Prepend the MJWhiteboard.submit helper to a widget's HTML source — EXACTLY once.
Idempotent: when the helper is already present (re-render of an injected source) the
input is returned unchanged, so memoized srcdocs never accumulate duplicate helpers.
Prepend the
MJWhiteboard.submithelper to a widget's HTML source — EXACTLY once. Idempotent: when the helper is already present (re-render of an injected source) the input is returned unchanged, so memoized srcdocs never accumulate duplicate helpers.