Just to mention: In OpenCascade v7.2.0 release notes there is note about Improved handling of translucent objects in 3D Viewer. It could be nice improvement as currently on version: 0.17.12847 (Git) OCC version: 7.1.0 there are some transparency flickering and sometimes it interferes with work. See how colours change after a little camera change in viewport:
The only problem is that we don't use OCC's 3D viewer but Coin3d (an OpenInventor clone) and Quarter (the successor of SoQt). The problem you show in the 2nd picture is well-known and happens when hovering over a transparent face. In this case the (pre-)selection superposes everything underneath.