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, 9 insertions, 1 deletions

diff --git a/src/types.h b/src/types.h
index 7464586..cf2411d 100644
--- a/src/types.h
+++ b/src/types.h
@@ -252,7 +252,15 @@ struct ComposeContext
     QString accountKey;          ///< Which account sends. Plain data here; the resolution rules live with whatever builds this context.
     Kind kind = Kind::New;
     QString originalPath;        ///< The .eml being replied to or forwarded. Empty for New.
-    QString inReplyTo;           ///< Message-ID of the original.
+    QString inReplyTo;           ///< Message-ID of the original. EMPTY for a Forward: carrying In-Reply-To would file the forward under the thread it left, in the recipient's client.
+
+    /// Message-ID of the message being answered, for flagging it afterwards.
+    ///
+    /// Item 68. Separate from inReplyTo because that is a THREADING header and
+    /// is deliberately empty on a Forward, while the P flag still has to land
+    /// on the message that was forwarded. Set for Reply, ReplyAll and Forward;
+    /// empty for New and for a resumed Draft.
+    QString sourceMessageId;
     QStringList references;      ///< The original's References plus its Message-ID.
     QStringList to;              ///< Pre-filled, the user's own addresses already stripped.
     QStringList cc;