As I've been doing extensive CSS work recently I have noticed that the SB content view often fails to update when an edit has been made to an external style sheet. I have isolated a simple test case.

Open the attached FXML in SB.
Open the attached CSS in your editor of choice.
Edit the value of the -fx-pref-height rule - you will see the Content Panel updates.
Edit the value of the -fx-pref-height rule again - bug - SB displays a info message saying the CSS file was updated but the Content Panel itself no longer updates. You have to close and re-open the FXML to see the updated version. Note that the Preview window does reflect CSS updates.