Some styling has been added to the SB UI - such as the invisible splitPanes as we had in 1.1. This is inside 'Common.css'.

If you drag a SplitPane onto the content panel you see that it is inheriting this styling which means that our CSS isolation is failing.

I also tested by putting the following into 'Common.css':

.button {
    -fx-background-color: red;
}

As I suspected, D&D a Button and it shows as red in the content panel.