Every Send control was disabled only by `canSend` (recipient/subject/body
validity), which never reflects an in-flight submission, so the composer
stayed interactive during the JMAP round-trip. Clicking Send quickly more
than once - or a click racing the keyboard send shortcut - invoked
handleSend once per click and sent the message multiple times (duplicate
deliveries and duplicate Sent entries), most easily hit on higher-latency
connections.
Add a synchronous re-entry guard: a ref (not state, which updates
asynchronously and wouldn't block a second click in the same tick) set once
handleSend clears its "don't send" early returns and reset in a finally,
plus an isSending state that disables every Send control. Covers all entry
points - the three Send buttons, the keyboard shortcut, the schedule dialog,
and the attachment-warning confirm.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
