Notifications don't become more transparent when passing the mouse cursor over them

[php4fan]

When FileZilla is finished transferring a bunch of files, or when Thunderbird has downloaded new emails, a notification pops up near the top-right corner of the screen.

Sometimes these notifications cover things on the screen that I need to see and interact with.

In gnome 3 and in whatever desktop environment was the default in Ubuntu 16.04 (and I think even a few releases before that), these notification popups used to significantly decrease in opacity when you passed the mouse cursor over them, making it easier to see what was behind them .

They don't do that in Cinnamon, and that's a big step back in usability that should be addressed.