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.
Best Answer
If one hasn't noticed, the said issue has been unsolved for the past two years to this answered date, according to this issue #4 on GitHub. The description is also very much alike:
The current maintainer, zakkak, seems to be aware of this issue from before and even quoted another project that had similar feature.
The implementation however, seems to be incomplete:
Several contributors had tried testing the implementation and latest attempt to this date was recorded in early-2016, which noted the limitation in this issue #23.
Note that this answer:
Disclaimer I do not use GNOME and not involved with the project.
TL;DR Try out the 'thumbnails' branch, if you haven't. Review the code and suggest improvements at the project repository on GitHub.