I've got Gnome 3.20 running with the "Workspace Grid" extension. Everything works well, except when pressing ctrl + alt + [arrow] to navigate my workspaces, the resulting overaly doesn't actually show what's on the workspace. If I go into the Activities are, the workspace thumbnails are displayed as they should be.
Here's what the overlay looks like currently (notice that the overaly is totally blank in each box):
Here's what it should look like:
I currently use the "Workspace Grid" plugin, also.