slice widget needs click to start highlight behavior
I tried this on ParaView 5.11.2 on Windows, but it was true in older slice widgets on ParaView 5.8.
The interior of the slice widget (the interior of the circle) is highlighted with you move the mouse over it, to show the user that it can drag on it to move the slice along its normal. However this does not happen the first time you add the slice. You have to click on the circle for the highlight behavior to start happening.
Same happens on ParaView master on Linux.
Edited by Dan Lipsa