https://javafx-jira.kenai.com/browse/RT-38922

The FX issue above is caused by the fact that on OSX, when Clipboard is modified not in Swing, the notification of the change is only triggered by re-activating of the Swing toplevel, assuming that any outside change could only happen outside of the Swing toplevel. However, this is not the case for JFXPanel, where Clipboard is modified without switching the activation. As a result, data copied to Clipboard from FX couldn't then be pasted to Swing in case Clipboard had contained some Swing data prior to that. Because nothing notifies it of the change, so it uses a cached content (on the java level).

(Note that FX and Swing has different Java clipboard implementations, but they share the system one.)