This is very hard to reproduce. Run HelloComboBox and cover it with another window. Then after a few seconds bring HelloComboBox to the foreground and use your mouse to randomly select comboboxes. After a while HelloComboBox will be stuck.

I traced the problem to here and after talking to Kevin this is the jira causing it RT-20656

ComboBoxListViewSkin.java
        textField.focusedProperty().addListener(new ChangeListener<Boolean>() {
            @Override public void changed(ObservableValue<? extends Boolean> ov, Boolean t, Boolean hasFocus) {
                if (hasFocus) {
                    // Using Platform.runLater here, as without it it seems we
                    // enter into some form of race condition where we are
                    // wanting to set focus on the comboBox whilst the textField
                    // is still notifying of its focus gain.
                    // This issue was identified in RT-21088.
                    Platform.runLater(new Runnable() {
                        @Override public void run() {
                            comboBox.requestFocus();
                        }
                    });
                }