Debian Plasma Desktop Active Corners Not Working

[tokyotea01]

Host: Debian Stretch, new install on new HDD, ported 3 virtual disks, and fresh install of VB 5.2.12
Guest: WIN 10 Insider Edition, WIN 7, WIN XP

Problem: I was experiencing the same no-keyboard input issue reported by another user, but, after reinstalling Guest Additions, that problem seems to be resolved. With Guest in Full Screen Mode (Host+F), the Active Corners of the Debian KDE Plasma desktop no longer work. Before upgrading from Jessie to Stretch, it was possible to push the pointer into a designated "hot" corner to see all desktops (and their contents) and click on one to return to Debian. Since upgrading, my current desktop change behaviour is to switch the Guest to Scaled Mode (Host+C), which brings up the Host taskbar, and select a different desktop there. If possible, I'd like to keep my Guest desktop in Full Screen and use the hot corners.

Anyone had this issue and have a solution?