It’s that thing where when you drag a window so it hits the edge of the screen, GNOME offers to maximise the window. Generally the number of times I will knowingly want to maximise a window by dragging it to the top of the screen is 0, while the number of times it happens accidentally is over 9,000 by lunch time.
Things that work
$ dconf write /org/gnome/mutter/edge-tiling false
It should take effect immediately.
If you like a pointy clicky way to do it then install dconf-editor package and run dconf-editor, but really all you will do is click down the tree org → gnome → mutter and then toggle edge-tiling so I don’t really see the point.
Things that people on the Internet say work, but don’t – a non-exhaustive list
These suggestions silently fail to do anything, as far as I can see. They may have been correct for earlier versions of GNOME, but I am using GNOME on Ubuntu 17.10 and they didn’t work for me.
dconf write /org/gnome/shell/extensions/classic-overrides/edge-tiling false
gsettings set org.gnome.shell.overrides edge-tiling false
dconf write /org/gnome/shell/overrides/edge-tiling false