Try using modifier keys with Tab. On my Mac, I was able to tap Option-Tab twice to get out of a rich text-enabled long text field, though there was an odd side-effect. The moment I was in another field, hitting Enter wouldn’t activate that field for editing, but would instead activate the popup menu for the primary field’s summary bar. Very strange. However, if I hit Option-Tab twice to jump to the next field, then just started typing, everything was fine.