How do I stop windows becoming 'full screen' when move to the screen edge

Its called window snapping. It should be an option in Ubuntu Tweak if memory serves.

I don't think ubuntu-tweak is available for 17.10, and I can't see anything about 'window snapping' in gnome-tweak-tool.
Might be in some other randomly named tool (or config file) I can't possibly know the name of.

It is a right PITA that these utilities aren't installed by default.

Or something similar to that. Whatever 17.10 can use to be configured

It is no use just telling me I need to configure it.

10 Things To Do After Installing Ubuntu 17.10
http://www.omgubuntu.co.uk/2017/10/things-to-do-after-installing-ubuntu-17-10

Use the link in that page. It has a tweak tool there. All I searched for was:

tweak app ubuntu 17.10

It was the top link.....

Yes, that tool exists and lets you set 'focus follows mouse' and disable 'raise on focus', but I can't find anything called 'window snapping' or anything else that might have the same effect.

try
dconf write /org/gnome/mutter/edge-tiling false
dconf write /org/gnome/shell/overrides/edge-tiling false
from
https://askubuntu.com/questions/968155/17-10-how-to-disable-window-to-window-snapping-while-having-edge-snapping-enable/969229

By the way, I do not rate this function as "stupid annoying behaviour" but as a very useful feature.

The first one of those changed the behaviour, it really ought to be in one of the config programs that is part of the standard release.

If I move a window to the edge - especially the right edge - it is because I want to look at something underneath without completely hiding the window being moved.

I could complain that I can't get a window partially off the top of the screen - useful to hide title bars you aren't interested in at all. And I don't seem to (easily) be able to create a window taller than the screen in order to hide the bottom parts.
(Yes there are some programs where you really do need to make both the top and bottom hidden!)