2 works in Windows, xfce and Unity too, mostly (and has been for years). The focus stays on the calculator if you wanna type into it, but you can still scroll whatever your mouse is over.
1 certainly works in any Linux DE I've ever used, as long as I make the terminal stay in the foreground whether it's focused or not. Personally, I prefer to use guake for sucha usecase, however.
2 works on macOS too, for what itβs worth. (My memory of this, which may not be entirely reliable, is that Linux and OS X both had 2 forever. I wish I could say the same for 1.)
Those were examples. The key point is that this is generic behavior for any window at any time. It doesn't have to be special-cased (with the calculator) and it happens all the time (making the terminal stay in the foreground is at least two clicks, for instance).
1 certainly works in any Linux DE I've ever used, as long as I make the terminal stay in the foreground whether it's focused or not. Personally, I prefer to use guake for sucha usecase, however.