fix(read): auto mark-read touches only the message on screen

Reported by the user: selecting an unexpanded thread root marked every
message in the thread read, replies included. maildir.synchronize_flags
is on, so removing `unread` rewrites Maildir filenames and the next sync
carries it to the server: mail the user never saw stops being unread
everywhere.

This was coherent while a root click rendered the whole conversation,
because everything marked read had been displayed. Removing that view
made a root render one message and left the thread-wide write in place,
so the defect arrived with the previous commit.

markCurrentThreadRead now sends m_currentMessageId, which is what the
pane rendered, through sendMessageTagChange. The thread-level `unread`
guard is dropped with it: a thread carries `unread` while ANY message in
it is unread, so it would pass a read root under unread replies and send
a write for a message already read. Scheduling still checks it, which
keeps a fully-read thread from arming a timer.

The test asserts on which worker entry point the window used, because
reading tags back cannot answer this. Three earlier versions passed
against the unfixed code: TagsRole is empty for a message row by design,
MessageOwnTagsRole subtracts thread tags and drops marks so it can never
hold `unread`, and raw node tags are not refreshed until onTagsApplied
confirms, which lands after the assertion. Mutation checked.

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

1 files changed, 26 insertions, 6 deletions

diff --git a/src/mainwindow.cpp b/src/mainwindow.cpp
index fb34fe2..17a5a65 100644
--- a/src/mainwindow.cpp
+++ b/src/mainwindow.cpp
@@ -3355,16 +3355,36 @@ void MainWindow::markCurrentThreadRead()
     }
 
     const ThreadSummary thread = m_model->threadAt(current.row());
-    if (thread.threadId != m_markReadThreadId
-        || !thread.tags.contains(QStringLiteral("unread"))) {
+    if (thread.threadId != m_markReadThreadId) {
         m_markReadThreadId.clear();
         return;
     }
 
-    const QStringList threadIds = { m_markReadThreadId };
+    // No thread-level `unread` check here any more. It would ask the wrong
+    // question now that one message is marked rather than the thread: a thread
+    // carries `unread` while ANY message in it is unread, so a read root under
+    // unread replies would pass this and a write would be sent for a message
+    // that is already read. The scheduling side still checks it, which stops a
+    // fully-read thread from arming a timer at all; what survives to here is
+    // decided per message below.
+
     m_markReadThreadId.clear();
 
-    // sendThreadTagChange, NOT tagSelected: this deliberately does not go on
+    // The MESSAGE on screen, not the thread it belongs to.
+    //
+    // This marked the whole thread until item 66, and that was coherent while
+    // a root click rendered the whole conversation: everything marked read had
+    // been displayed. Once a root began rendering a single message, the same
+    // code cleared `unread` from replies the user had never seen. Not a
+    // cosmetic slip: maildir.synchronize_flags is on, so removing `unread`
+    // rewrites Maildir filenames and the next sync carries it to the server.
+    //
+    // m_currentMessageId is what the pane actually rendered, set beside the
+    // loadMessage that produced it.
+    if (m_currentMessageId.isEmpty())
+        return;
+
+    // sendMessageTagChange, NOT tagSelected: this deliberately does not go on
     // the undo stack. The user never took this action, so hijacking Ctrl+Z to
     // reverse it would undo something they did not do, and toggle_unread
     // already gives them a direct way to put it back. Decided 2026-08-03.
@@ -3372,8 +3392,8 @@ void MainWindow::markCurrentThreadRead()
     // It still funnels through the one applyTags path, per CLAUDE.md; what
     // differs is only whether the inverse is pushed, which is a window-level
     // decision above the worker.
-    sendThreadTagChange(threadIds, {}, { QStringLiteral("unread") },
-                        tr("Mark read"));
+    sendMessageTagChange({ m_currentMessageId }, {},
+                         { QStringLiteral("unread") }, tr("Mark read"));
 }
 
 void MainWindow::editTagsOnSelection()