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): 
[![Picture of blank overlays (where there should be thumbnails of what's currently on the screen](https://i.stack.imgur.com/RsuxN.png)](https://i.stack.imgur.com/RsuxN.png)

Here's what it should look like:
[![Picture of correct overlays (where there are thumbnails of what's currently on the screen](https://i.stack.imgur.com/HOAPo.png)](https://i.stack.imgur.com/HOAPo.png)

I currently use the "Workspace Grid" plugin, also.