I'm using Finder's "Connect to Server" with a "vnc: // …" address to access a desktop remotely, from Mac to Mac (both run the latest Catalina), and I've always had a problem with The notification windows (in the upper right corner) fade / disappear, leaving a "ghost" of the window behind indefinitely. I can't click on things behind windows, and clicking on one of these doesn't do what I normally would if the window were completely opaque. For example, this shows a couple of notifications from MS Outlook / Update, and a notification from the App Store.
And here it is with an open menu below. You can only click a menu item by entering between the window openings (only the first item is accessible in this example):
I can make them disappear temporarily by activating the "DO NOT DISTURB" mode for notifications. But I am looking for a way to correct this behavior without disabling notifications. I still don't know if this happens with other VNC viewers besides the one integrated in MacOS / Finder. I can try that later. And I also plan to try to disable animations if no other solution is available. (How to disable all animations in OS X
Incidentally, this has also been happening to me with previous versions of MacOS.