Dom, I believe the issue is only happening to KDE user that install gtk-qt-engine.
By default, GTK application in KDE will use GTK widget, so that would explain why nobody is aware of the issue. Gnome user will sure not having this issue. As what I have tested, if I use other Raleigh style in that option, the issue goes away. Using other KDE style is also showing the same bug. I'm currently using Plastik theme. -- Sharuzzaman Ahmat Raslan
