Questioner Info
Description
As per title,

In the Adwaita, there is a highlighted border instead.

I don't think the new design indicates the current workspace more clearly than the original.
Thanks!
It seems you are using Materia gnome-shell theme for 3.38, not for 40. I'm assuming you installed/updated Materia before updating gnome-shell to 40.
By default our build system builds the gnome-shell theme based on your _current_ gnome-shell --version. So when you update the major version of gnome-shell, you need to reinstall the theme. Otherwise the gnome-shell theme remains for the previous version.
And the Arch package maintainers should've bump the materia-gtk-theme version e.g. to 20210322-2, with the build option -Dgnome_shell_version=40, when their gnome-shell package was updated to 40.0. Then this wasn't happening. Someone needs to tell this to the Arch package maintainers...
Thanks, I see it might be the same reason for #541 and #537. I've reported it.
Thanks for reporting it!
In INSTALL.md, I've added a note about it for package maintainers.
I think there is nothing left to do for this on our side, so I'm going to close this.
Thanks again for your contribution :)