All versions of Ubuntu, including Xubuntu, now use an overlay scrollbar in all GTK applications. The scrollbar is hidden, but a scroller appears when the mouse cursor is hovered on the right or bottom borders of a window.
If you would like the legacy scrollbar to be used instead of this overlay scrollbar, that can be done in a couple of ways:
The best solution is to remove the overlay scrollbar packages and restart your system:
$ sudo apt-get purge overlay-scrollbar*
This typically removes 3 packages:
Install the Unity Tweak Tool:
$ sudo apt-get install unity-tweak-tool
Next, open the Unity Tweak Tool and go to System > Scrolling. In the Scrollbars section, choose the Legacy radio button.
That is it, legacy scrollbars are enabled back in Ubuntu! :)
Tried with: Unity Tweak Tool 0.0.6 and Ubuntu 14.04