feat(pane): always render one message, never the conversationthread-view-removed

Selecting a thread root used to render the whole conversation, stubs
plus the last messages expanded, but only until the thread had been
expanded once. After that the identical click rendered a single message.
The user reported the inconsistency and asked for the single-message
behaviour throughout, and for the conversation view to go.

The cause was a timing one, not a race. The root card stands for the
thread's first message and onThreadSelected already preferred to load
just that, but the model learned the id only when the replies arrived,
so a fresh row fell through to a whole-thread render.

ThreadSummary now carries firstMessageId from the query itself, so the
id is known before any expansion and the fallback is unreachable. It is
free: notmuch_thread_get_toplevel_messages reads the index, not the
message files, and a walk with it is indistinguishable from one without
over a 36,615-thread database. Contrast recipients, which reads every
file and stays Sent-only.

The Sent view keeps showing what the user sent rather than the thread's
opening message, which is often someone else's. There is no
matched-messages iterator in libnotmuch, only a count, so that branch
walks oldest-first to the first NOTMUCH_MESSAGE_FLAG_MATCH and stops:
0.146s against a 0.143s baseline over 4,515 threads.

onThreadLoaded merges into renderMessages, since onMessageLoaded was
already delegating to it for the actual painting. It still takes a list
because MessageView renders a list; collapsing that is a separate change
to a class with its own tests.

NotmuchWorker::loadThread is kept and documented as having no UI caller.
It is a tested way to read a thread's messages with the match set
resolved, used as a helper by the worker's own tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/notmuchworker.h b/src/notmuchworker.h
index 9736ab8..f07e563 100644
--- a/src/notmuchworker.h
+++ b/src/notmuchworker.h
@@ -71,6 +71,19 @@ public slots:
     /// Loads the messages of one thread, oldest first. matchQuery is the
     /// user's current query; messages matching it render expanded, the rest
     /// as stubs.
+    /// **No UI caller since item 66, and that is deliberate.** This was how a
+    /// thread root rendered the whole conversation, stubs plus the last few
+    /// messages expanded. The user asked for that view to go: selecting any
+    /// row, root or reply, now renders exactly one message via loadMessage,
+    /// and `ThreadSummary::firstMessageId` is what makes the root's own
+    /// message known without expanding the thread first.
+    ///
+    /// Kept as a worker capability rather than deleted. It is a tested way to
+    /// read every message of a thread with the match set resolved, which the
+    /// worker's own tests use as a helper and a future feature may want. If
+    /// you are adding a caller, be sure you are not rebuilding the
+    /// conversation pane that was removed on purpose.
+    ///
     /// `matchedOnly` drops the messages that did not match `matchQuery` rather
     /// than rendering them as stubs. For the Sent view, where the thread is not
     /// the unit the user is reading: a sent message pulls in the replies it