Can't resize some elements in GTK+ programs

[Ben Aceler]

Steps to reproduce:

1) Install a GTK+ application that uses elements that changes their size. For example, I use Banshee.
2) Launch the application and try to resize an element. For example, I tried to move the delimeter between progress line and an album information

Expected result: the delimeter moves.
Actual result: all the application moves, but the delimeter stays.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: kwin 4:5.2.2a-0ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-13.13-generic 3.19.3
Uname: Linux 3.19.0-13-generic x86_64
ApportVersion: 2.16.2-0ubuntu4
Architecture: amd64
CurrentDesktop: KDE
Date: Sun Apr 12 22:18:06 2015
InstallationDate: Installed on 2015-04-11 (0 days ago)
InstallationMedia: Kubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
SourcePackage: kwin
UpgradeStatus: No upgrade log present (probably fresh install)