Comment 10 for bug 1792843

I have almost the same issue.
Using gdm3, when I lock my session (e.g. with <super>+<L>) the lock screen doesn't accept keyboard and mouse events at all.

This all began right after the upgrade from bionic to cosmic.

Another thing which might be related as it only happens when I use gdm3:
When I open the activities view, it never goes away. This means: I see the launcher on the left, my desktop wallpaper overlayed with transparent black (for this dimm-effect) and this stays for ever.
I can search for application, run programs either from the launcher or by searching for them or e.g. by pressing registered key combinations. The program-icons apear in the launcher as they should, but I don't see any actual program images as it appears they are all hidden beneath the activities background.

When using sddm or lightdm I don't have this issues (though I have others).

Software: Ubuntu cosmic amd64 with latest updates.
X-Session: Gnome on Xorg
Hardware: Lenovo Thinkpad x230