Yes, I see it in a few occasional places (it's consistent where it happens; one example is the password dialog when doing an ADB backup), but most apps I use don't have the problem. I don't have a non-Samsung device to test, so I'm not sure if it's Samsung-specific or not.
No idea. It's a stock system; no mods nor rooting, and a factory reset doesn't change the situation either (I had to factory reset both devices lately for an unrelated reason.) Most apps don't have this problem, but there are a few specific dialogs in the system and in some apps that behave this way. Maybe Samsung ships buggy firmware? I don't know.
The tablet (SM-P580) reports Android version 7.0 (there's an Oreo update out but I haven't updated yet), and the SM-J727U phone has 8.1.0 (“Samsung Experience version 9.5”). This seems to affect any of the keyboard apps I have (GBoard, Hacker's Keyboard, built-in Samsung keyboard are all affected). I also tried rebooting both devices, but the behavior did not change.
Virtual keyboard does not pop up in dialog boxes
Why should the user need to go through the trouble to do a one-off look at a file? Other Android file managers I've used have no problem with this, and it can be quite convenient. Example: You keep a large media collection on a remote system and want to be able to access files on the fly. Besides, 5M bytes is not particularly large by today's standards. We've moved beyond the 1990s. ;-)
Doubled mouse scroll events in terminal apps