feat: flag what you answered, mark what was forwarded to you

Item 68, which turned out to be three things once its premise was
measured. The note asked to extend a "passed" subject rule to "Fw:";
there was no subject rule, and the correlation it rested on did not
exist. What did exist was a gap nobody had reported.

Reply and forward now flag their source. The Maildir R and P flags,
which every other client sets and notmuch reads back as "replied" and
"passed", had never been written here: measured on the developer's
index, all 317 "replied" and all 6 "passed" came from other clients.
ComposeWindow emits sourceMessageAnswered after a successful send and
MainWindow routes it through sendMessageTagChange, message-scoped and
off the undo stack, for the reason auto mark-read is: the flag records
that the mail went, and the send cannot be undone.

ComposeContext carries sourceMessageId rather than reusing inReplyTo,
which is deliberately empty on a forward so the recipient's client does
not file it under the thread it left. Keying on it made the "passed"
half dead code that compiled and never fired. A resumed draft is
excluded: its kind records how the file was opened, not what the user is
doing, so flagging on it would set R from a guess.

A received forward gets its own mark. Derived from the subject at paint
time, storing nothing and reaching no server, because "passed" means "I
forwarded this" and setting it from a guess would assert something false
on 222 existing messages. subjectIsForwarded() shares forwardSubject()'s
prefix table so the two cannot disagree, strips a Re: chain first, and
takes extra locale spellings from [general] forward_prefixes, which
extends the built-in table rather than replacing it.

A mutation survived the first round and corrected a claim in the code:
QRegularExpression::escape already makes a punctuation prefix inert, so
the word guard is not about pattern validity. It stops a configured "-"
matching "-: x". The comment and test say that now.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LXCZFLXbAii5n5wtovpdhh

1 files changed, 20 insertions, 0 deletions

diff --git a/src/composecontext.h b/src/composecontext.h
index 6f311c7..b94d0f8 100644
--- a/src/composecontext.h
+++ b/src/composecontext.h
@@ -168,6 +168,26 @@ QString replySubject(const QString &original);
 
 QString forwardSubject(const QString &original);
 
+/// True when \p subject reads as a message someone forwarded TO the user.
+///
+/// Item 68. This is a DISPLAY predicate: the card draws a mark from it and
+/// stores nothing. It must never write a tag or a Maildir flag, because
+/// `passed` (the `P` flag) means "I forwarded this" and is a different fact
+/// about a different person. Setting it from a subject guess would assert
+/// something false and, with maildir.synchronize_flags on, propagate that to
+/// the server.
+///
+/// Matches the same prefix table forwardSubject() uses, so the set of
+/// recognised spellings cannot drift between "do not double the prefix" and
+/// "this is a forward". \p extraPrefixes adds locale spellings the built-in
+/// table omits, from `[general] forward_prefixes`; each is a bare word without
+/// its colon ("doorst", "vs"). An entry that is empty or not a word is ignored.
+///
+/// A `Re:` chain is stripped first, so `Re: Fwd: x` is recognised: a reply to
+/// a forward is still a forward the user received.
+bool subjectIsForwarded(const QString &subject,
+                        const QStringList &extraPrefixes = {});
+
 /// Builds the context that RESUMES a draft from its file.
 ///
 /// Unlike a reply, nothing here is derived: the recipients, the subject and