[heavily edited, was wrong about some assumptions]
Ok so the gnome team seem to have eventually realized that the screen shield implementation of GDM 3 is an abomination. (indeed it is absolutely atrocious, without any doubt whatsoever).
And have started working on a replacement. When that is finally ready, they will be able to retire the old screen shield. And any features of it would be re-implemented (presumably from scratch, we would hope). And integrated into the new login screen itself, as a single element.
So no more separate screen shield. Getting in the way, causing a nuisance.
The place to look at this new development work, while it is ongoing, seems to be over here:
Unfortunately the new design still seems to make the screen shield a mandatory part of the login screen. This is not actually what we wanted. A good place to complain about this would be at the link above ^ (login required). Or on alan's associated blog page / article about the new login screen design / proposal.
Until / unless they change their minds, then I can only recommend people try switching over to a different wayland compatible display manager. Researching the competition other than GDM3, only SDDM currently supports wayland too. Which is the login manager for KDE Plasma. So installing this on a stock version of ubuntu could mess up your installation. Due to all it's other KDE dependencies. Therefore a fresh install of kubuntu from scratch is safer. Try out the Kubuntu Live USB / ISO Disk Image before switching.
[how i reached this conclusion]
A breadcrumbs trail of prior gnome3 bugs leading up to it. Since the gnome3 developers say they are not planning to fix the existing / current / old screen shield included in 18.04 in the meantime...
These bugs are listed in a reverse order, from new to old: