How to fix what for me is a problem in the GNOME GUI
Disabling overlapping scroll bars means that the bar is displayed only when the content exceeds the graphic capacity of the window. To disable it you need to open the dconf editor, if we do not have it type the following command in the terminal to install it
apt-get install dconf-editor -y
then to disable it use the following command
gsettings set org.gnome.desktop.interface overlay-scrolling false
and to re-enable it the following command
gsettings set org.gnome.desktop.interface overlay-scrolling true