Okay is this a feature request? I'm sure this could be part of customization or extra configuration.

And there is no need to call this a disaster. If it's non-urgent then it's intended but if it's classified as non-urgent when it IS urgent then bring it up here.

It is a feature request, yes.
Unfortunately I found no way to add the corresponding label to it.

And it is not an urgent matter, nor is it something critical.
Though it would be nice to get this implemented soon

I want this. Doesn't seem excessively difficult to implement, so I'm going to look into it.

If one really needs this right now, a quick and dirty fix is to edit /usr/share/cinnamon/js/ui/messageTray.js#L1047 changing false to true. It'll get overwritten with Cinnamon updates, but at least it'll work until a proper option can be added.

Much obliged for the quickfix offered.
I got used to this over time, but nevertheless - I am pleased to see someone else who regards this as an actual issue.

If one really needs this right now, a quick and dirty fix is to edit /usr/share/cinnamon/js/ui/messageTray.js#L1047 changing false to true. It'll get overwritten with Cinnamon updates, but at least it'll work until a proper option can be added.

Has the line changed since? It's been almost 3 years since this reply, and I can't seem to find what to change, help would be greatly appreciated.

Cinnamon 6.0.4

Never mind, it looks like the line number has changed to /usr/share/cinnamon/js/ui/messageTray.js#L1012.
The line itself is
Main.layoutManager._chrome.modifyActorParams(this._notificationBin, { visibleInFullscreen: false });
if it changes again anytime in the future.

Has anyone here found an application or browser where the notifications didn't show over them in fullscreen? In my testing, I've had inconsistent results with Chromium but other browsers have been better (Firefox, LibreWolf, Vivaldi, etc.).