User feedback:
Undoing a style only change fires a change event where ContentChange.isEdit is true when it should be false.

The failure can be tested with the following unit test:

    @Test
    public void undoStyleChange() {
        ArrayList<ContentChange> changes = new ArrayList<>();
        TextPos p = control.appendText("BOLD");
        control.getModel().addListener((ch) -> {
            changes.add(ch);
        });
        control.applyStyle(TextPos.ZERO, p, BOLD);
        control.undo();
        assertEquals(2, changes.size());
        ContentChange ch1 = changes.get(0);
        assertFalse(ch1.isEdit());
        ContentChange ch2 = changes.get(1);
        assertFalse(ch2.isEdit());
    }