This one is hard to explain.
So basically, when I have two windows in GNOME 3 and make them side by side, so every window fits to one half of the screen, the behavior is not like I experienced it in the past and actually I miss it. The thing that is missing:
When I have the situation described above, but then have another window in focus, I was able to select (using the "Activities" menu) one of the two windows that were side-by-side to each other, and then magically the other-half-window would also come to front. They were kind of paired.
I don't know exactly when, but this "pairing behavior" vanished somehow and I would like to restore it.
Something about my system:
I use GNOME 3 since Ubuntu 17.10 and am now using Ubuntu 18.04. I did not modify it much, except that I use the Settings of Dash to Dock to modify the settings of the Ubuntu dock (The extension is disabled but it seems they still share the configs).
The changed settings include the separation of workspaces (meaning that I can i.e. click on the Firefox logo and instead of changing the workspace, it opens a new Firefox window). It's the only setting that looks related to me, but disabling it doesn't help. And actually the separation is a feature I really want to have. I also used it ever since, when everything was working as I expected. It may be possible that in the past I used the gnome tweak tool to configure it manually, so what I expect may not be the default behavior. I couldn't find anything, and I can't remember setting it up myself.