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, 10 insertions, 0 deletions

diff --git a/src/mainwindow.h b/src/mainwindow.h
index a5a8c31..cb8cec4 100644
--- a/src/mainwindow.h
+++ b/src/mainwindow.h
@@ -1104,6 +1104,16 @@ private:
 
     /// Confirms a move: applies the tags the move was asked to carry, with the
     /// origin placeholder resolved per message.
+    /// The single tag the CURRENT view's membership depends on, or empty.
+    ///
+    /// Only the three plain `tag:` filters (Unread, Inbox, Important) have
+    /// one: their query is exactly that tag, so a message losing it stops
+    /// belonging. Trash, Sent and Drafts are PATH queries, where a tag change
+    /// decides nothing, and a hand-typed query is not reasoned about at all.
+    /// Both answer empty, which is what keeps an optimistic row removal from
+    /// firing in a view it cannot judge.
+    QString viewFilterTag() const;
+
     void onMessagesMoved(const QMap<QString, QString> &originByMessageId,
                          const QString &destFolder);