Graphics bug in many animated flyouts (Win64; 0.5.5)

When an element with a flyout function is focused by MouseOver, the animated flyouts appear along with a 4 px wide vertical black artefact on the right side. Usually, this artefact is only visible during the short flyout animation time (less than second), but this short duration makes it even more annoying.
This bug is now found by me on Google+ and VK-com only, but I think is is a general graphics error, which is hiding deep inside the engine.

(Here goes the about:version info for my case (win7; Intel x64; 4 Gbyte RAM; ATI video)
{
Version numbers in brackets show the version used at runtime.

Command line F:\Program Files 2 (x86)\Midori\bin\midori.exe
Midori 0.5.5 ((null))
GTK+ 3.6.2 (3.6.2) Glib 2.34.3 (2.34.3)
WebKitGTK+ 1.10.1 (1.10.1) libSoup 2.40.2
cairo 1.10.2 (1.10.2) libnotify No
gcr No granite No
single instance Sockets
Platform Windows NT 6.1
Identification Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.6 (KHTML, like Gecko) Chrome/18.0.1025.133 Safari/537.6 Midori/0.5
Video Formats H264 [x] Ogg Theora [x] WebM [x]

Netscape Plugins:

Google Update Google Update
Java Deployment Toolkit 7.0.400.43 NPRuntime Script Plug-in Library for Java(TM) Deploy
Windows Activation Technologies Windows Activation Technologies Plugin for Mozilla
Shockwave Flash Shockwave Flash 11.8 r800
Java(TM) Platform SE 7 U40 Next Generation Java Plug-in 10.40.2 for Mozilla browsers
Unity Player Unity Player 3.5.5f3
Silverlight Plug-In 5.1.20513.0
}
)