitor config, when I activate TaskView the windows don't shrink down and the bar that's supposed to draw across the top doesn't display. Then the Windows don't respond to user interaction and it's difficult to return to non-taskview mode.
It seems that I had this problem once before with a earlier release of Windows 10 when the TaskView was very young. So perhaps it's a problem with one/more new features of TaskView.
Since the version of Windows I'm running is a late beta of the soon to be widely released April Windows Update maybe we can get it fixed before it becomes a widespread problem.