On Mac if I set an accelerator on a MenuItem that defines a modifier, say ctrl + a, and if I do so using KeyCharacterCombination, as soon as I press the modifier key the handler attached to the action defined for this menu entry is called many times.
That means, as of the code extract below, I see many output lines "menuitem1 got Event" even before I have time to press "a" key.
I should see this output only once and when both ctrl and "a" keys are pressed. At least this is the behavior I get on Windows.
For some reason when using KeyCodeCombination it does as expected on both Mac and Windows.

// KeyCodeCombination acceleratorKeyComboA =
// new KeyCodeCombination(KeyCode.A,
// KeyCombination.CONTROL_DOWN);
        KeyCharacterCombination acceleratorKeyComboA =
                new KeyCharacterCombination("a",
                                       KeyCombination.CONTROL_DOWN);

        MenuItem menuitem1 = new MenuItem();
        menuitem1.setText("_Open");
        menuitem1.setMnemonicParsing(true);
        menuitem1.setAccelerator((KeyCombination) acceleratorKeyComboA);
        menuitem1.setOnAction(new EventHandler<ActionEvent>() {
            @Override public void handle(ActionEvent e) {
                System.out.println("menuitem1 got Event: " + e);
            }
        });