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With Ubuntu 15.10 (currently in development, to be released on October 22), Unity will stop using its original overlay scrollbars for GTK3 apps and switch to Gnome's scrollbars instead.

This change affects GTK3 applications only and currently, the original Unity overlay scrollbars are still used for GTK2 applications (there's no info if this will change until the final release).

Unity's overlay scrollbars were announced back in March 2011 and were used by default starting with Ubuntu 11.04, being designed to maximize screen real estate.

In case you're not familiar with Unity's overlay scrollbars, these are the scrollbars you see in GTK apps (they aren't used in Firefox and LibreOffice, among others) under Unity (and not only) when you move your mouse over to the right side of the app:

Gnome's overlay scrollbars have the same goal - maximize screen real estate and remove the useless chrome -, and are displayed only when they are needed: a small scrollbar is displayed when the pointer is moved and a larger scrollbar is displayed when the user wants to interact with it:

Will Cooke, Ubuntu Desktop Manager says that "upstream Gnome now have some nicely designed ones [overlay scrollbars] of their own, and so to minimize the maintenance effort of the Unity ones we've decided to adopt the Gnome ones for 15.10 onwards". He adds that there are some bugs that need fixing, such as Dash needing an update to look like the Gnome overlay scrollbars and that it's all work in progress.

Besides minimizing the maintenance effort and better usability, a nasty bug should no longer occur thanks to this change: you shouldn't see a black / transparent background under Unity for some GNOME 3 apps, like Handbrake (screenshot), any more because this issue was caused by Unity's overlay scrollbars.

Below you can watch a video posted by Will Cooke today, showing Gnome's Overlay scrollbars in Unity (under Ubuntu 15.10 Wily Werewolf):