Faulty transparency in Gtk+ "About" dialog due to overlay scrollbar

Open the Help->About dialog of a Gtk+ app, e.g. gnome-terminal, gedit, evince...

Some text appears over the default gray background. Click on the Credits or License button. New text appears on white. Now click on this button again to close the info that just appeared.

Expected behivior: The initial look of the About dialog should be restored.

Actual behavior: The foreground text is there, but the background becomes black (Utopic) or transparent (Vivid beta).

If the dialog is closed and opened again, it'll be faulty right away for the rest of the app's lifetime. You have to restart the application to make it show correctly again.

Tested both with Gnome-shell and Unity; and both with Adwaita and Ambiance. In case of gnome-terminal, both with and without the unofficial transparency patch.

LIBOVERLAY_SCROLLBAR=0 makes the bug go away.

In https://bugzilla.gnome.org/show_bug.cgi?id=746707 mainsteam Gtk+ developer stated that he (presumably on Fedora) can't see the bug either, and his (apparently correct) guess was that it's caused by the overlay scrollbar.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: overlay-scrollbar-gtk3 0.2.16+r359+15.04.20150319-0ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-10.10-generic 3.19.2
Uname: Linux 3.19.0-10-generic x86_64
ApportVersion: 2.16.2-0ubuntu4
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Mar 25 01:34:25 2015
InstallationDate: Installed on 2012-05-30 (1028 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
SourcePackage: overlay-scrollbar
UpgradeStatus: Upgraded to vivid on 2015-03-17 (7 days ago)