Turn off smooth scrolling in GTK3

I solved this by disabling animations in the gnome shell. Disabling animations will disable all animations. This setting can be changed in many different ways. For the change to take effect in gedit, don't forget to restart gedit.

Using gsettings (command line)

Open a terminal and run this command:

gsettings set org.gnome.desktop.interface enable-animations false

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Using dconf-editor (GUI)

From the start screen, navigate to /org/gnome/desktop/interface/enable-animations

and set it to false.

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Using gnome-tweak-tool (GUI)

Open gnome-tweaks. In the appearance tab, set the following option to false: animations

Edit the file ~/.config/gtk-3.0/settings.ini to contain the following lines:

[Settings]
gtk-enable-animations=0

If the file doesn't exist, create it. If the file already exists and already has a [Settings] section, add gtk-enable-animations=0 in this section.

Thanks to DK Bose for suggesting .config/gtk-3.0/settings.ini and to pm-b for suggesting the option name enable-animations (I was looking for “scroll”).

You don't need to reboot or to log out, but you do need to restart the application.

Unfortunately, Gnome apps have made this MAJOR mess on Ubuntu 18.04. The correct setting depends on the Desktop environment you use. For Gnome 3.x it seems that the way to disable smooth scroll is to run

gsettings set org.gnome.desktop.interface enable-animations false

which disables other animations as well.

If you're running MATE you have to use different key

gsettings set org.mate.interface gtk-enable-animations false