Using ppa:gnome3-team/gnome3-staging dropdown menus in Libreoffice are messed up

[gunwald]

What did I do?
I installed ppa:gnome3-team/gnome3-staging and opened Libreoffice Writer:
   sudo add-apt-repository ppa:gnome3-team/gnome3-staging
   sudo add-apt-repository ppa:gnome3-team/gnome3
   sudo apt update && sudo apt dist-upgrade

What did happen?
All dropdown menus in LibreOffice Writer are a mess: The don't have a border, don't have a white background and if the selected entry changes (for example you move the cursor between to paragraphs with different fonts) the text becomes unreadable.

I gues, this is a problem with QT?

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: libreoffice-core 1:5.1.2-0ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-23.41-generic 4.4.10
Uname: Linux 4.4.0-23-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: GNOME
Date: Thu May 19 02:00:42 2016
ExecutablePath: /usr/lib/libreoffice/program/soffice.bin
SourcePackage: libreoffice
UpgradeStatus: Upgraded to xenial on 2016-04-22 (26 days ago)

[Björn Michaelsen (bjoern-michaelsen)]

FWIW, what I assume happening here is:
- something in the gnome ppa pulls in libreoffice-gtk3
- LibreOffice prefers the gtk3 backend over the gtk2 backend now (does so too on Debian btw)

workaround: set SAL_USE_VCLPLUGIN=gtk or SAL_USE_VCLPLUGIN=gen to force other plugins.

proper solution: Improve the gtk3 themes so they work proper in LibreOffice too.