from the FreeCAD forum:
https://forum.freecadweb.org/viewtopic.php?p=316137
Transparency issue again
Postby RalpES » Sat Jun 08, 2019 3:30 pm
OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.16117 (Git)
.....
AMD Radeon(TM) R7 Graphics
Radeon Softwareversion
19.5.2
Driver Packet Version
19.10.15.05-190522a-342855C-RadeonSoftwareAdrenalin2019
They rebuilt the software starting in version 17. I have version 15 also on my computer and it has no problem with making any solid object transparent and individual solids different levels of transparency. The display for an assembly makes one item choose and defined as transparent into the only item on the screen and in wire frame. Or just crashes. All other object are gone.