I've noticed that Latte Dock flickers on first hover and sort of black bar appears. It works ok for a while - at least until you log of then on , or change compositing. I found this bug report:

https://bugs.kde.org/show_bug.cgi?id=398257

It looks like exactly what I'm experiencing. It is marked as resolved - but I don't see a clue howto apply a possible fix. Has anyone figured this one out?