O.k. or not o.k., that's the question:
For me, these your mentioned steps are exclusive on here:
* pick the made constraint in tree-view: able to view associated parts by clicking magnifying lens (transparency of 80% applies then anyway) & NO toggleTransparency icon available
* none picked: toggleTransparency works
Please correct me if I did get something wrong. I just can't reproduce it on here for now.